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freebsd/sys/netinet/tcp_syncache.c
Andre Oppermann d2ddf5d4b0 Be more restrictive with segment validity checks in syncache_expand() 
and log check failures to syslog at LOG_DEBUG level.

Always prefill the sc->sc_ts field to use it in the checks.
2007-05-18 21:42:25 +00:00

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/*-
* Copyright (c) 2001 McAfee, Inc.
* Copyright (c) 2006 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.
*
* 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.
*
* $FreeBSD$
*/
#include "opt_inet.h"
#include "opt_inet6.h"
#include "opt_ipsec.h"
#include "opt_mac.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/malloc.h>
#include <sys/mbuf.h>
#include <sys/md5.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 <vm/uma.h>
#include <net/if.h>
#include <net/route.h>
#include <netinet/in.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_fsm.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#ifdef INET6
#include <netinet6/tcp6_var.h>
#endif
#ifdef IPSEC
#include <netinet6/ipsec.h>
#ifdef INET6
#include <netinet6/ipsec6.h>
#endif
#endif /*IPSEC*/
#ifdef FAST_IPSEC
#include <netipsec/ipsec.h>
#ifdef INET6
#include <netipsec/ipsec6.h>
#endif
#include <netipsec/key.h>
#endif /*FAST_IPSEC*/
#include <machine/in_cksum.h>
#include <security/mac/mac_framework.h>
static int tcp_syncookies = 1;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW,
&tcp_syncookies, 0,
"Use TCP SYN cookies if the syncache overflows");
static int tcp_syncookiesonly = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_RW,
&tcp_syncookiesonly, 0,
"Use only TCP SYN cookies");
#define SYNCOOKIE_SECRET_SIZE 8 /* dwords */
#define SYNCOOKIE_LIFETIME 16 /* seconds */
struct syncache {
TAILQ_ENTRY(syncache) sc_hash;
struct in_conninfo sc_inc; /* addresses */
u_long sc_rxttime; /* retransmit time */
u_int16_t sc_rxmits; /* retransmit counter */
u_int32_t sc_tsreflect; /* timestamp to reflect */
u_int32_t sc_ts; /* our timestamp to send */
u_int32_t sc_tsoff; /* ts offset w/ syncookies */
u_int32_t sc_flowlabel; /* IPv6 flowlabel */
tcp_seq sc_irs; /* seq from peer */
tcp_seq sc_iss; /* our ISS */
struct mbuf *sc_ipopts; /* source route */
u_int16_t sc_peer_mss; /* peer's MSS */
u_int16_t sc_wnd; /* advertised window */
u_int8_t sc_ip_ttl; /* IPv4 TTL */
u_int8_t sc_ip_tos; /* IPv4 TOS */
u_int8_t sc_requested_s_scale:4,
sc_requested_r_scale:4;
u_int8_t sc_flags;
#define SCF_NOOPT 0x01 /* no TCP options */
#define SCF_WINSCALE 0x02 /* negotiated window scaling */
#define SCF_TIMESTAMP 0x04 /* negotiated timestamps */
/* MSS is implicit */
#define SCF_UNREACH 0x10 /* icmp unreachable received */
#define SCF_SIGNATURE 0x20 /* send MD5 digests */
#define SCF_SACK 0x80 /* send SACK option */
#ifdef MAC
struct label *sc_label; /* MAC label reference */
#endif
};
struct syncache_head {
struct mtx sch_mtx;
TAILQ_HEAD(sch_head, syncache) sch_bucket;
struct callout sch_timer;
int sch_nextc;
u_int sch_length;
u_int sch_oddeven;
u_int32_t sch_secbits_odd[SYNCOOKIE_SECRET_SIZE];
u_int32_t sch_secbits_even[SYNCOOKIE_SECRET_SIZE];
u_int sch_reseed; /* time_uptime, seconds */
};
static void syncache_drop(struct syncache *, struct syncache_head *);
static void syncache_free(struct syncache *);
static void syncache_insert(struct syncache *, struct syncache_head *);
struct syncache *syncache_lookup(struct in_conninfo *, struct syncache_head **);
static int syncache_respond(struct syncache *);
static struct socket *syncache_socket(struct syncache *, struct socket *,
struct mbuf *m);
static void syncache_timer(void *);
static void syncookie_generate(struct syncache_head *, struct syncache *,
u_int32_t *);
static struct syncache
*syncookie_lookup(struct in_conninfo *, struct syncache_head *,
struct syncache *, struct tcpopt *, struct tcphdr *,
struct socket *);
/*
* Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
* 3 retransmits corresponds to a timeout of (1 + 2 + 4 + 8 == 15) seconds,
* 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
struct tcp_syncache {
struct syncache_head *hashbase;
uma_zone_t zone;
u_int hashsize;
u_int hashmask;
u_int bucket_limit;
u_int cache_count; /* XXX: unprotected */
u_int cache_limit;
u_int rexmt_limit;
u_int hash_secret;
};
static struct tcp_syncache tcp_syncache;
SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0, "TCP SYN cache");
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RDTUN,
&tcp_syncache.bucket_limit, 0, "Per-bucket hash limit for syncache");
