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freebsd-skq/sys/netinet/tcp_syncache.c
Michael Tuexen 283c76c7c3 RFC 7323 specifies that: 
* TCP segments without timestamps should be dropped when support for
  the timestamp option has been negotiated.
* TCP segments with timestamps should be processed normally if support
  for the timestamp option has not been negotiated.
This patch enforces the above.

PR:			250499
Reviewed by:		gnn, rrs
MFC after:		1 week
Sponsored by:		Netflix, Inc
Differential Revision:	https://reviews.freebsd.org/D27148
2020-11-09 21:49:40 +00:00

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/*-
* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
*
* Copyright (c) 2001 McAfee, Inc.
* Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
* All rights reserved.
*
* This software was developed for the FreeBSD Project by Jonathan Lemon
* and McAfee Research, the Security Research Division of McAfee, Inc. under
* DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
* DARPA CHATS research program. [2001 McAfee, Inc.]
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_pcbgroup.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/hash.h>
#include <sys/refcount.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/limits.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/proc.h> /* for proc0 declaration */
#include <sys/random.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/syslog.h>
#include <sys/ucred.h>
#include <sys/md5.h>
#include <crypto/siphash/siphash.h>
#include <vm/uma.h>
#include <net/if.h>
#include <net/if_var.h>
#include <net/route.h>
#include <net/vnet.h>
#include <netinet/in.h>
#include <netinet/in_kdtrace.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <netinet/in_var.h>
#include <netinet/in_pcb.h>
#include <netinet/ip_var.h>
#include <netinet/ip_options.h>
#ifdef INET6
#include <netinet/ip6.h>
#include <netinet/icmp6.h>
#include <netinet6/nd6.h>
#include <netinet6/ip6_var.h>
#include <netinet6/in6_pcb.h>
#endif
#include <netinet/tcp.h>
#include <netinet/tcp_fastopen.h>
#include <netinet/tcp_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/tcp_syncache.h>
#ifdef INET6
#include <netinet6/tcp6_var.h>
#endif
#ifdef TCP_OFFLOAD
#include <netinet/toecore.h>
#endif
#include <netipsec/ipsec_support.h>
#include <machine/in_cksum.h>
#include <security/mac/mac_framework.h>
VNET_DEFINE_STATIC(int, tcp_syncookies) = 1;
#define V_tcp_syncookies VNET(tcp_syncookies)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_VNET | CTLFLAG_RW,
&VNET_NAME(tcp_syncookies), 0,
"Use TCP SYN cookies if the syncache overflows");
VNET_DEFINE_STATIC(int, tcp_syncookiesonly) = 0;
#define V_tcp_syncookiesonly VNET(tcp_syncookiesonly)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_VNET | CTLFLAG_RW,
&VNET_NAME(tcp_syncookiesonly), 0,
"Use only TCP SYN cookies");
VNET_DEFINE_STATIC(int, functions_inherit_listen_socket_stack) = 1;
#define V_functions_inherit_listen_socket_stack \
VNET(functions_inherit_listen_socket_stack)
SYSCTL_INT(_net_inet_tcp, OID_AUTO, functions_inherit_listen_socket_stack,
CTLFLAG_VNET | CTLFLAG_RW,
&VNET_NAME(functions_inherit_listen_socket_stack), 0,
"Inherit listen socket's stack");
#ifdef TCP_OFFLOAD
#define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
#endif
static void syncache_drop(struct syncache *, struct syncache_head *);
static void syncache_free(struct syncache *);
static void syncache_insert(struct syncache *, struct syncache_head *);
static int syncache_respond(struct syncache *, const struct mbuf *, int);
static struct socket *syncache_socket(struct syncache *, struct socket *,
struct mbuf *m);
static void syncache_timeout(struct syncache *sc, struct syncache_head *sch,
int docallout);
static void syncache_timer(void *);
static uint32_t syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
uint8_t *, uintptr_t);
static tcp_seq syncookie_generate(struct syncache_head *, struct syncache *);
static struct syncache
*syncookie_lookup(struct in_conninfo *, struct syncache_head *,
struct syncache *, struct tcphdr *, struct tcpopt *,
struct socket *);
static void syncache_pause(struct in_conninfo *);
static void syncache_unpause(void *);
static void syncookie_reseed(void *);
#ifdef INVARIANTS
static int syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
struct socket *lso);
#endif
/*
* Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
* 3 retransmits corresponds to a timeout with default values of
* tcp_rexmit_initial * ( 1 +
* tcp_backoff[1] +
* tcp_backoff[2] +
* tcp_backoff[3]) + 3 * tcp_rexmit_slop,
* 1000 ms * (1 + 2 + 4 + 8) + 3 * 200 ms = 15600 ms,
* the odds are that the user has given up attempting to connect by then.
*/
#define SYNCACHE_MAXREXMTS 3
/* Arbitrary values */
#define TCP_SYNCACHE_HASHSIZE 512
#define TCP_SYNCACHE_BUCKETLIMIT 30
VNET_DEFINE_STATIC(struct tcp_syncache, tcp_syncache);
#define V_tcp_syncache VNET(tcp_syncache)
static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache,
CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
"TCP SYN cache");
SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
&VNET_NAME(tcp_syncache.bucket_limit), 0,
"Per-bucket hash limit for syncache");
SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_VNET | CTLFLAG_RDTUN,
&VNET_NAME(tcp_syncache.cache_limit), 0,
"Overall entry limit for syncache");
SYSCTL_UMA_CUR(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_VNET,
&VNET_NAME(tcp_syncache.zone), "Current number of entries in syncache");
SYSCTL_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_VNET | CTLFLAG_RDTUN,
&VNET_NAME(tcp_syncache.hashsize), 0,
"Size of TCP syncache hashtable");
static int
sysctl_net_inet_tcp_syncache_rexmtlimit_check(SYSCTL_HANDLER_ARGS)
{
int error;
u_int new;
new = V_tcp_syncache.rexmt_limit;
error = sysctl_handle_int(oidp, &new, 0, req);
if ((error == 0) && (req->newptr != NULL)) {
if (new > TCP_MAXRXTSHIFT)
error = EINVAL;
else
V_tcp_syncache.rexmt_limit = new;
}
return (error);
}
SYSCTL_PROC(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit,
CTLFLAG_VNET | CTLTYPE_UINT | CTLFLAG_RW | CTLFLAG_NEEDGIANT,
&VNET_NAME(tcp_syncache.rexmt_limit), 0,
sysctl_net_inet_tcp_syncache_rexmtlimit_check, "UI",
"Limit on SYN/ACK retransmissions");
VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
"Send reset on socket allocation failure");
static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
#define SCH_LOCK(sch) mtx_lock(&(sch)->sch_mtx)
#define SCH_UNLOCK(sch) mtx_unlock(&(sch)->sch_mtx)
#define SCH_LOCK_ASSERT(sch) mtx_assert(&(sch)->sch_mtx, MA_OWNED)
/*
* Requires the syncache entry to be already removed from the bucket list.
*/
static void
syncache_free(struct syncache *sc)
{
if (sc->sc_ipopts)
(void) m_free(sc->sc_ipopts);
if (sc->sc_cred)
crfree(sc->sc_cred);
#ifdef MAC
mac_syncache_destroy(&sc->sc_label);
#endif
uma_zfree(V_tcp_syncache.zone, sc);
}
void
syncache_init(void)
{
int i;
V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
V_tcp_syncache.hash_secret = arc4random();
TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
&V_tcp_syncache.hashsize);
TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
&V_tcp_syncache.bucket_limit);
if (!powerof2(V_tcp_syncache.hashsize) ||
V_tcp_syncache.hashsize == 0) {
printf("WARNING: syncache hash size is not a power of 2.\n");
V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
}
V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
/* Set limits. */
V_tcp_syncache.cache_limit =
V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
&V_tcp_syncache.cache_limit);
/* Allocate the hash table. */
V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
#ifdef VIMAGE
V_tcp_syncache.vnet = curvnet;
#endif
/* Initialize the hash buckets. */
for (i = 0; i < V_tcp_syncache.hashsize; i++) {
TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
NULL, MTX_DEF);
callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
&V_tcp_syncache.hashbase[i].sch_mtx, 0);
V_tcp_syncache.hashbase[i].sch_length = 0;
V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
V_tcp_syncache.hashbase[i].sch_last_overflow =
-(SYNCOOKIE_LIFETIME + 1);
}
/* Create the syncache entry zone. */
V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
V_tcp_syncache.cache_limit);
/* Start the SYN cookie reseeder callout. */
callout_init(&V_tcp_syncache.secret.reseed, 1);
arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
syncookie_reseed, &V_tcp_syncache);
/* Initialize the pause machinery. */
mtx_init(&V_tcp_syncache.pause_mtx, "tcp_sc_pause", NULL, MTX_DEF);
callout_init_mtx(&V_tcp_syncache.pause_co, &V_tcp_syncache.pause_mtx,
0);
V_tcp_syncache.pause_until = time_uptime - TCP_SYNCACHE_PAUSE_TIME;
V_tcp_syncache.pause_backoff = 0;
V_tcp_syncache.paused = false;
}
#ifdef VIMAGE
void
syncache_destroy(void)
{
struct syncache_head *sch;
struct syncache *sc, *nsc;
int i;
/*
* Stop the re-seed timer before freeing resources. No need to
* possibly schedule it another time.
