[PATCH net] tcp: ignore Fast Open on repair mode

[Yuchung Cheng <ycheng@google.com>]

The TCP repair sequence of operation is to first set the socket in
repair mode, then inject the TCP stats into the socket with repair
socket options, then call connect() to re-activate the socket. The
connect syscall simply returns and set state to ESTABLISHED
mode. As a result Fast Open is meaningless for TCP repair.

However allowing sendto() system call with MSG_FASTOPEN flag half-way
during the repair operation could unexpectedly cause data to be
sent, before the operation finishes changing the internal TCP stats
(e.g. MSS).  This in turn triggers TCP warnings on inconsistent
packet accounting.

The fix is to simply disallow Fast Open operation once the socket
is in the repair mode.

Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
---
 net/ipv4/tcp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 9ce1c726185e..4b18ad41d4df 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1204,7 +1204,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			uarg->zerocopy = 0;
 	}
 
-	if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect)) {
+	if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect) &&
+	    !tp->repair) {
 		err = tcp_sendmsg_fastopen(sk, msg, &copied_syn, size);
 		if (err == -EINPROGRESS && copied_syn > 0)
 			goto out;
-- 
2.17.0.441.gb46fe60e1d-goog