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RDTUN,
&tcp_syncache.cache_limit, 0, "Overall entry limit for syncache");
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, count, CTLFLAG_RD,
&tcp_syncache.cache_count, 0, "Current number of entries in syncache");
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RDTUN,
&tcp_syncache.hashsize, 0, "Size of TCP syncache hashtable");
SYSCTL_INT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW,
&tcp_syncache.rexmt_limit, 0, "Limit on SYN/ACK retransmissions");
static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
#define SYNCACHE_HASH(inc, mask) \
((tcp_syncache.hash_secret ^ \
(inc)->inc_faddr.s_addr ^ \
((inc)->inc_faddr.s_addr >> 16) ^ \
(inc)->inc_fport ^ (inc)->inc_lport) & mask)
#define SYNCACHE_HASH6(inc, mask) \
((tcp_syncache.hash_secret ^ \
(inc)->inc6_faddr.s6_addr32[0] ^ \
(inc)->inc6_faddr.s6_addr32[3] ^ \
(inc)->inc_fport ^ (inc)->inc_lport) & mask)
#define ENDPTS_EQ(a, b) ( \
(a)->ie_fport == (b)->ie_fport && \
(a)->ie_lport == (b)->ie_lport && \
(a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr && \
(a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr \
)
#define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0)
#define SYNCACHE_TIMEOUT(sc, sch, co) do { \
(sc)->sc_rxmits++; \
(sc)->sc_rxttime = ticks + \
TCPTV_RTOBASE * tcp_backoff[(sc)->sc_rxmits - 1]; \
if ((sch)->sch_nextc > (sc)->sc_rxttime) \
(sch)->sch_nextc = (sc)->sc_rxttime; \
if (!TAILQ_EMPTY(&(sch)->sch_bucket) && !(co)) \
callout_reset(&(sch)->sch_timer, \
(sch)->sch_nextc - ticks, \
syncache_timer, (void *)(sch)); \
} while (0)
#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);
#ifdef MAC
mac_destroy_syncache(&sc->sc_label);
#endif
uma_zfree(tcp_syncache.zone, sc);
}
void
syncache_init(void)
{
int i;
tcp_syncache.cache_count = 0;
tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
tcp_syncache.hash_secret = arc4random();
TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
&tcp_syncache.hashsize);
TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
&tcp_syncache.bucket_limit);
if (!powerof2(tcp_syncache.hashsize) || tcp_syncache.hashsize == 0) {
printf("WARNING: syncache hash size is not a power of 2.\n");
tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
}
tcp_syncache.hashmask = tcp_syncache.hashsize - 1;
/* Set limits. */
tcp_syncache.cache_limit =
tcp_syncache.hashsize * tcp_syncache.bucket_limit;
TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
&tcp_syncache.cache_limit);
/* Allocate the hash table. */
MALLOC(tcp_syncache.hashbase, struct syncache_head *,
tcp_syncache.hashsize * sizeof(struct syncache_head),
M_SYNCACHE, M_WAITOK | M_ZERO);
/* Initialize the hash buckets. */
for (i = 0; i < tcp_syncache.hashsize; i++) {
TAILQ_INIT(&tcp_syncache.hashbase[i].sch_bucket);
mtx_init(&tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
NULL, MTX_DEF);
callout_init_mtx(&tcp_syncache.hashbase[i].sch_timer,
&tcp_syncache.hashbase[i].sch_mtx, 0);
tcp_syncache.hashbase[i].sch_length = 0;
}
/* Create the syncache entry zone. */
tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
uma_zone_set_max(tcp_syncache.zone, tcp_syncache.cache_limit);
}
/*
* 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 >= tcp_syncache.bucket_limit) {
KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
("sch->sch_length incorrect"));
sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
syncache_drop(sc2, sch);
tcpstat.tcps_sc_bucketoverflow++;
}
/* Put it into the bucket. */
TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
sch->sch_length++;
/* Reinitialize the bucket row's timer. */
SYNCACHE_TIMEOUT(sc, sch, 1);
SCH_UNLOCK(sch);
tcp_syncache.cache_count++;
tcpstat.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);
TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
sch->sch_length--;
syncache_free(sc);
tcp_syncache.cache_count--;
}
/*
* 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;
/* NB: syncache_head has already been locked by the callout. */
SCH_LOCK_ASSERT(sch);
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 (sc->sc_rxttime >= tick) {
if (sc->sc_rxttime < sch->sch_nextc)
sch->sch_nextc = sc->sc_rxttime;
continue;
}
if (sc->sc_rxmits > tcp_syncache.rexmt_limit) {
if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
log(LOG_DEBUG, "%s; %s: Response timeout\n",
s, __func__);
free(s, M_TCPLOG);
}
syncache_drop(sc, sch);
tcpstat.tcps_sc_stale++;
continue;
}
(void) syncache_respond(sc);
tcpstat.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));
}
/*
* Find an entry in the syncache.
