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tcp: improve SEG.ACK validation

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Implement the improved SEG.ACK validation described in RFC 5961.
In addition to that, also detect ghost ACKs, which are ACKs for data
that has never been sent.
The additional checks are enabled by default, but can be disabled
by setting the sysctl-variable net.inet.tcp.insecure_ack to a
non-zero value.

PR:			250357
Reviewed by:		Peter Lei, rscheff (older version)
MFC after:		1 week
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D45894
This commit is contained in:
Michael Tuexen 2024-07-21 11:37:35 +02:00
parent 1cbd613f33
commit 646c28ea80
8 changed files with 143 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
share/man/man4/tcp.4
View File

@ -31,7 +31,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd November 30, 2023
.Dd July 21, 2024
.Dt TCP 4
.Os
.Sh NAME
@ -699,6 +699,9 @@ Default is false.
.It Va insecure_syn
Use criteria defined in RFC793 instead of RFC5961 for accepting SYN segments.
Default is false.
.It Va insecure_ack
Use criteria defined in RFC793 for validating SEG.ACK.
Default is false.
.It Va isn_reseed_interval
The interval (in seconds) specifying how often the secret data used in
RFC 1948 initial sequence number calculations should be reseeded.

2
sys/netinet/in_kdtrace.c
View File

@ -339,6 +339,8 @@ MIB_PROBE_TCP(tcps_ecn_sndect1);
MIB_PROBE_TCP(tcps_tlpresends);
MIB_PROBE_TCP(tcps_tlpresend_bytes);
MIB_PROBE_TCP(tcps_rcvghostack);
MIB_PROBE_TCP(tcps_rcvacktooold);
#endif
SDT_PROBE_DEFINE6_XLATE(ip, , , receive,

3
sys/netinet/in_kdtrace.h
View File

@ -330,6 +330,9 @@ SDT_PROBE_DECLARE(mib, tcp, count, tcps_ecn_sndect1);
SDT_PROBE_DECLARE(mib, tcp, count, tcps_tlpresends);
SDT_PROBE_DECLARE(mib, tcp, count, tcps_tlpresend_bytes);
SDT_PROBE_DECLARE(mib, tcp, count, tcps_rcvghostack);
SDT_PROBE_DECLARE(mib, tcp, count, tcps_rcvacktooold);
#endif
SDT_PROBE_DECLARE(ip, , , receive);

44
sys/netinet/tcp_input.c
View File

@ -202,6 +202,11 @@ SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_rst, CTLFLAG_VNET | CTLFLAG_RW,
&VNET_NAME(tcp_insecure_rst), 0,
"Follow RFC793 instead of RFC5961 criteria for accepting RST packets");
VNET_DEFINE(int, tcp_insecure_ack) = 0;
SYSCTL_INT(_net_inet_tcp, OID_AUTO, insecure_ack, CTLFLAG_VNET | CTLFLAG_RW,
&VNET_NAME(tcp_insecure_ack), 0,
"Follow RFC793 criteria for validating SEG.ACK");
VNET_DEFINE(int, tcp_recvspace) = 1024*64;
#define V_tcp_recvspace VNET(tcp_recvspace)
SYSCTL_INT(_net_inet_tcp, TCPCTL_RECVSPACE, recvspace, CTLFLAG_VNET | CTLFLAG_RW,
@ -2438,6 +2443,45 @@ tcp_do_segment(struct tcpcb *tp, struct mbuf *m, struct tcphdr *th,
/*
* Ack processing.
*/
if (SEQ_GEQ(tp->snd_una, tp->iss + (65535 << tp->snd_scale))) {
/* Checking SEG.ACK against ISS is definitely redundant. */
tp->t_flags2 |= TF2_NO_ISS_CHECK;
}
if (!V_tcp_insecure_ack) {
tcp_seq seq_min;
bool ghost_ack_check;
if (tp->t_flags2 & TF2_NO_ISS_CHECK) {
/* Check for too old ACKs (RFC 5961, Section 5.2). */
seq_min = tp->snd_una - tp->max_sndwnd;
ghost_ack_check = false;
} else {
if (SEQ_GT(tp->iss + 1, tp->snd_una - tp->max_sndwnd)) {
/* Checking for ghost ACKs is stricter. */
seq_min = tp->iss + 1;
ghost_ack_check = true;
} else {
/*
* Checking for too old ACKs (RFC 5961,
* Section 5.2) is stricter.
*/
seq_min = tp->snd_una - tp->max_sndwnd;
ghost_ack_check = false;
}
}
if (SEQ_LT(th->th_ack, seq_min)) {
if (ghost_ack_check)
TCPSTAT_INC(tcps_rcvghostack);
else
TCPSTAT_INC(tcps_rcvacktooold);
/* Send a challenge ACK. */
tcp_respond(tp, mtod(m, void *), th, m,
tp->rcv_nxt, tp->snd_nxt, TH_ACK);
tp->last_ack_sent = tp->rcv_nxt;
m = NULL;
goto drop;
}
}
switch (tp->t_state) {
/*
* In SYN_RECEIVED state, the ack ACKs our SYN, so enter

