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Add new functionality to switch to using cookies exclusively when we the

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syn cache overflows. Whether this is due to an attack or due to the system
having more legitimate connections than the syn cache can hold, this
situation can quickly impact performance.

To make the system perform better during these periods, the code will now
switch to exclusively using cookies until the syn cache stops overflowing.
In order for this to occur, the system must be configured to use the syn
cache with syn cookie fallback. If syn cookies are completely disabled,
this change should have no functional impact.

When the system is exclusively using syn cookies (either due to
configuration or the overflow detection enabled by this change), the
code will now skip acquiring a lock on the syn cache bucket. Additionally,
the code will now skip lookups in several places (such as when the system
receives a RST in response to a SYN|ACK frame).

Reviewed by:	rrs, gallatin (previous version)
Discussed with:	tuexen
Sponsored by:	Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D21644
This commit is contained in:
Jonathan T. Looney 2019-09-26 15:18:57 +00:00
parent 0bee4d631a
commit 0b18fb0798
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=352746
2 changed files with 214 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

224
sys/netinet/tcp_syncache.c
View File

@ -144,6 +144,8 @@ 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,
@ -300,6 +302,14 @@ syncache_init(void)
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
@ -316,6 +326,14 @@ syncache_destroy(void)
*/
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++) {
@ -339,6 +357,7 @@ syncache_destroy(void)
/* 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
@ -360,10 +379,10 @@ syncache_insert(struct syncache *sc, struct syncache_head *sch)
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);
TCPSTAT_INC(tcps_sc_bucketoverflow);
}
/* Put it into the bucket. */
@ -450,6 +469,7 @@ syncache_timer(void *xsch)
struct syncache *sc, *nsc;
int tick = ticks;
char *s;
bool paused;
CURVNET_SET(sch->sch_sc->vnet);
@ -462,7 +482,19 @@ syncache_timer(void *xsch)
*/
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
@ -505,14 +537,24 @@ syncache_timer(void *xsch)
}
/*
* Find an entry in the syncache.
* Returns always with locked syncache_head plus a matching entry or NULL.
* 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 struct syncache *
syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
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)
{
struct syncache *sc;
struct syncache_head *sch;
uint32_t hash;
/*
@ -525,8 +567,20 @@ syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
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;
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. */
@ -551,6 +605,8 @@ syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th, struct mbuf *m)
struct syncache_head *sch;
char *s = NULL;
if (syncache_cookiesonly())
return;
sc = syncache_lookup(inc, &sch); /* returns locked sch */
SCH_LOCK_ASSERT(sch);
@ -654,6 +710,8 @@ 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) {
@ -669,6 +727,8 @@ 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)
@ -1029,6 +1089,7 @@ syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
struct syncache_head *sch;
struct syncache scs;
char *s;
bool locked;
/*
* Global TCP locks are held because we manipulate the PCB lists
@ -1038,8 +1099,15 @@ syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
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);
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
/*
@ -1063,7 +1131,7 @@ syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
* C. check that the syncookie is valid. If it is, then
* cobble up a fake syncache entry, and return.
*/
if (!V_tcp_syncookies) {
if (locked && !V_tcp_syncookies) {
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Spurious ACK, "
@ -1071,7 +1139,7 @@ syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
s, __func__);
goto failed;
}
if (!V_tcp_syncookiesonly &&
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)))
@ -1082,7 +1150,8 @@ syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
}
bzero(&scs, sizeof(scs));
sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
SCH_UNLOCK(sch);
if (locked)
SCH_UNLOCK(sch);
if (sc == NULL) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Segment failed "
@ -1331,6 +1400,7 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
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,
@ -1436,8 +1506,15 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
* 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 (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);
@ -1490,7 +1567,15 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
goto skip_alloc;
}
sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
/*
* 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.
@ -1501,6 +1586,7 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
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) {
@ -1508,6 +1594,9 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
bzero(&scs, sizeof(scs));
sc = &scs;
} else {
KASSERT(locked,
("%s: bucket unexpectedly unlocked",
__func__));
SCH_UNLOCK(sch);
if (ipopts)
(void) m_free(ipopts);
@ -1626,7 +1715,8 @@ syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
}
#endif
SCH_UNLOCK(sch);
if (locked)
SCH_UNLOCK(sch);
if (tfo_cookie_valid) {
syncache_tfo_expand(sc, lsop, m, tfo_response_cookie);
@ -2262,6 +2352,104 @@ syncookie_reseed(void *arg)
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

8
sys/netinet/tcp_syncache.h
View File

@ -111,6 +111,9 @@ struct syncookie_secret {
u_int lifetime;
};
#define TCP_SYNCACHE_PAUSE_TIME SYNCOOKIE_LIFETIME
#define TCP_SYNCACHE_MAX_BACKOFF 6 /* 16 minutes */
struct tcp_syncache {
struct syncache_head *hashbase;
uma_zone_t zone;
@ -122,6 +125,11 @@ struct tcp_syncache {
uint32_t hash_secret;
struct vnet *vnet;
struct syncookie_secret secret;
struct mtx pause_mtx;
struct callout pause_co;
time_t pause_until;
uint8_t pause_backoff;
volatile bool paused;
};
/* Internal use for the syncookie functions. */