* Returns always with locked syncache_head plus a matching entry or NULL.
*/
struct syncache *
syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
{
struct syncache *sc;
struct syncache_head *sch;
#ifdef INET6
if (inc->inc_isipv6) {
sch = &tcp_syncache.hashbase[
SYNCACHE_HASH6(inc, tcp_syncache.hashmask)];
*schp = sch;
SCH_LOCK(sch);
/* Circle through bucket row to find matching entry. */
TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
return (sc);
}
} else
#endif
{
sch = &tcp_syncache.hashbase[
SYNCACHE_HASH(inc, tcp_syncache.hashmask)];
*schp = sch;
SCH_LOCK(sch);
/* Circle through bucket row to find matching entry. */
TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
#ifdef INET6
if (sc->sc_inc.inc_isipv6)
continue;
#endif
if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
return (sc);
}
}
SCH_LOCK_ASSERT(*schp);
return (NULL); /* 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.
*/
void
syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
{
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 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.
*
* The sequence number in the reset segment is normally an
* echo of our outgoing acknowlegement numbers, but some hosts
* send a reset with the sequence number at the rightmost edge
* of our receive window, and we have to handle this case.
*/
if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
syncache_drop(sc, sch);
tcpstat.tcps_sc_reset++;
}
done:
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.tcps_sc_badack++;
}
SCH_UNLOCK(sch);
}
void
syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
{
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->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.tcps_sc_unreach++;
done:
SCH_UNLOCK(sch);
}
/*
* Build a new TCP socket structure from a syncache entry.
*/
static struct socket *
syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
{
struct inpcb *inp = NULL;
struct socket *so;
struct tcpcb *tp;
char *s;
NET_ASSERT_GIANT();
INP_INFO_WLOCK_ASSERT(&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, SS_ISCONNECTED);
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.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
SOCK_LOCK(so);
mac_set_socket_peer_from_mbuf(m, so);
SOCK_UNLOCK(so);
#endif
inp = sotoinpcb(so);
INP_LOCK(inp);
/* Insert new socket into PCB hash list. */
inp->inp_inc.inc_isipv6 = sc->sc_inc.inc_isipv6;
#ifdef INET6
if (sc->sc_inc.inc_isipv6) {
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
inp->inp_lport = sc->sc_inc.inc_lport;
if (in_pcbinshash(inp) != 0) {
/*
* Undo the assignments above if we failed to
* put the PCB on the hash lists.
