6.1-stable review patch. If anyone has any objections, please let me know.
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From: Matthieu Baerts (NGI0) matttbe@kernel.org
commit c1668292689ad2ee16c9c1750a8044b0b0aad663 upstream.
The 'Fixes' commit recently changed the behaviour of TCP by skipping the processing of the 3rd ACK when a sk->sk_socket is set. The goal was to skip tcp_ack_snd_check() in tcp_rcv_state_process() not to send an unnecessary ACK in case of simultaneous connect(). Unfortunately, that had an impact on TFO and MPTCP.
I started to look at the impact on MPTCP, because the MPTCP CI found some issues with the MPTCP Packetdrill tests [1]. Then Paolo Abeni suggested me to look at the impact on TFO with "plain" TCP.
For MPTCP, when receiving the 3rd ACK of a request adding a new path (MP_JOIN), sk->sk_socket will be set, and point to the MPTCP sock that has been created when the MPTCP connection got established before with the first path. The newly added 'goto' will then skip the processing of the segment text (step 7) and not go through tcp_data_queue() where the MPTCP options are validated, and some actions are triggered, e.g. sending the MPJ 4th ACK [2] as demonstrated by the new errors when running a packetdrill test [3] establishing a second subflow.
This doesn't fully break MPTCP, mainly the 4th MPJ ACK that will be delayed. Still, we don't want to have this behaviour as it delays the switch to the fully established mode, and invalid MPTCP options in this 3rd ACK will not be caught any more. This modification also affects the MPTCP + TFO feature as well, and being the reason why the selftests started to be unstable the last few days [4].
For TFO, the existing 'basic-cookie-not-reqd' test [5] was no longer passing: if the 3rd ACK contains data, and the connection is accept()ed before receiving them, these data would no longer be processed, and thus not ACKed.
One last thing about MPTCP, in case of simultaneous connect(), a fallback to TCP will be done, which seems fine:
`../common/defaults.sh`
0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_MPTCP) = 3 +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+0 > S 0:0(0) <mss 1460, sackOK, TS val 100 ecr 0, nop, wscale 8, mpcapable v1 flags[flag_h] nokey> +0 < S 0:0(0) win 1000 <mss 1460, sackOK, TS val 407 ecr 0, nop, wscale 8, mpcapable v1 flags[flag_h] nokey> +0 > S. 0:0(0) ack 1 <mss 1460, sackOK, TS val 330 ecr 0, nop, wscale 8, mpcapable v1 flags[flag_h] nokey> +0 < S. 0:0(0) ack 1 win 65535 <mss 1460, sackOK, TS val 700 ecr 100, nop, wscale 8, mpcapable v1 flags[flag_h] key[skey=2]> +0 > . 1:1(0) ack 1 <nop, nop, TS val 845707014 ecr 700, nop, nop, sack 0:1>
Simultaneous SYN-data crossing is also not supported by TFO, see [6].
Kuniyuki Iwashima suggested to restrict the processing to SYN+ACK only: that's a more generic solution than the one initially proposed, and also enough to fix the issues described above.
Later on, Eric Dumazet mentioned that an ACK should still be sent in reaction to the second SYN+ACK that is received: not sending a DUPACK here seems wrong and could hurt:
0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3 +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+0 > S 0:0(0) <mss 1460, sackOK, TS val 1000 ecr 0,nop,wscale 8> +0 < S 0:0(0) win 1000 <mss 1000, sackOK, nop, nop> +0 > S. 0:0(0) ack 1 <mss 1460, sackOK, TS val 3308134035 ecr 0,nop,wscale 8> +0 < S. 0:0(0) ack 1 win 1000 <mss 1000, sackOK, nop, nop> +0 > . 1:1(0) ack 1 <nop, nop, sack 0:1> // <== Here
So in this version, the 'goto consume' is dropped, to always send an ACK when switching from TCP_SYN_RECV to TCP_ESTABLISHED. This ACK will be seen as a DUPACK -- with DSACK if SACK has been negotiated -- in case of simultaneous SYN crossing: that's what is expected here.
Link: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/9936227696 [1] Link: https://datatracker.ietf.org/doc/html/rfc8684#fig_tokens [2] Link: https://github.com/multipath-tcp/packetdrill/blob/mptcp-net-next/gtests/net/... [3] Link: https://netdev.bots.linux.dev/contest.html?executor=vmksft-mptcp-dbg&tes... [4] Link: https://github.com/google/packetdrill/blob/master/gtests/net/tcp/fastopen/se... [5] Link: https://github.com/google/packetdrill/blob/master/gtests/net/tcp/fastopen/cl... [6] Fixes: 23e89e8ee7be ("tcp: Don't drop SYN+ACK for simultaneous connect().") Suggested-by: Paolo Abeni pabeni@redhat.com Suggested-by: Kuniyuki Iwashima kuniyu@amazon.com Suggested-by: Eric Dumazet edumazet@google.com Signed-off-by: Matthieu Baerts (NGI0) matttbe@kernel.org Reviewed-by: Eric Dumazet edumazet@google.com Link: https://patch.msgid.link/20240724-upstream-net-next-20240716-tcp-3rd-ack-con... Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/tcp_input.c | 3 --- 1 file changed, 3 deletions(-)
--- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -6641,9 +6641,6 @@ int tcp_rcv_state_process(struct sock *s tcp_fast_path_on(tp); if (sk->sk_shutdown & SEND_SHUTDOWN) tcp_shutdown(sk, SEND_SHUTDOWN); - - if (sk->sk_socket) - goto consume; break;
case TCP_FIN_WAIT1: {