*/
callout_drain(&V_tcp_syncache.secret.reseed);
/* Stop the SYN cache pause callout. */
mtx_lock(&V_tcp_syncache.pause_mtx);
if (callout_stop(&V_tcp_syncache.pause_co) == 0) {
mtx_unlock(&V_tcp_syncache.pause_mtx);
callout_drain(&V_tcp_syncache.pause_co);
} else
mtx_unlock(&V_tcp_syncache.pause_mtx);
/* Cleanup hash buckets: stop timers, free entries, destroy locks. */
for (i = 0; i < V_tcp_syncache.hashsize; i++) {
sch = &V_tcp_syncache.hashbase[i];
callout_drain(&sch->sch_timer);
SCH_LOCK(sch);
TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
syncache_drop(sc, sch);
SCH_UNLOCK(sch);
KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
("%s: sch->sch_bucket not empty", __func__));
KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
__func__, sch->sch_length));
mtx_destroy(&sch->sch_mtx);
}
KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
("%s: cache_count not 0", __func__));
/* Free the allocated global resources. */
uma_zdestroy(V_tcp_syncache.zone);
free(V_tcp_syncache.hashbase, M_SYNCACHE);
mtx_destroy(&V_tcp_syncache.pause_mtx);
}
#endif
/*
* Inserts a syncache entry into the specified bucket row.
* Locks and unlocks the syncache_head autonomously.
*/
static void
syncache_insert(struct syncache *sc, struct syncache_head *sch)
{
struct syncache *sc2;
SCH_LOCK(sch);
/*
* Make sure that we don't overflow the per-bucket limit.
* If the bucket is full, toss the oldest element.
*/
if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
("sch->sch_length incorrect"));
syncache_pause(&sc->sc_inc);
sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
sch->sch_last_overflow = time_uptime;
syncache_drop(sc2, sch);
}
/* Put it into the bucket. */
TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
sch->sch_length++;
#ifdef TCP_OFFLOAD
if (ADDED_BY_TOE(sc)) {
struct toedev *tod = sc->sc_tod;
tod->tod_syncache_added(tod, sc->sc_todctx);
}
#endif
/* Reinitialize the bucket row's timer. */
if (sch->sch_length == 1)
sch->sch_nextc = ticks + INT_MAX;
syncache_timeout(sc, sch, 1);
SCH_UNLOCK(sch);
TCPSTATES_INC(TCPS_SYN_RECEIVED);
TCPSTAT_INC(tcps_sc_added);
}
/*
* Remove and free entry from syncache bucket row.
* Expects locked syncache head.
*/
static void
syncache_drop(struct syncache *sc, struct syncache_head *sch)
{
SCH_LOCK_ASSERT(sch);
TCPSTATES_DEC(TCPS_SYN_RECEIVED);
TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
sch->sch_length--;
#ifdef TCP_OFFLOAD
if (ADDED_BY_TOE(sc)) {
struct toedev *tod = sc->sc_tod;
tod->tod_syncache_removed(tod, sc->sc_todctx);
}
#endif
syncache_free(sc);
}
/*
* Engage/reengage time on bucket row.
*/
static void
syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
{
int rexmt;
if (sc->sc_rxmits == 0)
rexmt = tcp_rexmit_initial;
else
TCPT_RANGESET(rexmt,
tcp_rexmit_initial * tcp_backoff[sc->sc_rxmits],
tcp_rexmit_min, TCPTV_REXMTMAX);
sc->sc_rxttime = ticks + rexmt;
sc->sc_rxmits++;
if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
sch->sch_nextc = sc->sc_rxttime;
if (docallout)
callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
syncache_timer, (void *)sch);
}
}
/*
* Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
* If we have retransmitted an entry the maximum number of times, expire it.
* One separate timer for each bucket row.
*/
static void
syncache_timer(void *xsch)
{
struct syncache_head *sch = (struct syncache_head *)xsch;
struct syncache *sc, *nsc;
struct epoch_tracker et;
int tick = ticks;
char *s;
bool paused;
CURVNET_SET(sch->sch_sc->vnet);
/* NB: syncache_head has already been locked by the callout. */
SCH_LOCK_ASSERT(sch);
/*
* In the following cycle we may remove some entries and/or
* advance some timeouts, so re-initialize the bucket timer.
*/
sch->sch_nextc = tick + INT_MAX;
/*
* If we have paused processing, unconditionally remove
* all syncache entries.
*/
mtx_lock(&V_tcp_syncache.pause_mtx);
paused = V_tcp_syncache.paused;
mtx_unlock(&V_tcp_syncache.pause_mtx);
TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
if (paused) {
syncache_drop(sc, sch);
continue;
}
/*
* We do not check if the listen socket still exists
* and accept the case where the listen socket may be
* gone by the time we resend the SYN/ACK. We do
* not expect this to happens often. If it does,
* then the RST will be sent by the time the remote
* host does the SYN/ACK->ACK.
*/
if (TSTMP_GT(sc->sc_rxttime, tick)) {
if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
sch->sch_nextc = sc->sc_rxttime;
continue;
}
if (sc->sc_rxmits > V_tcp_ecn_maxretries) {
sc->sc_flags &= ~SCF_ECN;
}
if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
"giving up and removing syncache entry\n",
s, __func__);
free(s, M_TCPLOG);
}
syncache_drop(sc, sch);
TCPSTAT_INC(tcps_sc_stale);
continue;
}
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Response timeout, "
"retransmitting (%u) SYN|ACK\n",
s, __func__, sc->sc_rxmits);
free(s, M_TCPLOG);
}
NET_EPOCH_ENTER(et);
syncache_respond(sc, NULL, TH_SYN|TH_ACK);
NET_EPOCH_EXIT(et);
TCPSTAT_INC(tcps_sc_retransmitted);
syncache_timeout(sc, sch, 0);
}
if (!TAILQ_EMPTY(&(sch)->sch_bucket))
callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
syncache_timer, (void *)(sch));
CURVNET_RESTORE();
}
/*
* Returns true if the system is only using cookies at the moment.
* This could be due to a sysadmin decision to only use cookies, or it
* could be due to the system detecting an attack.
*/
static inline bool
syncache_cookiesonly(void)
{
return (V_tcp_syncookies && (V_tcp_syncache.paused ||
V_tcp_syncookiesonly));
}
/*
* Find the hash bucket for the given connection.
*/
static struct syncache_head *
syncache_hashbucket(struct in_conninfo *inc)
{
uint32_t hash;
/*
* The hash is built on foreign port + local port + foreign address.
* We rely on the fact that struct in_conninfo starts with 16 bits
* of foreign port, then 16 bits of local port then followed by 128
* bits of foreign address. In case of IPv4 address, the first 3
* 32-bit words of the address always are zeroes.
*/
hash = jenkins_hash32((uint32_t *)&inc->inc_ie, 5,
V_tcp_syncache.hash_secret) & V_tcp_syncache.hashmask;
return (&V_tcp_syncache.hashbase[hash]);
}
/*
* Find an entry in the syncache.
* Returns always with locked syncache_head plus a matching entry or NULL.
*/
static struct syncache *
syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
{
struct syncache *sc;
struct syncache_head *sch;
*schp = sch = syncache_hashbucket(inc);
SCH_LOCK(sch);
/* Circle through bucket row to find matching entry. */
TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash)
if (bcmp(&inc->inc_ie, &sc->sc_inc.inc_ie,
sizeof(struct in_endpoints)) == 0)
break;
return (sc); /* Always returns with locked sch. */
}
/*
* This function is called when we get a RST for a
* non-existent connection, so that we can see if the
* connection is in the syn cache. If it is, zap it.
* If required send a challenge ACK.
*/
void
syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th, struct mbuf *m)
{
struct syncache *sc;
struct syncache_head *sch;
char *s = NULL;
if (syncache_cookiesonly())
return;
sc = syncache_lookup(inc, &sch); /* returns locked sch */
SCH_LOCK_ASSERT(sch);
/*
* Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
* See RFC 793 page 65, section SEGMENT ARRIVES.
*/
if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
"FIN flag set, segment ignored\n", s, __func__);
TCPSTAT_INC(tcps_badrst);
goto done;
}
/*
* No corresponding connection was found in syncache.
* If syncookies are enabled and possibly exclusively
* used, or we are under memory pressure, a valid RST
* may not find a syncache entry. In that case we're
* done and no SYN|ACK retransmissions will happen.
* Otherwise the RST was misdirected or spoofed.
*/
if (sc == NULL) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
"syncache entry (possibly syncookie only), "
"segment ignored\n", s, __func__);
TCPSTAT_INC(tcps_badrst);
goto done;
}
/*
* If the RST bit is set, check the sequence number to see
* if this is a valid reset segment.
*
* RFC 793 page 37:
* In all states except SYN-SENT, all reset (RST) segments
* are validated by checking their SEQ-fields. A reset is
* valid if its sequence number is in the window.
*
* RFC 793 page 69:
* There are four cases for the acceptability test for an incoming
* segment:
*
* Segment Receive Test
* Length Window
* ------- ------- -------------------------------------------
* 0 0 SEG.SEQ = RCV.NXT
* 0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
* >0 0 not acceptable
* >0 >0 RCV.NXT =< SEG.SEQ < RCV.NXT+RCV.WND
* or RCV.NXT =< SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND
*
* Note that when receiving a SYN segment in the LISTEN state,
* IRS is set to SEG.SEQ and RCV.NXT is set to SEG.SEQ+1, as
* described in RFC 793, page 66.