*/
#ifdef INET6
if (sc->sc_inc.inc_isipv6)
inp->in6p_laddr = in6addr_any;
else
#endif
inp->inp_laddr.s_addr = INADDR_ANY;
inp->inp_lport = 0;
goto abort;
}
#ifdef IPSEC
/* Copy old policy into new socket's. */
if (ipsec_copy_pcbpolicy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
printf("syncache_socket: could not copy policy\n");
#endif
#ifdef FAST_IPSEC
/* Copy old policy into new socket's. */
if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
printf("syncache_socket: could not copy policy\n");
#endif
#ifdef INET6
if (sc->sc_inc.inc_isipv6) {
struct inpcb *oinp = sotoinpcb(lso);
struct in6_addr laddr6;
struct sockaddr_in6 sin6;
/*
* 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);
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 (in6_pcbconnect(inp, (struct sockaddr *)&sin6,
thread0.td_ucred)) {
inp->in6p_laddr = laddr6;
goto abort;
}
/* Override flowlabel from in6_pcbconnect. */
inp->in6p_flowinfo &= ~IPV6_FLOWLABEL_MASK;
inp->in6p_flowinfo |= sc->sc_flowlabel;
} else
#endif
{
struct in_addr laddr;
struct sockaddr_in sin;
inp->inp_options = ip_srcroute(m);
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 (in_pcbconnect(inp, (struct sockaddr *)&sin,
thread0.td_ucred)) {
inp->inp_laddr = laddr;
goto abort;
}
}
tp = intotcpcb(inp);
tp->t_state = TCPS_SYN_RECEIVED;
tp->iss = sc->sc_iss;
tp->irs = sc->sc_irs;
tcp_rcvseqinit(tp);
tcp_sendseqinit(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 = ticks;
tp->ts_offset = sc->sc_tsoff;
}
#ifdef 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;
}
/*
* 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, reset cwnd to 1 segment.
*/
if (sc->sc_rxmits > 1)
tp->snd_cwnd = tp->t_maxseg;
tcp_timer_activate(tp, TT_KEEP, tcp_keepinit);
INP_UNLOCK(inp);
tcpstat.tcps_accepts++;
return (so);
abort:
INP_UNLOCK(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.
*/
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_WLOCK_ASSERT(&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);
if (sc == NULL) {
/*
* There is no syncache entry, so see if this ACK is
* a returning syncookie. To do this, first:
* A. See if this socket has had a syncache entry dropped in
* the past. We don't want to accept a bogus syncookie
* if we've never received a SYN.
* B. check that the syncookie is valid. If it is, then
* cobble up a fake syncache entry, and return.
*/
if (!tcp_syncookies) {
SCH_UNLOCK(sch);
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: Spurious ACK\n",
s, __func__);
goto failed;
}
bzero(&scs, sizeof(scs));
sc = syncookie_lookup(inc, sch, &scs, to, th, *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\n",
s, __func__);
goto failed;
}
tcpstat.tcps_sc_recvcookie++;
} else {
/* Pull out the entry to unlock the bucket row. */
TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
sch->sch_length--;
tcp_syncache.cache_count--;
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\n",
s, __func__, th->th_ack, sc->sc_iss);
goto failed;
}
/*
* The SEQ must match the received initial receive sequence
* number + 1 (the SYN) because we didn't ACK any data that
* may have come with the SYN.
*/
if (th->th_seq != sc->sc_irs + 1) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u\n",
s, __func__, th->th_ack, sc->sc_iss);
goto failed;
}
/*
* If timestamps were present in the SYN and we accepted
* them in our SYN|ACK we require them to be present from
* now on. And vice versa.
*/
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\n",
s, __func__);
goto failed;
}
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\n",
s, __func__);
goto failed;
}
/*
* If timestamps were negotiated the reflected timestamp
* must be equal to what we actually sent in the SYN|ACK.
*/
if ((to->to_flags & TOF_TS) && to->to_tsecr != sc->sc_ts) {
if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u\n",
s, __func__, to->to_tsecr, sc->sc_ts);
goto failed;
}
*lsop = syncache_socket(sc, *lsop, m);
if (*lsop == NULL)
tcpstat.tcps_sc_aborted++;
else
tcpstat.tcps_sc_completed++;
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);
}
/*
* 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.
*/
void
syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
struct inpcb *inp, struct socket **lsop, struct mbuf *m)
{
struct tcpcb *tp;
struct socket *so;
struct syncache *sc = NULL;
struct syncache_head *sch;
struct mbuf *ipopts = NULL;
u_int32_t flowtmp;
int win, sb_hiwat, ip_ttl, ip_tos, noopt;
#ifdef INET6
int autoflowlabel = 0;
#endif
#ifdef MAC
struct label *maclabel;
#endif
struct syncache scs;
INP_INFO_WLOCK_ASSERT(&tcbinfo);
INP_LOCK_ASSERT(inp); /* listen socket */
/*
* Combine all so/tp operations very early to drop the INP lock as
* soon as possible.
