From 3ba075278c11cdb19e2dbb80362042f1b0c08f74 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 25 Feb 2025 17:10:48 +0000 Subject: [PATCH] tcp: be less liberal in TSEcr received while in SYN_RECV state Yong-Hao Zou mentioned that linux was not strict as other OS in 3WHS, for flows using TCP TS option (RFC 7323) As hinted by an old comment in tcp_check_req(), we can check the TSEcr value in the incoming packet corresponds to one of the SYNACK TSval values we have sent. In this patch, I record the oldest and most recent values that SYNACK packets have used. Send a challenge ACK if we receive a TSEcr outside of this range, and increase a new SNMP counter. nstat -az | grep TSEcrRejected TcpExtTSEcrRejected 0 0.0 Due to TCP fastopen implementation, do not apply yet these checks for fastopen flows. v2: No longer use req->num_timeout, but treq->snt_tsval_first to detect when first SYNACK is prepared. This means we make sure to not send an initial zero TSval. Make sure MPTCP and TCP selftests are passing. Change MIB name to TcpExtTSEcrRejected v1: https://lore.kernel.org/netdev/CADVnQykD8i4ArpSZaPKaoNxLJ2if2ts9m4As+=Jvdkrgx1qMHw@mail.gmail.com/T/ Reported-by: Yong-Hao Zou Signed-off-by: Eric Dumazet Reviewed-by: Matthieu Baerts (NGI0) Reviewed-by: Neal Cardwell Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20250225171048.3105061-1-edumazet@google.com Signed-off-by: Jakub Kicinski --- .../networking/net_cachelines/snmp.rst | 1 + include/linux/tcp.h | 2 ++ include/uapi/linux/snmp.h | 1 + net/ipv4/proc.c | 1 + net/ipv4/syncookies.c | 1 + net/ipv4/tcp_input.c | 1 + net/ipv4/tcp_minisocks.c | 26 +++++++++++-------- net/ipv4/tcp_output.c | 6 +++++ 8 files changed, 28 insertions(+), 11 deletions(-) diff --git a/Documentation/networking/net_cachelines/snmp.rst b/Documentation/networking/net_cachelines/snmp.rst index 90ca2d92547d4..bc96efc92cf5b 100644 --- a/Documentation/networking/net_cachelines/snmp.rst +++ b/Documentation/networking/net_cachelines/snmp.rst @@ -36,6 +36,7 @@ unsigned_long LINUX_MIB_TIMEWAITRECYCLED unsigned_long LINUX_MIB_TIMEWAITKILLED unsigned_long LINUX_MIB_PAWSACTIVEREJECTED unsigned_long LINUX_MIB_PAWSESTABREJECTED +unsigned_long LINUX_MIB_TSECR_REJECTED unsigned_long LINUX_MIB_DELAYEDACKLOST unsigned_long LINUX_MIB_LISTENOVERFLOWS unsigned_long LINUX_MIB_LISTENDROPS diff --git a/include/linux/tcp.h b/include/linux/tcp.h index f88daaa76d836..159b2c59eb627 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -160,6 +160,8 @@ struct tcp_request_sock { u32 rcv_isn; u32 snt_isn; u32 ts_off; + u32 snt_tsval_first; + u32 snt_tsval_last; u32 last_oow_ack_time; /* last SYNACK */ u32 rcv_nxt; /* the ack # by SYNACK. For * FastOpen it's the seq# diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h index 848c7784e684c..eb9fb776fdc3e 100644 --- a/include/uapi/linux/snmp.h +++ b/include/uapi/linux/snmp.h @@ -186,6 +186,7 @@ enum LINUX_MIB_TIMEWAITKILLED, /* TimeWaitKilled */ LINUX_MIB_PAWSACTIVEREJECTED, /* PAWSActiveRejected */ LINUX_MIB_PAWSESTABREJECTED, /* PAWSEstabRejected */ + LINUX_MIB_TSECRREJECTED, /* TSEcrRejected */ LINUX_MIB_PAWS_OLD_ACK, /* PAWSOldAck */ LINUX_MIB_DELAYEDACKS, /* DelayedACKs */ LINUX_MIB_DELAYEDACKLOCKED, /* DelayedACKLocked */ diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c index affd21a0f5728..10cbeb76c2745 100644 --- a/net/ipv4/proc.c +++ b/net/ipv4/proc.c @@ -189,6 +189,7 @@ static const struct snmp_mib snmp4_net_list[] = { SNMP_MIB_ITEM("TWKilled", LINUX_MIB_TIMEWAITKILLED), SNMP_MIB_ITEM("PAWSActive", LINUX_MIB_PAWSACTIVEREJECTED), SNMP_MIB_ITEM("PAWSEstab", LINUX_MIB_PAWSESTABREJECTED), + SNMP_MIB_ITEM("TSEcrRejected", LINUX_MIB_TSECRREJECTED), SNMP_MIB_ITEM("PAWSOldAck", LINUX_MIB_PAWS_OLD_ACK), SNMP_MIB_ITEM("DelayedACKs", LINUX_MIB_DELAYEDACKS), SNMP_MIB_ITEM("DelayedACKLocked", LINUX_MIB_DELAYEDACKLOCKED), diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c