*/
if ((SEQ_GEQ(th->th_seq, sc->sc_irs + 1) &&
SEQ_LT(th->th_seq, sc->sc_irs + 1 + sc->sc_wnd)) ||
(sc->sc_wnd == 0 && th->th_seq == sc->sc_irs + 1)) {
if (V_tcp_insecure_rst ||
th->th_seq == sc->sc_irs + 1) {
syncache_drop(sc, sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG,
"%s; %s: Our SYN|ACK was rejected, "
"connection attempt aborted by remote "
"endpoint\n",
s, __func__);
TCPSTAT_INC(tcps_sc_reset);
} else {
TCPSTAT_INC(tcps_badrst);
/* Send challenge ACK. */
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: RST with invalid "
" SEQ %u != NXT %u (+WND %u), "
"sending challenge ACK\n",
s, __func__,
th->th_seq, sc->sc_irs + 1, sc->sc_wnd);
syncache_respond(sc, m, TH_ACK);
}
} else {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
"NXT %u (+WND %u), segment ignored\n",
s, __func__,
th->th_seq, sc->sc_irs + 1, sc->sc_wnd);
TCPSTAT_INC(tcps_badrst);
}
done:
if (s != NULL)
free(s, M_TCPLOG);
SCH_UNLOCK(sch);
}
void
syncache_badack(struct in_conninfo *inc)
{
struct syncache *sc;
struct syncache_head *sch;
if (syncache_cookiesonly())
return;
sc = syncache_lookup(inc, &sch); /* returns locked sch */
SCH_LOCK_ASSERT(sch);
if (sc != NULL) {
syncache_drop(sc, sch);
TCPSTAT_INC(tcps_sc_badack);
}
SCH_UNLOCK(sch);
}
void
syncache_unreach(struct in_conninfo *inc, tcp_seq th_seq)
{
struct syncache *sc;
struct syncache_head *sch;
if (syncache_cookiesonly())
return;
sc = syncache_lookup(inc, &sch); /* returns locked sch */
SCH_LOCK_ASSERT(sch);
if (sc == NULL)
goto done;
/* If the sequence number != sc_iss, then it's a bogus ICMP msg */
if (ntohl(th_seq) != sc->sc_iss)
goto done;
/*
* If we've rertransmitted 3 times and this is our second error,
* we remove the entry. Otherwise, we allow it to continue on.
* This prevents us from incorrectly nuking an entry during a
* spurious network outage.
*
* See tcp_notify().
*/
if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
sc->sc_flags |= SCF_UNREACH;
goto done;
}
syncache_drop(sc, sch);
TCPSTAT_INC(tcps_sc_unreach);
done:
SCH_UNLOCK(sch);
}
/*
* Build a new TCP socket structure from a syncache entry.
*
* On success return the newly created socket with its underlying inp locked.
*/
static struct socket *
syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
{
struct tcp_function_block *blk;
struct inpcb *inp = NULL;
struct socket *so;
struct tcpcb *tp;
int error;
char *s;
NET_EPOCH_ASSERT();
/*
* Ok, create the full blown connection, and set things up
* as they would have been set up if we had created the
* connection when the SYN arrived. If we can't create
* the connection, abort it.
*/
so = sonewconn(lso, 0);
if (so == NULL) {
/*
* Drop the connection; we will either send a RST or
* have the peer retransmit its SYN again after its
* RTO and try again.
*/
TCPSTAT_INC(tcps_listendrop);
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Socket create failed "
"due to limits or memory shortage\n",
s, __func__);
free(s, M_TCPLOG);
}
goto abort2;
}
#ifdef MAC
mac_socketpeer_set_from_mbuf(m, so);
#endif
inp = sotoinpcb(so);
inp->inp_inc.inc_fibnum = so->so_fibnum;
INP_WLOCK(inp);
/*
* Exclusive pcbinfo lock is not required in syncache socket case even
* if two inpcb locks can be acquired simultaneously:
* - the inpcb in LISTEN state,
* - the newly created inp.
*
* In this case, an inp cannot be at same time in LISTEN state and
* just created by an accept() call.
*/
INP_HASH_WLOCK(&V_tcbinfo);
/* Insert new socket into PCB hash list. */
inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
#ifdef INET6
if (sc->sc_inc.inc_flags & INC_ISIPV6) {
inp->inp_vflag &= ~INP_IPV4;
inp->inp_vflag |= INP_IPV6;
inp->in6p_laddr = sc->sc_inc.inc6_laddr;
} else {
inp->inp_vflag &= ~INP_IPV6;
inp->inp_vflag |= INP_IPV4;
#endif
inp->inp_ip_ttl = sc->sc_ip_ttl;
inp->inp_ip_tos = sc->sc_ip_tos;
inp->inp_laddr = sc->sc_inc.inc_laddr;
#ifdef INET6
}
#endif
/*
* If there's an mbuf and it has a flowid, then let's initialise the
* inp with that particular flowid.
*/
if (m != NULL && M_HASHTYPE_GET(m) != M_HASHTYPE_NONE) {
inp->inp_flowid = m->m_pkthdr.flowid;
inp->inp_flowtype = M_HASHTYPE_GET(m);
#ifdef NUMA
inp->inp_numa_domain = m->m_pkthdr.numa_domain;
#endif
}
inp->inp_lport = sc->sc_inc.inc_lport;
#ifdef INET6
if (inp->inp_vflag & INP_IPV6PROTO) {
struct inpcb *oinp = sotoinpcb(lso);
/*
* Inherit socket options from the listening socket.
* Note that in6p_inputopts are not (and should not be)
* copied, since it stores previously received options and is
* used to detect if each new option is different than the
* previous one and hence should be passed to a user.
* If we copied in6p_inputopts, a user would not be able to
* receive options just after calling the accept system call.
*/
inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
if (oinp->in6p_outputopts)
inp->in6p_outputopts =
ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
inp->in6p_hops = oinp->in6p_hops;
}
if (sc->sc_inc.inc_flags & INC_ISIPV6) {
struct in6_addr laddr6;
struct sockaddr_in6 sin6;
sin6.sin6_family = AF_INET6;
sin6.sin6_len = sizeof(sin6);
sin6.sin6_addr = sc->sc_inc.inc6_faddr;
sin6.sin6_port = sc->sc_inc.inc_fport;
sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
laddr6 = inp->in6p_laddr;
if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
inp->in6p_laddr = sc->sc_inc.inc6_laddr;
if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
thread0.td_ucred, m, false)) != 0) {
inp->in6p_laddr = laddr6;
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
"with error %i\n",
s, __func__, error);
free(s, M_TCPLOG);
}
INP_HASH_WUNLOCK(&V_tcbinfo);
goto abort;
}
/* Override flowlabel from in6_pcbconnect. */
inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
inp->inp_flow |= sc->sc_flowlabel;
}
#endif /* INET6 */
#if defined(INET) && defined(INET6)
else
#endif
#ifdef INET
{
struct in_addr laddr;
struct sockaddr_in sin;
inp->inp_options = (m) ? ip_srcroute(m) : NULL;
if (inp->inp_options == NULL) {
inp->inp_options = sc->sc_ipopts;
sc->sc_ipopts = NULL;
}
sin.sin_family = AF_INET;
sin.sin_len = sizeof(sin);
sin.sin_addr = sc->sc_inc.inc_faddr;
sin.sin_port = sc->sc_inc.inc_fport;
bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
laddr = inp->inp_laddr;
if (inp->inp_laddr.s_addr == INADDR_ANY)
inp->inp_laddr = sc->sc_inc.inc_laddr;
if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
thread0.td_ucred, m, false)) != 0) {
inp->inp_laddr = laddr;
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
"with error %i\n",
s, __func__, error);
free(s, M_TCPLOG);
}
INP_HASH_WUNLOCK(&V_tcbinfo);
goto abort;
}
}
#endif /* INET */
#if defined(IPSEC) || defined(IPSEC_SUPPORT)
/* Copy old policy into new socket's. */
if (ipsec_copy_pcbpolicy(sotoinpcb(lso), inp) != 0)
printf("syncache_socket: could not copy policy\n");
#endif
INP_HASH_WUNLOCK(&V_tcbinfo);
tp = intotcpcb(inp);
tcp_state_change(tp, TCPS_SYN_RECEIVED);
tp->iss = sc->sc_iss;
tp->irs = sc->sc_irs;
tcp_rcvseqinit(tp);
tcp_sendseqinit(tp);
blk = sototcpcb(lso)->t_fb;
if (V_functions_inherit_listen_socket_stack && blk != tp->t_fb) {
/*
* Our parents t_fb was not the default,
* we need to release our ref on tp->t_fb and
* pickup one on the new entry.
*/
struct tcp_function_block *rblk;
rblk = find_and_ref_tcp_fb(blk);
KASSERT(rblk != NULL,
("cannot find blk %p out of syncache?", blk));
if (tp->t_fb->tfb_tcp_fb_fini)
(*tp->t_fb->tfb_tcp_fb_fini)(tp, 0);
refcount_release(&tp->t_fb->tfb_refcnt);
tp->t_fb = rblk;
/*
* XXXrrs this is quite dangerous, it is possible
* for the new function to fail to init. We also
* are not asking if the handoff_is_ok though at
* the very start thats probalbly ok.