*/
so = *lsop;
tp = sototcpcb(so);
#ifdef INET6
if (inc->inc_isipv6 &&
(inp->in6p_flags & IN6P_AUTOFLOWLABEL))
autoflowlabel = 1;
#endif
ip_ttl = inp->inp_ip_ttl;
ip_tos = inp->inp_ip_tos;
win = sbspace(&so->so_rcv);
sb_hiwat = so->so_rcv.sb_hiwat;
noopt = (tp->t_flags & TF_NOOPT);
so = NULL;
tp = NULL;
#ifdef MAC
if (mac_init_syncache(&maclabel) != 0) {
INP_UNLOCK(inp);
INP_INFO_WUNLOCK(&tcbinfo);
goto done;
} else
mac_init_syncache_from_inpcb(maclabel, inp);
#endif
INP_UNLOCK(inp);
INP_INFO_WUNLOCK(&tcbinfo);
/*
* Remember the IP options, if any.
*/
#ifdef INET6
if (!inc->inc_isipv6)
#endif
ipopts = ip_srcroute(m);
/*
* 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?)
*/
sc = syncache_lookup(inc, &sch); /* returns locked entry */
SCH_LOCK_ASSERT(sch);
if (sc != NULL) {
tcpstat.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_destroy_syncache(&maclabel);
KASSERT(sc->sc_label != NULL,
("%s: label not initialized", __func__));
#endif
if (syncache_respond(sc) == 0) {
SYNCACHE_TIMEOUT(sc, sch, 1);
tcpstat.tcps_sndacks++;
tcpstat.tcps_sndtotal++;
}
SCH_UNLOCK(sch);
goto done;
}
sc = uma_zalloc(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.tcps_sc_zonefail++;
if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL)
syncache_drop(sc, sch);
sc = uma_zalloc(tcp_syncache.zone, M_NOWAIT | M_ZERO);
if (sc == NULL) {
if (tcp_syncookies) {
bzero(&scs, sizeof(scs));
sc = &scs;
} else {
SCH_UNLOCK(sch);
if (ipopts)
(void) m_free(ipopts);
goto done;
}
}
}
/*
* Fill in the syncache values.
*/
#ifdef MAC
sc->sc_label = maclabel;
#endif
sc->sc_ipopts = ipopts;
bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
#ifdef INET6
if (!inc->inc_isipv6)
#endif
{
sc->sc_ip_tos = ip_tos;
sc->sc_ip_ttl = ip_ttl;
}
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 (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_ts = ticks;
sc->sc_flags |= SCF_TIMESTAMP;
}
if (to->to_flags & TOF_SCALE) {
int wscale = 0;
/*
* Compute proper scaling value from buffer space.
* Leave enough room for the socket buffer to grow
* with auto sizing. This allows us to scale the
* receive buffer over a wide range while not losing
* any efficiency or fine granularity.
*
* RFC1323: The Window field in a SYN (i.e., a <SYN>
* or <SYN,ACK>) segment itself is never scaled.
*/
while (wscale < TCP_MAX_WINSHIFT &&
(0x1 << wscale) < tcp_minmss)
wscale++;
sc->sc_requested_r_scale = wscale;
sc->sc_requested_s_scale = to->to_wscale;
sc->sc_flags |= SCF_WINSCALE;
}
}
#ifdef TCP_SIGNATURE
/*
* If listening socket requested TCP digests, and received SYN
* contains the option, flag this in the syncache so that
* syncache_respond() will do the right thing with the SYN+ACK.
* XXX: Currently we always record the option by default and will
* attempt to use it in syncache_respond().
*/
if (to->to_flags & TOF_SIGNATURE)
sc->sc_flags |= SCF_SIGNATURE;
#endif
if (to->to_flags & TOF_SACK)
sc->sc_flags |= SCF_SACK;
if (to->to_flags & TOF_MSS)
sc->sc_peer_mss = to->to_mss; /* peer mss may be zero */
if (noopt)
sc->sc_flags |= SCF_NOOPT;
if (tcp_syncookies) {
syncookie_generate(sch, sc, &flowtmp);
#ifdef INET6
if (autoflowlabel)
sc->sc_flowlabel = flowtmp;
#endif
} else {
#ifdef INET6
if (autoflowlabel)
sc->sc_flowlabel =
(htonl(ip6_randomflowlabel()) & IPV6_FLOWLABEL_MASK);
#endif
}
SCH_UNLOCK(sch);
/*
* Do a standard 3-way handshake.
