Re: [PATCH] tcp: Reject invalid ack_seq to Fast Open sockets

On Mon, Oct 22, 2012 at 5:26 PM, H.K. Jerry Chu [off-list ref] wrote:

From: Jerry Chu <redacted>

A packet with an invalid ack_seq may cause a TCP Fast Open socket to switch
to the unexpected TCP_CLOSING state, triggering a BUG_ON kernel panic.

When a FIN packet with an invalid ack_seq# arrives at a socket in
the TCP_FIN_WAIT1 state, rather than discarding the packet, the current
code will accept the FIN, causing state transition to TCP_CLOSING.

This may be a small deviation from RFC793, which seems to say that the
packet should be dropped. Unfortunately I did not expect this case for
Fast Open hence it will trigger a BUG_ON panic.

It turns out there is really nothing bad about a TFO socket going into
TCP_CLOSING state so I could just remove the BUG_ON statements. But after
some thought I think it's better to treat this case like TCP_SYN_RECV
and return a RST to the confused peer who caused the unacceptable ack_seq
to be generated in the first place.

Signed-off-by: H.K. Jerry Chu <redacted>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <redacted>

Acked-by: Neal Cardwell <ncardwell@google.com>