*/
if (tp->t_fb->tfb_tcp_fb_init) {
(*tp->t_fb->tfb_tcp_fb_init)(tp);
}
}
tp->snd_wl1 = sc->sc_irs;
tp->snd_max = tp->iss + 1;
tp->snd_nxt = tp->iss + 1;
tp->rcv_up = sc->sc_irs + 1;
tp->rcv_wnd = sc->sc_wnd;
tp->rcv_adv += tp->rcv_wnd;
tp->last_ack_sent = tp->rcv_nxt;
tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
if (sc->sc_flags & SCF_NOOPT)
tp->t_flags |= TF_NOOPT;
else {
if (sc->sc_flags & SCF_WINSCALE) {
tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
tp->snd_scale = sc->sc_requested_s_scale;
tp->request_r_scale = sc->sc_requested_r_scale;
}
if (sc->sc_flags & SCF_TIMESTAMP) {
tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
tp->ts_recent = sc->sc_tsreflect;
tp->ts_recent_age = tcp_ts_getticks();
tp->ts_offset = sc->sc_tsoff;
}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
if (sc->sc_flags & SCF_SIGNATURE)
tp->t_flags |= TF_SIGNATURE;
#endif
if (sc->sc_flags & SCF_SACK)
tp->t_flags |= TF_SACK_PERMIT;
}
if (sc->sc_flags & SCF_ECN)
tp->t_flags2 |= TF2_ECN_PERMIT;
/*
* Set up MSS and get cached values from tcp_hostcache.
* This might overwrite some of the defaults we just set.
*/
tcp_mss(tp, sc->sc_peer_mss);
/*
* If the SYN,ACK was retransmitted, indicate that CWND to be
* limited to one segment in cc_conn_init().
* NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
*/
if (sc->sc_rxmits > 1)
tp->snd_cwnd = 1;
#ifdef TCP_OFFLOAD
/*
* Allow a TOE driver to install its hooks. Note that we hold the
* pcbinfo lock too and that prevents tcp_usr_accept from accepting a
* new connection before the TOE driver has done its thing.
*/
if (ADDED_BY_TOE(sc)) {
struct toedev *tod = sc->sc_tod;
tod->tod_offload_socket(tod, sc->sc_todctx, so);
}
#endif
/*
* Copy and activate timers.
*/
tp->t_keepinit = sototcpcb(lso)->t_keepinit;
tp->t_keepidle = sototcpcb(lso)->t_keepidle;
tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
TCPSTAT_INC(tcps_accepts);
return (so);
abort:
INP_WUNLOCK(inp);
abort2:
if (so != NULL)
soabort(so);
return (NULL);
}
/*
* This function gets called when we receive an ACK for a
* socket in the LISTEN state. We look up the connection
* in the syncache, and if its there, we pull it out of
* the cache and turn it into a full-blown connection in
* the SYN-RECEIVED state.
*
* On syncache_socket() success the newly created socket
* has its underlying inp locked.
*/
int
syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
struct socket **lsop, struct mbuf *m)
{
struct syncache *sc;
struct syncache_head *sch;
struct syncache scs;
char *s;
bool locked;
NET_EPOCH_ASSERT();
KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
("%s: can handle only ACK", __func__));
if (syncache_cookiesonly()) {
sc = NULL;
sch = syncache_hashbucket(inc);
locked = false;
} else {
sc = syncache_lookup(inc, &sch); /* returns locked sch */
locked = true;
SCH_LOCK_ASSERT(sch);
}
#ifdef INVARIANTS
/*
* Test code for syncookies comparing the syncache stored
* values with the reconstructed values from the cookie.
*/
if (sc != NULL)
syncookie_cmp(inc, sch, sc, th, to, *lsop);
#endif
if (sc == NULL) {
/*
* There is no syncache entry, so see if this ACK is
* a returning syncookie. To do this, first:
* A. Check if syncookies are used in case of syncache
* overflows
* B. See if this socket has had a syncache entry dropped in
* the recent past. We don't want to accept a bogus
* syncookie if we've never received a SYN or accept it
* twice.
* C. check that the syncookie is valid. If it is, then
* cobble up a fake syncache entry, and return.
*/
if (locked && !V_tcp_syncookies) {
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Spurious ACK, "
"segment rejected (syncookies disabled)\n",
s, __func__);
goto failed;
}
if (locked && !V_tcp_syncookiesonly &&
sch->sch_last_overflow < time_uptime - SYNCOOKIE_LIFETIME) {
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Spurious ACK, "
"segment rejected (no syncache entry)\n",
s, __func__);
goto failed;
}
bzero(&scs, sizeof(scs));
sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
if (locked)
SCH_UNLOCK(sch);
if (sc == NULL) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Segment failed "
"SYNCOOKIE authentication, segment rejected "
"(probably spoofed)\n", s, __func__);
goto failed;
}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
/* If received ACK has MD5 signature, check it. */
if ((to->to_flags & TOF_SIGNATURE) != 0 &&
(!TCPMD5_ENABLED() ||
TCPMD5_INPUT(m, th, to->to_signature) != 0)) {
/* Drop the ACK. */
if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Segment rejected, "
"MD5 signature doesn't match.\n",
s, __func__);
free(s, M_TCPLOG);
}
TCPSTAT_INC(tcps_sig_err_sigopt);
return (-1); /* Do not send RST */
}
#endif /* TCP_SIGNATURE */
} else {
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
/*
* If listening socket requested TCP digests, check that
* received ACK has signature and it is correct.
* If not, drop the ACK and leave sc entry in th cache,
* because SYN was received with correct signature.
*/
if (sc->sc_flags & SCF_SIGNATURE) {
if ((to->to_flags & TOF_SIGNATURE) == 0) {
/* No signature */
TCPSTAT_INC(tcps_sig_err_nosigopt);
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Segment "
"rejected, MD5 signature wasn't "
"provided.\n", s, __func__);
free(s, M_TCPLOG);
}
return (-1); /* Do not send RST */
}
if (!TCPMD5_ENABLED() ||
TCPMD5_INPUT(m, th, to->to_signature) != 0) {
/* Doesn't match or no SA */
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Segment "
"rejected, MD5 signature doesn't "
"match.\n", s, __func__);
free(s, M_TCPLOG);
}
return (-1); /* Do not send RST */
}
}
#endif /* TCP_SIGNATURE */
/*
* RFC 7323 PAWS: If we have a timestamp on this segment and
* it's less than ts_recent, drop it.
* XXXMT: RFC 7323 also requires to send an ACK.
* In tcp_input.c this is only done for TCP segments
* with user data, so be consistent here and just drop
* the segment.
*/
if (sc->sc_flags & SCF_TIMESTAMP && to->to_flags & TOF_TS &&
TSTMP_LT(to->to_tsval, sc->sc_tsreflect)) {
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
log(LOG_DEBUG,
"%s; %s: SEG.TSval %u < TS.Recent %u, "
"segment dropped\n", s, __func__,
to->to_tsval, sc->sc_tsreflect);
free(s, M_TCPLOG);
}
return (-1); /* Do not send RST */
}
/*
* If timestamps were not negotiated during SYN/ACK and a
* segment without a timestamp is received, ignore the
* timestamp and process the packet normally.
* See section 3.2 of RFC 7323.
*/
if (!(sc->sc_flags & SCF_TIMESTAMP) &&
(to->to_flags & TOF_TS)) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Timestamp not "
"expected, segment processed normally\n",
s, __func__);
free(s, M_TCPLOG);
s = NULL;
}
}
/*
* If timestamps were negotiated during SYN/ACK and a
* segment without a timestamp is received, silently drop
* the segment.
* See section 3.2 of RFC 7323.
*/
if ((sc->sc_flags & SCF_TIMESTAMP) &&
!(to->to_flags & TOF_TS)) {
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Timestamp missing, "
"segment silently dropped\n", s, __func__);
free(s, M_TCPLOG);
}
return (-1); /* Do not send RST */
}
/*
* Pull out the entry to unlock the bucket row.
*
* NOTE: We must decrease TCPS_SYN_RECEIVED count here, not
* tcp_state_change(). The tcpcb is not existent at this
* moment. A new one will be allocated via syncache_socket->
* sonewconn->tcp_usr_attach in TCPS_CLOSED state, then
* syncache_socket() will change it to TCPS_SYN_RECEIVED.
*/
TCPSTATES_DEC(TCPS_SYN_RECEIVED);
TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
sch->sch_length--;
#ifdef TCP_OFFLOAD
if (ADDED_BY_TOE(sc)) {
struct toedev *tod = sc->sc_tod;
tod->tod_syncache_removed(tod, sc->sc_todctx);
}
#endif
SCH_UNLOCK(sch);
}
/*
* Segment validation:
* ACK must match our initial sequence number + 1 (the SYN|ACK).
*/
if (th->th_ack != sc->sc_iss + 1) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
"rejected\n", s, __func__, th->th_ack, sc->sc_iss);
goto failed;
}
/*
* The SEQ must fall in the window starting at the received
* initial receive sequence number + 1 (the SYN).