*/
if (syncache_respond(sc) == 0) {
if (tcp_syncookies && tcp_syncookiesonly && sc != &scs)
syncache_free(sc);
else if (sc != &scs)
syncache_insert(sc, sch); /* locks and unlocks sch */
tcpstat.tcps_sndacks++;
tcpstat.tcps_sndtotal++;
} else {
if (sc != &scs)
syncache_free(sc);
tcpstat.tcps_sc_dropped++;
}
done:
#ifdef MAC
if (sc == &scs)
mac_destroy_syncache(&maclabel);
#endif
*lsop = NULL;
m_freem(m);
return;
}
static int
syncache_respond(struct syncache *sc)
{
struct ip *ip = NULL;
struct mbuf *m;
struct tcphdr *th;
int optlen, error;
u_int16_t hlen, tlen, mssopt;
struct tcpopt to;
#ifdef INET6
struct ip6_hdr *ip6 = NULL;
#endif
hlen =
#ifdef INET6
(sc->sc_inc.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 = tcp_mssopt(&sc->sc_inc);
if (sc->sc_peer_mss)
mssopt = max( min(sc->sc_peer_mss, mssopt), 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_DONTWAIT, MT_DATA);
if (m == NULL)
return (ENOBUFS);
#ifdef MAC
mac_create_mbuf_from_syncache(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_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);
} else
#endif
{
ip = mtod(m, struct ip *);
ip->ip_v = IPVERSION;
ip->ip_hl = sizeof(struct ip) >> 2;
ip->ip_len = 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 (path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
ip->ip_off |= IP_DF;
th = (struct tcphdr *)(ip + 1);
}
th->th_sport = sc->sc_inc.inc_lport;
th->th_dport = sc->sc_inc.inc_fport;
th->th_seq = htonl(sc->sc_iss);
th->th_ack = htonl(sc->sc_irs + 1);
th->th_off = sizeof(struct tcphdr) >> 2;
th->th_x2 = 0;
th->th_flags = TH_SYN|TH_ACK;
th->th_win = htons(sc->sc_wnd);
th->th_urp = 0;
/* Tack on the TCP options. */
if ((sc->sc_flags & SCF_NOOPT) == 0) {
to.to_flags = 0;
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_TIMESTAMP) {
/* Virgin timestamp or TCP cookie enhanced one. */
to.to_tsval = sc->sc_ts;
to.to_tsecr = sc->sc_tsreflect;
to.to_flags |= TOF_TS;
}
if (sc->sc_flags & SCF_SACK)
to.to_flags |= TOF_SACKPERM;
#ifdef TCP_SIGNATURE
if (sc->sc_flags & SCF_SIGNATURE)
to.to_flags |= TOF_SIGNATURE;
#endif
optlen = tcp_addoptions(&to, (u_char *)(th + 1));
#ifdef TCP_SIGNATURE
tcp_signature_compute(m, sizeof(struct ip), 0, optlen,
to.to_signature, IPSEC_DIR_OUTBOUND);
#endif
/* 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_isipv6)
ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
else
#endif
ip->ip_len += optlen;
} else
optlen = 0;
#ifdef INET6
if (sc->sc_inc.inc_isipv6) {
th->th_sum = 0;
th->th_sum = in6_cksum(m, IPPROTO_TCP, hlen,
tlen + optlen - hlen);
ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
} else
#endif
{
th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
htons(tlen + optlen - hlen + IPPROTO_TCP));
m->m_pkthdr.csum_flags = CSUM_TCP;
m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
}
return (error);
}
/*
* The purpose of SYN cookies is to avoid keeping track of all SYN's we
* receive and to be able to handle SYN floods from bogus source addresses
* (where we will never receive any reply). SYN floods try to exhaust all
* our memory and available slots in the SYN cache table to cause a denial
* of service to legitimate users of the local host.
*
* The idea of SYN cookies is to encode and include all necessary information
* about the connection setup state within the SYN-ACK we send back and thus
* to get along without keeping any local state until the ACK to the SYN-ACK
* arrives (if ever). Everything we need to know should be available from
* the information we encoded in the SYN-ACK.
*
* More information about the theory behind SYN cookies and its first
* discussion and specification can be found at:
* http://cr.yp.to/syncookies.html (overview)
* http://cr.yp.to/syncookies/archive (gory details)
*
* This implementation extends the orginal idea and first implementation
* of FreeBSD by using not only the initial sequence number field to store
* information but also the timestamp field if present. This way we can
* keep track of the entire state we need to know to recreate the session in
* its original form. Almost all TCP speakers implement RFC1323 timestamps
* these days. For those that do not we still have to live with the known
* shortcomings of the ISN only SYN cookies.