37
sys/netinet/tcp_stacks/bbr.c
View File

@ -7711,6 +7711,43 @@ bbr_process_ack(struct mbuf *m, struct tcphdr *th, struct socket *so,
bbr = (struct tcp_bbr *)tp->t_fb_ptr;
lost = bbr->r_ctl.rc_lost;
nsegs = max(1, m->m_pkthdr.lro_nsegs);
if (SEQ_GEQ(tp->snd_una, tp->iss + (65535 << tp->snd_scale))) {
/* Checking SEG.ACK against ISS is definitely redundant. */
tp->t_flags2 |= TF2_NO_ISS_CHECK;
}
if (!V_tcp_insecure_ack) {
tcp_seq seq_min;
bool ghost_ack_check;
if (tp->t_flags2 & TF2_NO_ISS_CHECK) {
/* Check for too old ACKs (RFC 5961, Section 5.2). */
seq_min = tp->snd_una - tp->max_sndwnd;
ghost_ack_check = false;
} else {
if (SEQ_GT(tp->iss + 1, tp->snd_una - tp->max_sndwnd)) {
/* Checking for ghost ACKs is stricter. */
seq_min = tp->iss + 1;
ghost_ack_check = true;
} else {
/*
* Checking for too old ACKs (RFC 5961,
* Section 5.2) is stricter.
*/
seq_min = tp->snd_una - tp->max_sndwnd;
ghost_ack_check = false;
}
}
if (SEQ_LT(th->th_ack, seq_min)) {
if (ghost_ack_check)
TCPSTAT_INC(tcps_rcvghostack);
else
TCPSTAT_INC(tcps_rcvacktooold);
/* Send challenge ACK. */
ctf_do_dropafterack(m, tp, th, thflags, tlen, ret_val);
bbr->r_wanted_output = 1;
return (1);
}
}
if (SEQ_GT(th->th_ack, tp->snd_max)) {
ctf_do_dropafterack(m, tp, th, thflags, tlen, ret_val);
bbr->r_wanted_output = 1;

39
sys/netinet/tcp_stacks/rack.c
View File

@ -12472,6 +12472,45 @@ rack_process_ack(struct mbuf *m, struct tcphdr *th, struct socket *so,
INP_WLOCK_ASSERT(tptoinpcb(tp));
rack = (struct tcp_rack *)tp->t_fb_ptr;
if (SEQ_GEQ(tp->snd_una, tp->iss + (65535 << tp->snd_scale))) {
/* Checking SEG.ACK against ISS is definitely redundant. */
tp->t_flags2 |= TF2_NO_ISS_CHECK;
}
if (!V_tcp_insecure_ack) {
tcp_seq seq_min;
bool ghost_ack_check;
if (tp->t_flags2 & TF2_NO_ISS_CHECK) {
/* Check for too old ACKs (RFC 5961, Section 5.2). */
seq_min = tp->snd_una - tp->max_sndwnd;
ghost_ack_check = false;
} else {
if (SEQ_GT(tp->iss + 1, tp->snd_una - tp->max_sndwnd)) {
/* Checking for ghost ACKs is stricter. */
seq_min = tp->iss + 1;
ghost_ack_check = true;
} else {
/*
* Checking for too old ACKs (RFC 5961,
* Section 5.2) is stricter.
*/
seq_min = tp->snd_una - tp->max_sndwnd;
ghost_ack_check = false;
}
}
if (SEQ_LT(th->th_ack, seq_min)) {
if (ghost_ack_check)
TCPSTAT_INC(tcps_rcvghostack);
else
TCPSTAT_INC(tcps_rcvacktooold);
/* Send challenge ACK. */
__ctf_do_dropafterack(m, tp, th, thflags, tlen, ret_val,
&rack->r_ctl.challenge_ack_ts,
&rack->r_ctl.challenge_ack_cnt);
rack->r_wanted_output = 1;
return (1);
}
}
if (SEQ_GT(th->th_ack, tp->snd_max)) {
__ctf_do_dropafterack(m, tp, th, thflags, tlen, ret_val,
&rack->r_ctl.challenge_ack_ts,