*/
if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
"rejected\n", s, __func__, th->th_seq, sc->sc_irs);
goto failed;
}
*lsop = syncache_socket(sc, *lsop, m);
if (*lsop == NULL)
TCPSTAT_INC(tcps_sc_aborted);
else
TCPSTAT_INC(tcps_sc_completed);
/* how do we find the inp for the new socket? */
if (sc != &scs)
syncache_free(sc);
return (1);
failed:
if (sc != NULL && sc != &scs)
syncache_free(sc);
if (s != NULL)
free(s, M_TCPLOG);
*lsop = NULL;
return (0);
}
static void
syncache_tfo_expand(struct syncache *sc, struct socket **lsop, struct mbuf *m,
uint64_t response_cookie)
{
struct inpcb *inp;
struct tcpcb *tp;
unsigned int *pending_counter;
NET_EPOCH_ASSERT();
pending_counter = intotcpcb(sotoinpcb(*lsop))->t_tfo_pending;
*lsop = syncache_socket(sc, *lsop, m);
if (*lsop == NULL) {
TCPSTAT_INC(tcps_sc_aborted);
atomic_subtract_int(pending_counter, 1);
} else {
soisconnected(*lsop);
inp = sotoinpcb(*lsop);
tp = intotcpcb(inp);
tp->t_flags |= TF_FASTOPEN;
tp->t_tfo_cookie.server = response_cookie;
tp->snd_max = tp->iss;
tp->snd_nxt = tp->iss;
tp->t_tfo_pending = pending_counter;
TCPSTAT_INC(tcps_sc_completed);
}
}
/*
* Given a LISTEN socket and an inbound SYN request, add
* this to the syn cache, and send back a segment:
* <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
* to the source.
*
* IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
* Doing so would require that we hold onto the data and deliver it
* to the application. However, if we are the target of a SYN-flood
* DoS attack, an attacker could send data which would eventually
* consume all available buffer space if it were ACKed. By not ACKing
* the data, we avoid this DoS scenario.
*
* The exception to the above is when a SYN with a valid TCP Fast Open (TFO)
* cookie is processed and a new socket is created. In this case, any data
* accompanying the SYN will be queued to the socket by tcp_input() and will
* be ACKed either when the application sends response data or the delayed
* ACK timer expires, whichever comes first.
*/
int
syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
void *todctx, uint8_t iptos)
{
struct tcpcb *tp;
struct socket *so;
struct syncache *sc = NULL;
struct syncache_head *sch;
struct mbuf *ipopts = NULL;
u_int ltflags;
int win, ip_ttl, ip_tos;
char *s;
int rv = 0;
#ifdef INET6
int autoflowlabel = 0;
#endif
#ifdef MAC
struct label *maclabel;
#endif
struct syncache scs;
struct ucred *cred;
uint64_t tfo_response_cookie;
unsigned int *tfo_pending = NULL;
int tfo_cookie_valid = 0;
int tfo_response_cookie_valid = 0;
bool locked;
INP_WLOCK_ASSERT(inp); /* listen socket */
KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
("%s: unexpected tcp flags", __func__));
/*
* Combine all so/tp operations very early to drop the INP lock as
* soon as possible.
*/
so = *lsop;
KASSERT(SOLISTENING(so), ("%s: %p not listening", __func__, so));
tp = sototcpcb(so);
cred = crhold(so->so_cred);
#ifdef INET6
if (inc->inc_flags & INC_ISIPV6) {
if (inp->inp_flags & IN6P_AUTOFLOWLABEL) {
autoflowlabel = 1;
}
ip_ttl = in6_selecthlim(inp, NULL);
if ((inp->in6p_outputopts == NULL) ||
(inp->in6p_outputopts->ip6po_tclass == -1)) {
ip_tos = 0;
} else {
ip_tos = inp->in6p_outputopts->ip6po_tclass;
}
}
#endif
#if defined(INET6) && defined(INET)
else
#endif
#ifdef INET
{
ip_ttl = inp->inp_ip_ttl;
ip_tos = inp->inp_ip_tos;
}
#endif
win = so->sol_sbrcv_hiwat;
ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
if (V_tcp_fastopen_server_enable && IS_FASTOPEN(tp->t_flags) &&
(tp->t_tfo_pending != NULL) &&
(to->to_flags & TOF_FASTOPEN)) {
/*
* Limit the number of pending TFO connections to
* approximately half of the queue limit. This prevents TFO
* SYN floods from starving the service by filling the
* listen queue with bogus TFO connections.
*/
if (atomic_fetchadd_int(tp->t_tfo_pending, 1) <=
(so->sol_qlimit / 2)) {
int result;
result = tcp_fastopen_check_cookie(inc,
to->to_tfo_cookie, to->to_tfo_len,
&tfo_response_cookie);
tfo_cookie_valid = (result > 0);
tfo_response_cookie_valid = (result >= 0);
}
/*
* Remember the TFO pending counter as it will have to be
* decremented below if we don't make it to syncache_tfo_expand().
*/
tfo_pending = tp->t_tfo_pending;
}
/* By the time we drop the lock these should no longer be used. */
so = NULL;
tp = NULL;
#ifdef MAC
if (mac_syncache_init(&maclabel) != 0) {
INP_WUNLOCK(inp);
goto done;
} else
mac_syncache_create(maclabel, inp);
#endif
if (!tfo_cookie_valid)
INP_WUNLOCK(inp);
/*
* Remember the IP options, if any.
*/
#ifdef INET6
if (!(inc->inc_flags & INC_ISIPV6))
#endif
#ifdef INET
ipopts = (m) ? ip_srcroute(m) : NULL;
#else
ipopts = NULL;
#endif
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
/*
* If listening socket requested TCP digests, check that received
* SYN has signature and it is correct. If signature doesn't match
* or TCP_SIGNATURE support isn't enabled, drop the packet.
*/
if (ltflags & TF_SIGNATURE) {
if ((to->to_flags & TOF_SIGNATURE) == 0) {
TCPSTAT_INC(tcps_sig_err_nosigopt);
goto done;
}
if (!TCPMD5_ENABLED() ||
TCPMD5_INPUT(m, th, to->to_signature) != 0)
goto done;
}
#endif /* TCP_SIGNATURE */
/*
* See if we already have an entry for this connection.
* If we do, resend the SYN,ACK, and reset the retransmit timer.
*
* XXX: should the syncache be re-initialized with the contents
* of the new SYN here (which may have different options?)
*
* XXX: We do not check the sequence number to see if this is a
* real retransmit or a new connection attempt. The question is
* how to handle such a case; either ignore it as spoofed, or
* drop the current entry and create a new one?
*/
if (syncache_cookiesonly()) {
sc = NULL;
sch = syncache_hashbucket(inc);
locked = false;
} else {
sc = syncache_lookup(inc, &sch); /* returns locked sch */
locked = true;
SCH_LOCK_ASSERT(sch);
}
if (sc != NULL) {
if (tfo_cookie_valid)
INP_WUNLOCK(inp);
TCPSTAT_INC(tcps_sc_dupsyn);
if (ipopts) {
/*
* If we were remembering a previous source route,
* forget it and use the new one we've been given.
*/
if (sc->sc_ipopts)
(void) m_free(sc->sc_ipopts);
sc->sc_ipopts = ipopts;
}
/*
* Update timestamp if present.
*/
if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
sc->sc_tsreflect = to->to_tsval;
else
sc->sc_flags &= ~SCF_TIMESTAMP;
/*
* Disable ECN if needed.
*/
if ((sc->sc_flags & SCF_ECN) &&
((th->th_flags & (TH_ECE|TH_CWR)) != (TH_ECE|TH_CWR))) {
sc->sc_flags &= ~SCF_ECN;
}
#ifdef MAC
/*
* Since we have already unconditionally allocated label
* storage, free it up. The syncache entry will already
* have an initialized label we can use.
*/
mac_syncache_destroy(&maclabel);
#endif
TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
/* Retransmit SYN|ACK and reset retransmit count. */
if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
"resetting timer and retransmitting SYN|ACK\n",
s, __func__);
free(s, M_TCPLOG);
}
if (syncache_respond(sc, m, TH_SYN|TH_ACK) == 0) {
sc->sc_rxmits = 0;
syncache_timeout(sc, sch, 1);
TCPSTAT_INC(tcps_sndacks);
TCPSTAT_INC(tcps_sndtotal);
}
SCH_UNLOCK(sch);
goto donenoprobe;
}
if (tfo_cookie_valid) {
bzero(&scs, sizeof(scs));
sc = &scs;
goto skip_alloc;
}
/*
* Skip allocating a syncache entry if we are just going to discard
* it later.
*/
if (!locked) {
bzero(&scs, sizeof(scs));
sc = &scs;
} else
sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
if (sc == NULL) {
/*
* The zone allocator couldn't provide more entries.
* Treat this as if the cache was full; drop the oldest
* entry and insert the new one.
*/
TCPSTAT_INC(tcps_sc_zonefail);
if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL) {
sch->sch_last_overflow = time_uptime;
syncache_drop(sc, sch);
syncache_pause(inc);
}
sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
if (sc == NULL) {
if (V_tcp_syncookies) {
bzero(&scs, sizeof(scs));
sc = &scs;
} else {
KASSERT(locked,
("%s: bucket unexpectedly unlocked",
__func__));
SCH_UNLOCK(sch);
if (ipopts)
(void) m_free(ipopts);
goto done;
}
}
}
skip_alloc:
if (!tfo_cookie_valid && tfo_response_cookie_valid)
sc->sc_tfo_cookie = &tfo_response_cookie;
/*
* Fill in the syncache values.