*
* Cookie layers:
*
* Initial sequence number we send:
* 31|................................|0
* DDDDDDDDDDDDDDDDDDDDDDDDDMMMRRRP
* D = MD5 Digest (first dword)
* M = MSS index
* R = Rotation of secret
* P = Odd or Even secret
*
* The MD5 Digest is computed with over following parameters:
* a) randomly rotated secret
* b) struct in_conninfo containing the remote/local ip/port (IPv4&IPv6)
* c) the received initial sequence number from remote host
* d) the rotation offset and odd/even bit
*
* Timestamp we send:
* 31|................................|0
* DDDDDDDDDDDDDDDDDDDDDDSSSSRRRRA5
* D = MD5 Digest (third dword) (only as filler)
* S = Requested send window scale
* R = Requested receive window scale
* A = SACK allowed
* 5 = TCP-MD5 enabled (not implemented yet)
* XORed with MD5 Digest (forth dword)
*
* The timestamp isn't cryptographically secure and doesn't need to be.
* The double use of the MD5 digest dwords ties it to a specific remote/
* local host/port, remote initial sequence number and our local time
* limited secret. A received timestamp is reverted (XORed) and then
* the contained MD5 dword is compared to the computed one to ensure the
* timestamp belongs to the SYN-ACK we sent. The other parameters may
* have been tampered with but this isn't different from supplying bogus
* values in the SYN in the first place.
*
* Some problems with SYN cookies remain however:
* Consider the problem of a recreated (and retransmitted) cookie. If the
* original SYN was accepted, the connection is established. The second
* SYN is inflight, and if it arrives with an ISN that falls within the
* receive window, the connection is killed.
*
* Notes:
* A heuristic to determine when to accept syn cookies is not necessary.
* An ACK flood would cause the syncookie verification to be attempted,
* but a SYN flood causes syncookies to be generated. Both are of equal
* cost, so there's no point in trying to optimize the ACK flood case.
* Also, if you don't process certain ACKs for some reason, then all someone
* would have to do is launch a SYN and ACK flood at the same time, which
* would stop cookie verification and defeat the entire purpose of syncookies.
*/
static int tcp_sc_msstab[] = { 0, 256, 468, 536, 996, 1452, 1460, 8960 };
static void
syncookie_generate(struct syncache_head *sch, struct syncache *sc,
u_int32_t *flowlabel)
{
MD5_CTX ctx;
u_int32_t md5_buffer[MD5_DIGEST_LENGTH / sizeof(u_int32_t)];
u_int32_t data;
u_int32_t *secbits;
u_int off, pmss, mss;
int i;
SCH_LOCK_ASSERT(sch);
/* Which of the two secrets to use. */
secbits = sch->sch_oddeven ?
sch->sch_secbits_odd : sch->sch_secbits_even;
/* Reseed secret if too old. */
if (sch->sch_reseed < time_uptime) {
sch->sch_oddeven = sch->sch_oddeven ? 0 : 1; /* toggle */
secbits = sch->sch_oddeven ?
sch->sch_secbits_odd : sch->sch_secbits_even;
for (i = 0; i < SYNCOOKIE_SECRET_SIZE; i++)
secbits[i] = arc4random();
sch->sch_reseed = time_uptime + SYNCOOKIE_LIFETIME;
}
/* Secret rotation offset. */
off = sc->sc_iss & 0x7; /* iss was randomized before */
/* Maximum segment size calculation. */
pmss = max( min(sc->sc_peer_mss, tcp_mssopt(&sc->sc_inc)), tcp_minmss);
for (mss = sizeof(tcp_sc_msstab) / sizeof(int) - 1; mss > 0; mss--)
if (tcp_sc_msstab[mss] <= pmss)
break;
/* Fold parameters and MD5 digest into the ISN we will send. */
data = sch->sch_oddeven;/* odd or even secret, 1 bit */
data |= off << 1; /* secret offset, derived from iss, 3 bits */
data |= mss << 4; /* mss, 3 bits */
MD5Init(&ctx);
