Re: [PATCH net] tcp: fastopen: fix on syn-data transmit failure

From: Eric Dumazet <redacted>
Date: Tue, 19 Sep 2017 10:05:57 -0700

From: Eric Dumazet <edumazet@google.com>

Our recent change exposed a bug in TCP Fastopen Client that syzkaller
found right away [1]

When we prepare skb with SYN+DATA, we attempt to transmit it,
and we update socket state as if the transmit was a success.

In socket RTX queue we have two skbs, one with the SYN alone,
and a second one containing the DATA.

When (malicious) ACK comes in, we now complain that second one had no
skb_mstamp.

The proper fix is to make sure that if the transmit failed, we do not
pretend we sent the DATA skb, and make it our send_head.

When 3WHS completes, we can now send the DATA right away, without having
to wait for a timeout.

[1]

 ...

Fixes: 8c72c65b426b ("tcp: update skb->skb_mstamp more carefully")
Fixes: 783237e8daf1 ("net-tcp: Fast Open client - sending SYN-data")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>

Applied, thanks Eric.