*/
#ifdef MAC
sc->sc_label = maclabel;
#endif
sc->sc_cred = cred;
cred = NULL;
sc->sc_ipopts = ipopts;
bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
sc->sc_ip_tos = ip_tos;
sc->sc_ip_ttl = ip_ttl;
#ifdef TCP_OFFLOAD
sc->sc_tod = tod;
sc->sc_todctx = todctx;
#endif
sc->sc_irs = th->th_seq;
sc->sc_flags = 0;
sc->sc_flowlabel = 0;
/*
* Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
* win was derived from socket earlier in the function.
*/
win = imax(win, 0);
win = imin(win, TCP_MAXWIN);
sc->sc_wnd = win;
if (V_tcp_do_rfc1323) {
/*
* A timestamp received in a SYN makes
* it ok to send timestamp requests and replies.
*/
if (to->to_flags & TOF_TS) {
sc->sc_tsreflect = to->to_tsval;
sc->sc_flags |= SCF_TIMESTAMP;
sc->sc_tsoff = tcp_new_ts_offset(inc);
}
if (to->to_flags & TOF_SCALE) {
int wscale = 0;
/*
* Pick the smallest possible scaling factor that
* will still allow us to scale up to sb_max, aka
* kern.ipc.maxsockbuf.
*
* We do this because there are broken firewalls that
* will corrupt the window scale option, leading to
* the other endpoint believing that our advertised
* window is unscaled. At scale factors larger than
* 5 the unscaled window will drop below 1500 bytes,
* leading to serious problems when traversing these
* broken firewalls.
*
* With the default maxsockbuf of 256K, a scale factor
* of 3 will be chosen by this algorithm. Those who
* choose a larger maxsockbuf should watch out
* for the compatibility problems mentioned above.
*
* RFC1323: The Window field in a SYN (i.e., a <SYN>
* or <SYN,ACK>) segment itself is never scaled.
*/
while (wscale < TCP_MAX_WINSHIFT &&
(TCP_MAXWIN << wscale) < sb_max)
wscale++;
sc->sc_requested_r_scale = wscale;
sc->sc_requested_s_scale = to->to_wscale;
sc->sc_flags |= SCF_WINSCALE;
}
}
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
/*
* If listening socket requested TCP digests, flag this in the
* syncache so that syncache_respond() will do the right thing
* with the SYN+ACK.
*/
if (ltflags & TF_SIGNATURE)
sc->sc_flags |= SCF_SIGNATURE;
#endif /* TCP_SIGNATURE */
if (to->to_flags & TOF_SACKPERM)
sc->sc_flags |= SCF_SACK;
if (to->to_flags & TOF_MSS)
sc->sc_peer_mss = to->to_mss; /* peer mss may be zero */
if (ltflags & TF_NOOPT)
sc->sc_flags |= SCF_NOOPT;
if (((th->th_flags & (TH_ECE|TH_CWR)) == (TH_ECE|TH_CWR)) &&
V_tcp_do_ecn)
sc->sc_flags |= SCF_ECN;
if (V_tcp_syncookies)
sc->sc_iss = syncookie_generate(sch, sc);
else
sc->sc_iss = arc4random();
#ifdef INET6
if (autoflowlabel) {
if (V_tcp_syncookies)
sc->sc_flowlabel = sc->sc_iss;
else
sc->sc_flowlabel = ip6_randomflowlabel();
sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
}
#endif
if (locked)
SCH_UNLOCK(sch);
if (tfo_cookie_valid) {
syncache_tfo_expand(sc, lsop, m, tfo_response_cookie);
/* INP_WUNLOCK(inp) will be performed by the caller */
rv = 1;
goto tfo_expanded;
}
TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
/*
* Do a standard 3-way handshake.
*/
if (syncache_respond(sc, m, TH_SYN|TH_ACK) == 0) {
if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
syncache_free(sc);
else if (sc != &scs)
syncache_insert(sc, sch); /* locks and unlocks sch */
TCPSTAT_INC(tcps_sndacks);
TCPSTAT_INC(tcps_sndtotal);
} else {
if (sc != &scs)
syncache_free(sc);
TCPSTAT_INC(tcps_sc_dropped);
}
goto donenoprobe;
done:
TCP_PROBE5(receive, NULL, NULL, m, NULL, th);
donenoprobe:
if (m) {
*lsop = NULL;
m_freem(m);
}
/*
* If tfo_pending is not NULL here, then a TFO SYN that did not
* result in a new socket was processed and the associated pending
* counter has not yet been decremented. All such TFO processing paths
* transit this point.
*/
if (tfo_pending != NULL)
tcp_fastopen_decrement_counter(tfo_pending);
tfo_expanded:
if (cred != NULL)
crfree(cred);
#ifdef MAC
if (sc == &scs)
mac_syncache_destroy(&maclabel);
#endif
return (rv);
}
/*
* Send SYN|ACK or ACK to the peer. Either in response to a peer's segment,
* i.e. m0 != NULL, or upon 3WHS ACK timeout, i.e. m0 == NULL.
*/
static int
syncache_respond(struct syncache *sc, const struct mbuf *m0, int flags)
{
struct ip *ip = NULL;
struct mbuf *m;
struct tcphdr *th = NULL;
int optlen, error = 0; /* Make compiler happy */
u_int16_t hlen, tlen, mssopt;
struct tcpopt to;
#ifdef INET6
struct ip6_hdr *ip6 = NULL;
#endif
NET_EPOCH_ASSERT();
hlen =
#ifdef INET6
(sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
#endif
sizeof(struct ip);
tlen = hlen + sizeof(struct tcphdr);
/* Determine MSS we advertize to other end of connection. */
mssopt = max(tcp_mssopt(&sc->sc_inc), V_tcp_minmss);
/* XXX: Assume that the entire packet will fit in a header mbuf. */
KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
("syncache: mbuf too small"));
/* Create the IP+TCP header from scratch. */
m = m_gethdr(M_NOWAIT, MT_DATA);
if (m == NULL)
return (ENOBUFS);
#ifdef MAC
mac_syncache_create_mbuf(sc->sc_label, m);
#endif
m->m_data += max_linkhdr;
m->m_len = tlen;
m->m_pkthdr.len = tlen;
m->m_pkthdr.rcvif = NULL;
#ifdef INET6
if (sc->sc_inc.inc_flags & INC_ISIPV6) {
ip6 = mtod(m, struct ip6_hdr *);
ip6->ip6_vfc = IPV6_VERSION;
ip6->ip6_nxt = IPPROTO_TCP;
ip6->ip6_src = sc->sc_inc.inc6_laddr;
ip6->ip6_dst = sc->sc_inc.inc6_faddr;
ip6->ip6_plen = htons(tlen - hlen);
/* ip6_hlim is set after checksum */
/* Zero out traffic class and flow label. */
ip6->ip6_flow &= ~IPV6_FLOWINFO_MASK;
ip6->ip6_flow |= sc->sc_flowlabel;
ip6->ip6_flow |= htonl(sc->sc_ip_tos << 20);
th = (struct tcphdr *)(ip6 + 1);
}
#endif
#if defined(INET6) && defined(INET)
else
#endif
#ifdef INET
{
ip = mtod(m, struct ip *);
ip->ip_v = IPVERSION;
ip->ip_hl = sizeof(struct ip) >> 2;
ip->ip_len = htons(tlen);
ip->ip_id = 0;
ip->ip_off = 0;
ip->ip_sum = 0;
ip->ip_p = IPPROTO_TCP;
ip->ip_src = sc->sc_inc.inc_laddr;
ip->ip_dst = sc->sc_inc.inc_faddr;
ip->ip_ttl = sc->sc_ip_ttl;
ip->ip_tos = sc->sc_ip_tos;
/*
* See if we should do MTU discovery. Route lookups are
* expensive, so we will only unset the DF bit if:
*
* 1) path_mtu_discovery is disabled
* 2) the SCF_UNREACH flag has been set
*/
if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
ip->ip_off |= htons(IP_DF);
th = (struct tcphdr *)(ip + 1);
}
#endif /* INET */
th->th_sport = sc->sc_inc.inc_lport;
th->th_dport = sc->sc_inc.inc_fport;
if (flags & TH_SYN)
th->th_seq = htonl(sc->sc_iss);
else
th->th_seq = htonl(sc->sc_iss + 1);
th->th_ack = htonl(sc->sc_irs + 1);
th->th_off = sizeof(struct tcphdr) >> 2;
th->th_x2 = 0;
th->th_flags = flags;
th->th_win = htons(sc->sc_wnd);
th->th_urp = 0;
if ((flags & TH_SYN) && (sc->sc_flags & SCF_ECN)) {
th->th_flags |= TH_ECE;
TCPSTAT_INC(tcps_ecn_shs);
}
/* Tack on the TCP options. */
if ((sc->sc_flags & SCF_NOOPT) == 0) {
to.to_flags = 0;
if (flags & TH_SYN) {
to.to_mss = mssopt;
to.to_flags = TOF_MSS;
if (sc->sc_flags & SCF_WINSCALE) {
to.to_wscale = sc->sc_requested_r_scale;
to.to_flags |= TOF_SCALE;
}
if (sc->sc_flags & SCF_SACK)
to.to_flags |= TOF_SACKPERM;
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
if (sc->sc_flags & SCF_SIGNATURE)
to.to_flags |= TOF_SIGNATURE;
#endif
if (sc->sc_tfo_cookie) {
to.to_flags |= TOF_FASTOPEN;
to.to_tfo_len = TCP_FASTOPEN_COOKIE_LEN;
to.to_tfo_cookie = sc->sc_tfo_cookie;
/* don't send cookie again when retransmitting response */
sc->sc_tfo_cookie = NULL;
}
}
if (sc->sc_flags & SCF_TIMESTAMP) {
to.to_tsval = sc->sc_tsoff + tcp_ts_getticks();
to.to_tsecr = sc->sc_tsreflect;
to.to_flags |= TOF_TS;
}
optlen = tcp_addoptions(&to, (u_char *)(th + 1));
/* Adjust headers by option size. */
th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
m->m_len += optlen;
m->m_pkthdr.len += optlen;
#ifdef INET6
if (sc->sc_inc.inc_flags & INC_ISIPV6)
ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
else
#endif
ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
#if defined(IPSEC_SUPPORT) || defined(TCP_SIGNATURE)
if (sc->sc_flags & SCF_SIGNATURE) {
KASSERT(to.to_flags & TOF_SIGNATURE,
("tcp_addoptions() didn't set tcp_signature"));
/* NOTE: to.to_signature is inside of mbuf */
if (!TCPMD5_ENABLED() ||
TCPMD5_OUTPUT(m, th, to.to_signature) != 0) {
m_freem(m);
return (EACCES);
}
}
#endif
} else
optlen = 0;
M_SETFIB(m, sc->sc_inc.inc_fibnum);
m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
/*
* If we have peer's SYN and it has a flowid, then let's assign it to
* our SYN|ACK. ip6_output() and ip_output() will not assign flowid
* to SYN|ACK due to lack of inp here.