index 26816b876dd8b..5459a78b98095 100644 --- a/net/ipv4/syncookies.c +++ b/net/ipv4/syncookies.c @@ -279,6 +279,7 @@ static int cookie_tcp_reqsk_init(struct sock *sk, struct sk_buff *skb, ireq->smc_ok = 0; treq->snt_synack = 0; + treq->snt_tsval_first = 0; treq->tfo_listener = false; treq->txhash = net_tx_rndhash(); treq->rcv_isn = ntohl(th->seq) - 1; diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 217a8747a79b5..d22ad553b45b1 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -7081,6 +7081,7 @@ static void tcp_openreq_init(struct request_sock *req, tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq; tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1; tcp_rsk(req)->snt_synack = 0; + tcp_rsk(req)->snt_tsval_first = 0; tcp_rsk(req)->last_oow_ack_time = 0; req->mss = rx_opt->mss_clamp; req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0; diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c index 1eccc518b957e..4f87406ddbcd4 100644 --- a/net/ipv4/tcp_minisocks.c +++ b/net/ipv4/tcp_minisocks.c @@ -663,6 +663,7 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, struct sock *child; const struct tcphdr *th = tcp_hdr(skb); __be32 flg = tcp_flag_word(th) & (TCP_FLAG_RST|TCP_FLAG_SYN|TCP_FLAG_ACK); + bool tsecr_reject = false; bool paws_reject = false; bool own_req; @@ -672,8 +673,13 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, if (tmp_opt.saw_tstamp) { tmp_opt.ts_recent = READ_ONCE(req->ts_recent); - if (tmp_opt.rcv_tsecr) + if (tmp_opt.rcv_tsecr) { + if (inet_rsk(req)->tstamp_ok && !fastopen) + tsecr_reject = !between(tmp_opt.rcv_tsecr, + tcp_rsk(req)->snt_tsval_first, + READ_ONCE(tcp_rsk(req)->snt_tsval_last)); tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off; + } /* We do not store true stamp, but it is not required, * it can be estimated (approximately) * from another data. @@ -788,18 +794,14 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, tcp_rsk(req)->snt_isn + 1)) return sk; - /* Also, it would be not so bad idea to check rcv_tsecr, which - * is essentially ACK extension and too early or too late values - * should cause reset in unsynchronized states. - */ - /* RFC793: "first check sequence number". */ - if (paws_reject || !tcp_in_window(TCP_SKB_CB(skb)->seq, - TCP_SKB_CB(skb)->end_seq, - tcp_rsk(req)->rcv_nxt, - tcp_rsk(req)->rcv_nxt + - tcp_synack_window(req))) { + if (paws_reject || tsecr_reject || + !tcp_in_window(TCP_SKB_CB(skb)->seq, + TCP_SKB_CB(skb)->end_seq, + tcp_rsk(req)->rcv_nxt, + tcp_rsk(req)->rcv_nxt + + tcp_synack_window(req))) { /* Out of window: send ACK and drop. */ if (!(flg & TCP_FLAG_RST) && !tcp_oow_rate_limited(sock_net(sk), skb, @@ -808,6 +810,8 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb, req->rsk_ops->send_ack(sk, skb, req); if (paws_reject) NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED); + else if (tsecr_reject) + NET_INC_STATS(sock_net(sk), LINUX_MIB_TSECRREJECTED); return NULL; } diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 9a3cf51eab787..0a660075add5b 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -943,6 +943,12 @@ static unsigned int tcp_synack_options(const struct sock *sk, opts->options |= OPTION_TS; opts->tsval = tcp_skb_timestamp_ts(tcp_rsk(req)->req_usec_ts, skb) + tcp_rsk(req)->ts_off; + if (!tcp_rsk(req)->snt_tsval_first) { + if (!opts->tsval) + opts->tsval = ~0U; + tcp_rsk(req)->snt_tsval_first = opts->tsval; + } + WRITE_ONCE(tcp_rsk(req)->snt_tsval_last, opts->tsval); opts->tsecr = READ_ONCE(req->ts_recent); remaining -= TCPOLEN_TSTAMP_ALIGNED; }