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freebsd-nq/sys/netinet/tcp_syncache.c
Michael Tuexen bc35229fad When an ACK segment as the third message of the three way handshake is 
received and support for time stamps was negotiated in the SYN/SYNACK
exchange, perform the PAWS check and only expand the syn cache entry if
the check is passed.
Without this check, endpoints may get stuck on the incomplete queue.

Reviewed by:		jtl@
MFC after:		3 days
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D20374
2019-05-26 17:18:14 +00:00

2323 lines
68 KiB
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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 *, struct syncache_head *,
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 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, 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,
&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);
}
#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);
/* 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);
}
#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"));
sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
sch->sch_last_overflow = time_uptime;
syncache_drop(sc2, sch);
TCPSTAT_INC(tcps_sc_bucketoverflow);
}
/* 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;
int tick = ticks;
char *s;
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;
TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
/*
* 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_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);
}
syncache_respond(sc, sch, NULL, TH_SYN|TH_ACK);
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();
}
/*
* 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;
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;
sch = &V_tcp_syncache.hashbase[hash];
*schp = sch;
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;
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, sch, 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;
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;
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;
INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
/*
* 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_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
}
/*
* Install in the reservation hash table for now, but don't yet
* install a connection group since the full 4-tuple isn't yet
* configured.
*/
inp->inp_lport = sc->sc_inc.inc_lport;
if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
/*
* Undo the assignments above if we failed to
* put the PCB on the hash lists.
*/
#ifdef INET6
if (sc->sc_inc.inc_flags & INC_ISIPV6)
inp->in6p_laddr = in6addr_any;
else
#endif
inp->inp_laddr.s_addr = INADDR_ANY;
inp->inp_lport = 0;
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
"with error %i\n",
s, __func__, error);
free(s, M_TCPLOG);
}
INP_HASH_WUNLOCK(&V_tcbinfo);
goto abort;
}
#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);
}
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)) != 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)) != 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_flags |= TF_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;
/*
* Global TCP locks are held because we manipulate the PCB lists
* and create a new socket.
*/
INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
("%s: can handle only ACK", __func__));
sc = syncache_lookup(inc, &sch); /* returns locked sch */
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 (!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 (!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);
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 */
}
/*
* 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;
}
/*
* If timestamps were not negotiated during SYN/ACK they
* must not appear on any segment during this session.
*/
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 rejected\n", s, __func__);
goto failed;
}
/*
* If timestamps were negotiated during SYN/ACK they should
* appear on every segment during this session.
* XXXAO: This is only informal as there have been unverified
* reports of non-compliants stacks.
*/
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 missing, "
"no action\n", s, __func__);
free(s, M_TCPLOG);
s = NULL;
}
}
*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;
/*
* Global TCP locks are held because we manipulate the PCB lists
* and create a new socket.
*/
INP_INFO_RLOCK_ASSERT(&V_tcbinfo);
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)
{
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;
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) &&
(inp->inp_flags & IN6P_AUTOFLOWLABEL))
autoflowlabel = 1;
#endif
ip_ttl = inp->inp_ip_ttl;
ip_tos = inp->inp_ip_tos;
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?
*/
sc = syncache_lookup(inc, &sch); /* returns locked entry */
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;
#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, sch, 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;
}
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);
}
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 {
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));
#ifdef INET6
if (!(inc->inc_flags & INC_ISIPV6))
#endif
{
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_iss = arc4random();
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)) && V_tcp_do_ecn)
sc->sc_flags |= SCF_ECN;
if (V_tcp_syncookies)
sc->sc_iss = syncookie_generate(sch, sc);
#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
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, sch, 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, struct syncache_head *sch,
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
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 */
ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
ip6->ip6_flow |= sc->sc_flowlabel;
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 = in6_selecthlim(NULL, NULL);
#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;
SCH_LOCK_ASSERT(sch);
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 = sch->sch_sc->secret.oddeven & 0x1;
cookie.flags.odd_even = secbit;
secbits = sch->sch_sc->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;
SCH_LOCK_ASSERT(sch);
/*
* 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 = sch->sch_sc->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 = 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);
}
/*
* 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, int max_pcbs, int *pcbs_exported)
{
struct xtcpcb xt;
struct syncache *sc;
struct syncache_head *sch;
int count, error, i;
for (count = 0, error = 0, 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 (count >= max_pcbs) {
SCH_UNLOCK(sch);
goto exit;
}
if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
continue;
bzero(&xt, sizeof(xt));
xt.xt_len = sizeof(xt);
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));
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;
error = SYSCTL_OUT(req, &xt, sizeof xt);
if (error) {
SCH_UNLOCK(sch);
goto exit;
}
count++;
}
SCH_UNLOCK(sch);
}
exit:
*pcbs_exported = count;
return error;
}
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