*/
if (m0 != NULL && M_HASHTYPE_GET(m0) != M_HASHTYPE_NONE) {
m->m_pkthdr.flowid = m0->m_pkthdr.flowid;
M_HASHTYPE_SET(m, M_HASHTYPE_GET(m0));
}
#ifdef INET6
if (sc->sc_inc.inc_flags & INC_ISIPV6) {
m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
IPPROTO_TCP, 0);
ip6->ip6_hlim = sc->sc_ip_ttl;
#ifdef TCP_OFFLOAD
if (ADDED_BY_TOE(sc)) {
struct toedev *tod = sc->sc_tod;
error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
return (error);
}
#endif
TCP_PROBE5(send, NULL, NULL, ip6, NULL, th);
error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
}
#endif
#if defined(INET6) && defined(INET)
else
#endif
#ifdef INET
{
m->m_pkthdr.csum_flags = CSUM_TCP;
th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
htons(tlen + optlen - hlen + IPPROTO_TCP));
#ifdef TCP_OFFLOAD
if (ADDED_BY_TOE(sc)) {
struct toedev *tod = sc->sc_tod;
error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
return (error);
}
#endif
TCP_PROBE5(send, NULL, NULL, ip, NULL, th);
error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
}
#endif
return (error);
}
/*
* The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
* that exceed the capacity of the syncache by avoiding the storage of any
* of the SYNs we receive. Syncookies defend against blind SYN flooding
* attacks where the attacker does not have access to our responses.
*
* Syncookies encode and include all necessary information about the
* connection setup within the SYN|ACK that we send back. That way we
* can avoid keeping any local state until the ACK to our SYN|ACK returns
* (if ever). Normally the syncache and syncookies are running in parallel
* with the latter taking over when the former is exhausted. When matching
* syncache entry is found the syncookie is ignored.
*
* The only reliable information persisting the 3WHS is our initial sequence
* number ISS of 32 bits. Syncookies embed a cryptographically sufficient
* strong hash (MAC) value and a few bits of TCP SYN options in the ISS
* of our SYN|ACK. The MAC can be recomputed when the ACK to our SYN|ACK
* returns and signifies a legitimate connection if it matches the ACK.
*
* The available space of 32 bits to store the hash and to encode the SYN
* option information is very tight and we should have at least 24 bits for
* the MAC to keep the number of guesses by blind spoofing reasonably high.
*
* SYN option information we have to encode to fully restore a connection:
* MSS: is imporant to chose an optimal segment size to avoid IP level
* fragmentation along the path. The common MSS values can be encoded
* in a 3-bit table. Uncommon values are captured by the next lower value
* in the table leading to a slight increase in packetization overhead.
* WSCALE: is necessary to allow large windows to be used for high delay-
* bandwidth product links. Not scaling the window when it was initially
* negotiated is bad for performance as lack of scaling further decreases
* the apparent available send window. We only need to encode the WSCALE
* we received from the remote end. Our end can be recalculated at any
* time. The common WSCALE values can be encoded in a 3-bit table.
* Uncommon values are captured by the next lower value in the table
* making us under-estimate the available window size halving our
* theoretically possible maximum throughput for that connection.
* SACK: Greatly assists in packet loss recovery and requires 1 bit.
* TIMESTAMP and SIGNATURE is not encoded because they are permanent options
* that are included in all segments on a connection. We enable them when
* the ACK has them.
*
* Security of syncookies and attack vectors:
*
* The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
* together with the gloabl secret to make it unique per connection attempt.
* Thus any change of any of those parameters results in a different MAC output
* in an unpredictable way unless a collision is encountered. 24 bits of the
* MAC are embedded into the ISS.
*
* To prevent replay attacks two rotating global secrets are updated with a
* new random value every 15 seconds. The life-time of a syncookie is thus
* 15-30 seconds.
*
* Vector 1: Attacking the secret. This requires finding a weakness in the
* MAC itself or the way it is used here. The attacker can do a chosen plain
* text attack by varying and testing the all parameters under his control.
* The strength depends on the size and randomness of the secret, and the
* cryptographic security of the MAC function. Due to the constant updating
* of the secret the attacker has at most 29.999 seconds to find the secret
* and launch spoofed connections. After that he has to start all over again.
*
* Vector 2: Collision attack on the MAC of a single ACK. With a 24 bit MAC
* size an average of 4,823 attempts are required for a 50% chance of success
* to spoof a single syncookie (birthday collision paradox). However the
* attacker is blind and doesn't know if one of his attempts succeeded unless
* he has a side channel to interfere success from. A single connection setup
* success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
* This many attempts are required for each one blind spoofed connection. For
* every additional spoofed connection he has to launch another N attempts.
* Thus for a sustained rate 100 spoofed connections per second approximately
* 1,800,000 packets per second would have to be sent.
*
* NB: The MAC function should be fast so that it doesn't become a CPU
* exhaustion attack vector itself.
*
* References:
* RFC4987 TCP SYN Flooding Attacks and Common Mitigations
* SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
* http://cr.yp.to/syncookies.html (overview)
* http://cr.yp.to/syncookies/archive (details)
*
*
* Schematic construction of a syncookie enabled Initial Sequence Number:
* 0 1 2 3
* 12345678901234567890123456789012
* |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
*
* x 24 MAC (truncated)
* W 3 Send Window Scale index
* M 3 MSS index
* S 1 SACK permitted
* P 1 Odd/even secret
*/
/*
* Distribution and probability of certain MSS values. Those in between are
* rounded down to the next lower one.
* [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
* .2% .3% 5% 7% 7% 20% 15% 45%
*/
static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
/*
* Distribution and probability of certain WSCALE values. We have to map the
* (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
* bits based on prevalence of certain values. Where we don't have an exact
* match for are rounded down to the next lower one letting us under-estimate
* the true available window. At the moment this would happen only for the
* very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
* and window size). The absence of the WSCALE option (no scaling in either
* direction) is encoded with index zero.
* [WSCALE values histograms, Allman, 2012]
* X 10 10 35 5 6 14 10% by host
* X 11 4 5 5 18 49 3% by connections
*/
static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
/*
* Compute the MAC for the SYN cookie. SIPHASH-2-4 is chosen for its speed
* and good cryptographic properties.
*/
static uint32_t
syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
uint8_t *secbits, uintptr_t secmod)
{
SIPHASH_CTX ctx;
uint32_t siphash[2];
SipHash24_Init(&ctx);
SipHash_SetKey(&ctx, secbits);
switch (inc->inc_flags & INC_ISIPV6) {
#ifdef INET
case 0:
SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
break;
#endif
#ifdef INET6
case INC_ISIPV6:
SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
break;
#endif
}
SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
SipHash_Update(&ctx, &irs, sizeof(irs));
SipHash_Update(&ctx, &flags, sizeof(flags));
SipHash_Update(&ctx, &secmod, sizeof(secmod));
SipHash_Final((u_int8_t *)&siphash, &ctx);
return (siphash[0] ^ siphash[1]);
}
static tcp_seq
syncookie_generate(struct syncache_head *sch, struct syncache *sc)
{
u_int i, secbit, wscale;
uint32_t iss, hash;
uint8_t *secbits;
union syncookie cookie;
cookie.cookie = 0;
/* Map our computed MSS into the 3-bit index. */
for (i = nitems(tcp_sc_msstab) - 1;
tcp_sc_msstab[i] > sc->sc_peer_mss && i > 0;
i--)
;
cookie.flags.mss_idx = i;
/*
* Map the send window scale into the 3-bit index but only if
* the wscale option was received.