9
sys/netinet/tcp_var.h
View File

@ -848,6 +848,7 @@ tcp_packets_this_ack(struct tcpcb *tp, tcp_seq ack)
#define TF2_CANNOT_DO_ECN 0x00080000 /* The stack does not do ECN */
#define TF2_PROC_SACK_PROHIBIT 0x00100000 /* Due to small MSS size do not process sack's */
#define TF2_IPSEC_TSO 0x00200000 /* IPSEC + TSO supported */
#define TF2_NO_ISS_CHECK 0x00200000 /* Don't check SEG.ACK against ISS */
/*
* Structure to hold TCP options that are only used during segment
@ -1089,8 +1090,12 @@ struct tcpstat {
uint64_t tcps_tlpresends; /* number of tlp resends */
uint64_t tcps_tlpresend_bytes; /* number of bytes resent by tlp */
/* SEG.ACK validation failures */
uint64_t tcps_rcvghostack; /* received ACK for data never sent */
uint64_t tcps_rcvacktooold; /* received ACK for data too long ago */
uint64_t _pad[3]; /* 3 TBD placeholder for STABLE */
uint64_t _pad[1]; /* 1 TBD placeholder for STABLE */
};
#define tcps_rcvmemdrop tcps_rcvreassfull /* compat */
@ -1280,6 +1285,7 @@ VNET_DECLARE(int, tcp_ecn_maxretries);
VNET_DECLARE(int, tcp_initcwnd_segments);
VNET_DECLARE(int, tcp_insecure_rst);
VNET_DECLARE(int, tcp_insecure_syn);
VNET_DECLARE(int, tcp_insecure_ack);
VNET_DECLARE(uint32_t, tcp_map_entries_limit);
VNET_DECLARE(uint32_t, tcp_map_split_limit);
VNET_DECLARE(int, tcp_minmss);
@ -1327,6 +1333,7 @@ VNET_DECLARE(struct inpcbinfo, tcbinfo);
#define V_tcp_initcwnd_segments VNET(tcp_initcwnd_segments)
#define V_tcp_insecure_rst VNET(tcp_insecure_rst)
#define V_tcp_insecure_syn VNET(tcp_insecure_syn)
#define V_tcp_insecure_ack VNET(tcp_insecure_ack)
#define V_tcp_map_entries_limit VNET(tcp_map_entries_limit)
#define V_tcp_map_split_limit VNET(tcp_map_split_limit)
#define V_tcp_minmss VNET(tcp_minmss)

8
usr.bin/netstat/inet.c
View File

@ -642,8 +642,12 @@ tcp_stats(u_long off, const char *name, int af1 __unused, int proto __unused)
"{N:/UDP tunneled pkt%s}\n");
p(tcps_tunneled_errs, "\t\t{:received-bad-udp-tunneled-pkts/%ju} "
"{N:/UDP tunneled pkt cnt with error%s}\n");
p(tcps_rcvacktoomuch, "\t\t{:received-acks-for-unsent-data/%ju} "
"{N:/ack%s for unsent data}\n");
p(tcps_rcvacktoomuch, "\t\t{:received-acks-for-data-not-yet-sent/%ju} "
"{N:/ack%s for data not yet sent}\n");
p(tcps_rcvghostack, "\t\t{:received-acks-for-data-never-been-sent/%ju} "
"{N:/ack%s for data never been sent (ghost acks)}\n");
p(tcps_rcvacktooold, "\t\t{:received-acks-for-data-being-too-old/%ju} "
"{N:/ack%s for data being too old}\n");
p2(tcps_rcvpack, tcps_rcvbyte, "\t\t"
"{:received-in-sequence-packets/%ju} {N:/packet%s} "
"({:received-in-sequence-bytes/%ju} {N:/byte%s}) "