MD5Update(&ctx, ((u_int8_t *)secbits) + off,
SYNCOOKIE_SECRET_SIZE * sizeof(*secbits) - off);
MD5Update(&ctx, secbits, off);
MD5Update(&ctx, &sc->sc_inc, sizeof(sc->sc_inc));
MD5Update(&ctx, &sc->sc_irs, sizeof(sc->sc_irs));
MD5Update(&ctx, &data, sizeof(data));
MD5Final((u_int8_t *)&md5_buffer, &ctx);
data |= (md5_buffer[0] << 7);
sc->sc_iss = data;
#ifdef INET6
*flowlabel = md5_buffer[1] & IPV6_FLOWLABEL_MASK;
#endif
/* Additional parameters are stored in the timestamp if present. */
if (sc->sc_flags & SCF_TIMESTAMP) {
data = ((sc->sc_flags & SCF_SIGNATURE) ? 1 : 0); /* TCP-MD5, 1 bit */
data |= ((sc->sc_flags & SCF_SACK) ? 1 : 0) << 1; /* SACK, 1 bit */
data |= sc->sc_requested_s_scale << 2; /* SWIN scale, 4 bits */
data |= sc->sc_requested_r_scale << 6; /* RWIN scale, 4 bits */
data |= md5_buffer[2] << 10; /* more digest bits */
data ^= md5_buffer[3];
sc->sc_ts = data;
sc->sc_tsoff = data - ticks; /* after XOR */
}
return;
}
static struct syncache *
syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch,
struct syncache *sc, struct tcpopt *to, struct tcphdr *th,
struct socket *so)
{
MD5_CTX ctx;
u_int32_t md5_buffer[MD5_DIGEST_LENGTH / sizeof(u_int32_t)];
u_int32_t data = 0;
u_int32_t *secbits;
tcp_seq ack, seq;
int off, mss, wnd, flags;
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;
off = (ack >> 1) & 0x7;
mss = (ack >> 4) & 0x7;
flags = ack & 0x7f;
/* Which of the two secrets to use. */
secbits = (flags & 0x1) ? sch->sch_secbits_odd : sch->sch_secbits_even;
/*
* The secret wasn't updated for the lifetime of a syncookie,
* so this SYN-ACK/ACK is either too old (replay) or totally bogus.
*/
if (sch->sch_reseed < time_uptime) {
return (NULL);
}
/* Recompute the digest so we can compare it. */
MD5Init(&ctx);
MD5Update(&ctx, ((u_int8_t *)secbits) + off,
SYNCOOKIE_SECRET_SIZE * sizeof(*secbits) - off);
MD5Update(&ctx, secbits, off);
MD5Update(&ctx, inc, sizeof(*inc));
MD5Update(&ctx, &seq, sizeof(seq));
MD5Update(&ctx, &flags, sizeof(flags));
MD5Final((u_int8_t *)&md5_buffer, &ctx);
/* Does the digest part of or ACK'ed ISS match? */
if ((ack & (~0x7f)) != (md5_buffer[0] << 7))
return (NULL);
/* Does the digest part of our reflected timestamp match? */
if (to->to_flags & TOF_TS) {
data = md5_buffer[3] ^ to->to_tsecr;
if ((data & (~0x3ff)) != (md5_buffer[2] << 10))
return (NULL);
}
/* Fill in the syncache values. */
bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
sc->sc_ipopts = NULL;
sc->sc_irs = seq;
sc->sc_iss = ack;
#ifdef INET6
if (inc->inc_isipv6) {
if (sotoinpcb(so)->in6p_flags & IN6P_AUTOFLOWLABEL)
sc->sc_flowlabel = md5_buffer[1] & IPV6_FLOWLABEL_MASK;
} else
#endif
{
sc->sc_ip_ttl = sotoinpcb(so)->inp_ip_ttl;
sc->sc_ip_tos = sotoinpcb(so)->inp_ip_tos;
}
/* Additional parameters that were encoded in the timestamp. */
if (data) {
sc->sc_flags |= SCF_TIMESTAMP;
sc->sc_tsreflect = to->to_tsval;
sc->sc_ts = to->to_tsecr;
sc->sc_tsoff = to->to_tsecr - ticks;
sc->sc_flags |= (data & 0x1) ? SCF_SIGNATURE : 0;
sc->sc_flags |= ((data >> 1) & 0x1) ? SCF_SACK : 0;
sc->sc_requested_s_scale = min((data >> 2) & 0xf,
TCP_MAX_WINSHIFT);
sc->sc_requested_r_scale = min((data >> 6) & 0xf,
TCP_MAX_WINSHIFT);
if (sc->sc_requested_s_scale || sc->sc_requested_r_scale)
sc->sc_flags |= SCF_WINSCALE;
} else
sc->sc_flags |= SCF_NOOPT;
wnd = sbspace(&so->so_rcv);
wnd = imax(wnd, 0);
wnd = imin(wnd, TCP_MAXWIN);
sc->sc_wnd = wnd;
sc->sc_rxmits = 0;
sc->sc_peer_mss = tcp_sc_msstab[mss];
return (sc);
}
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