*/
if (sc->sc_flags & SCF_WINSCALE) {
wscale = sc->sc_requested_s_scale;
for (i = nitems(tcp_sc_wstab) - 1;
tcp_sc_wstab[i] > wscale && i > 0;
i--)
;
cookie.flags.wscale_idx = i;
}
/* Can we do SACK? */
if (sc->sc_flags & SCF_SACK)
cookie.flags.sack_ok = 1;
/* Which of the two secrets to use. */
secbit = V_tcp_syncache.secret.oddeven & 0x1;
cookie.flags.odd_even = secbit;
secbits = V_tcp_syncache.secret.key[secbit];
hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
(uintptr_t)sch);
/*
* Put the flags into the hash and XOR them to get better ISS number
* variance. This doesn't enhance the cryptographic strength and is
* done to prevent the 8 cookie bits from showing up directly on the
* wire.
*/
iss = hash & ~0xff;
iss |= cookie.cookie ^ (hash >> 24);
TCPSTAT_INC(tcps_sc_sendcookie);
return (iss);
}
static struct syncache *
syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
struct socket *lso)
{
uint32_t hash;
uint8_t *secbits;
tcp_seq ack, seq;
int wnd, wscale = 0;
union syncookie cookie;
/*
* Pull information out of SYN-ACK/ACK and revert sequence number
* advances.
*/
ack = th->th_ack - 1;
seq = th->th_seq - 1;
/*
* Unpack the flags containing enough information to restore the
* connection.
*/
cookie.cookie = (ack & 0xff) ^ (ack >> 24);
/* Which of the two secrets to use. */
secbits = V_tcp_syncache.secret.key[cookie.flags.odd_even];
hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
/* The recomputed hash matches the ACK if this was a genuine cookie. */
if ((ack & ~0xff) != (hash & ~0xff))
return (NULL);
/* Fill in the syncache values. */
sc->sc_flags = 0;
bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
sc->sc_ipopts = NULL;
sc->sc_irs = seq;
sc->sc_iss = ack;
switch (inc->inc_flags & INC_ISIPV6) {
#ifdef INET
case 0:
sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
break;
#endif
#ifdef INET6
case INC_ISIPV6:
if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
sc->sc_flowlabel =
htonl(sc->sc_iss) & IPV6_FLOWLABEL_MASK;
break;
#endif
}
sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
/* We can simply recompute receive window scale we sent earlier. */
while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
wscale++;
/* Only use wscale if it was enabled in the orignal SYN. */
if (cookie.flags.wscale_idx > 0) {
sc->sc_requested_r_scale = wscale;
sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
sc->sc_flags |= SCF_WINSCALE;
}
wnd = lso->sol_sbrcv_hiwat;
wnd = imax(wnd, 0);
wnd = imin(wnd, TCP_MAXWIN);
sc->sc_wnd = wnd;
if (cookie.flags.sack_ok)
sc->sc_flags |= SCF_SACK;
if (to->to_flags & TOF_TS) {
sc->sc_flags |= SCF_TIMESTAMP;
sc->sc_tsreflect = to->to_tsval;
sc->sc_tsoff = tcp_new_ts_offset(inc);
}
if (to->to_flags & TOF_SIGNATURE)
sc->sc_flags |= SCF_SIGNATURE;
sc->sc_rxmits = 0;
TCPSTAT_INC(tcps_sc_recvcookie);
return (sc);
}
#ifdef INVARIANTS
static int
syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
struct socket *lso)
{
struct syncache scs, *scx;
char *s;
bzero(&scs, sizeof(scs));
scx = syncookie_lookup(inc, sch, &scs, th, to, lso);
if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
return (0);
if (scx != NULL) {
if (sc->sc_peer_mss != scx->sc_peer_mss)
log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
s, __func__, sc->sc_requested_r_scale,
scx->sc_requested_r_scale);
if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
s, __func__, sc->sc_requested_s_scale,
scx->sc_requested_s_scale);
if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
}
if (s != NULL)
free(s, M_TCPLOG);
return (0);
}
#endif /* INVARIANTS */
static void
syncookie_reseed(void *arg)
{
struct tcp_syncache *sc = arg;
uint8_t *secbits;
int secbit;
/*
* Reseeding the secret doesn't have to be protected by a lock.
* It only must be ensured that the new random values are visible
* to all CPUs in a SMP environment. The atomic with release
* semantics ensures that.
*/
secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
secbits = sc->secret.key[secbit];
arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
atomic_add_rel_int(&sc->secret.oddeven, 1);
/* Reschedule ourself. */
callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
}
/*
* We have overflowed a bucket. Let's pause dealing with the syncache.
* This function will increment the bucketoverflow statistics appropriately
* (once per pause when pausing is enabled; otherwise, once per overflow).
*/
static void
syncache_pause(struct in_conninfo *inc)
{
time_t delta;
const char *s;
/* XXX:
* 2. Add sysctl read here so we don't get the benefit of this
* change without the new sysctl.
*/
/*
* Try an unlocked read. If we already know that another thread
* has activated the feature, there is no need to proceed.
*/
if (V_tcp_syncache.paused)
return;
/* Are cookied enabled? If not, we can't pause. */
if (!V_tcp_syncookies) {
TCPSTAT_INC(tcps_sc_bucketoverflow);
return;
}
/*
* We may be the first thread to find an overflow. Get the lock
* and evaluate if we need to take action.
*/
mtx_lock(&V_tcp_syncache.pause_mtx);
if (V_tcp_syncache.paused) {
mtx_unlock(&V_tcp_syncache.pause_mtx);
return;
}
/* Activate protection. */
V_tcp_syncache.paused = true;
TCPSTAT_INC(tcps_sc_bucketoverflow);
/*
* Determine the last backoff time. If we are seeing a re-newed
* attack within that same time after last reactivating the syncache,
* consider it an extension of the same attack.
*/
delta = TCP_SYNCACHE_PAUSE_TIME << V_tcp_syncache.pause_backoff;
if (V_tcp_syncache.pause_until + delta - time_uptime > 0) {
if (V_tcp_syncache.pause_backoff < TCP_SYNCACHE_MAX_BACKOFF) {
delta <<= 1;
V_tcp_syncache.pause_backoff++;
}
} else {
delta = TCP_SYNCACHE_PAUSE_TIME;
V_tcp_syncache.pause_backoff = 0;
}
/* Log a warning, including IP addresses, if able. */
if (inc != NULL)
s = tcp_log_addrs(inc, NULL, NULL, NULL);
else
s = (const char *)NULL;
log(LOG_WARNING, "TCP syncache overflow detected; using syncookies for "
"the next %lld seconds%s%s%s\n", (long long)delta,
(s != NULL) ? " (last SYN: " : "", (s != NULL) ? s : "",
(s != NULL) ? ")" : "");
free(__DECONST(void *, s), M_TCPLOG);
/* Use the calculated delta to set a new pause time. */
V_tcp_syncache.pause_until = time_uptime + delta;
callout_reset(&V_tcp_syncache.pause_co, delta * hz, syncache_unpause,
&V_tcp_syncache);
mtx_unlock(&V_tcp_syncache.pause_mtx);
}
/* Evaluate whether we need to unpause. */
static void
syncache_unpause(void *arg)
{
struct tcp_syncache *sc;
time_t delta;
sc = arg;
mtx_assert(&sc->pause_mtx, MA_OWNED | MA_NOTRECURSED);
callout_deactivate(&sc->pause_co);
/*
* Check to make sure we are not running early. If the pause
* time has expired, then deactivate the protection.
*/
if ((delta = sc->pause_until - time_uptime) > 0)
callout_schedule(&sc->pause_co, delta * hz);
else
sc->paused = false;
}
/*
* Exports the syncache entries to userland so that netstat can display
* them alongside the other sockets. This function is intended to be
* called only from tcp_pcblist.
*
* Due to concurrency on an active system, the number of pcbs exported
* may have no relation to max_pcbs. max_pcbs merely indicates the
* amount of space the caller allocated for this function to use.
*/
int
syncache_pcblist(struct sysctl_req *req)
{
struct xtcpcb xt;
struct syncache *sc;
struct syncache_head *sch;
int error, i;
bzero(&xt, sizeof(xt));
xt.xt_len = sizeof(xt);
xt.t_state = TCPS_SYN_RECEIVED;
xt.xt_inp.xi_socket.xso_protocol = IPPROTO_TCP;
xt.xt_inp.xi_socket.xso_len = sizeof (struct xsocket);
xt.xt_inp.xi_socket.so_type = SOCK_STREAM;
xt.xt_inp.xi_socket.so_state = SS_ISCONNECTING;
for (i = 0; i < V_tcp_syncache.hashsize; i++) {
sch = &V_tcp_syncache.hashbase[i];
SCH_LOCK(sch);
TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
continue;
if (sc->sc_inc.inc_flags & INC_ISIPV6)
xt.xt_inp.inp_vflag = INP_IPV6;
else
xt.xt_inp.inp_vflag = INP_IPV4;
bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc,
sizeof (struct in_conninfo));
error = SYSCTL_OUT(req, &xt, sizeof xt);
if (error) {
SCH_UNLOCK(sch);
return (0);
}
}
SCH_UNLOCK(sch);
}
return (0);
}
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