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

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>
---
 net/ipv4/tcp_input.c |   12 ++++++++++--
 net/ipv4/tcp_timer.c |    2 +-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 60cf836..ce3e3c7 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c

@@ -5970,7 +5970,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
 
 	req = tp->fastopen_rsk;
 	if (req != NULL) {
-		BUG_ON(sk->sk_state != TCP_SYN_RECV &&
+		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
 		    sk->sk_state != TCP_FIN_WAIT1);
 
 		if (tcp_check_req(sk, skb, req, NULL, true) == NULL)

@@ -6059,7 +6059,15 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
 			 * ACK we have received, this would have acknowledged
 			 * our SYNACK so stop the SYNACK timer.
 			 */
-			if (acceptable && req != NULL) {
+			if (req != NULL) {
+				/* Return RST if ack_seq is invalid.
+				 * Note that RFC793 only says to generate a
+				 * DUPACK for it but for TCP Fast Open it seems
+				 * better to treat this case like TCP_SYN_RECV
+				 * above.
+				 */
+				if (!acceptable)
+					return 1;
 				/* We no longer need the request sock. */
 				reqsk_fastopen_remove(sk, req, false);
 				tcp_rearm_rto(sk);

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index fc04711..a24f4a4 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c

@@ -347,7 +347,7 @@ void tcp_retransmit_timer(struct sock *sk)
 		return;
 	}
 	if (tp->fastopen_rsk) {
-		BUG_ON(sk->sk_state != TCP_SYN_RECV &&
+		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
 		    sk->sk_state != TCP_FIN_WAIT1);
 		tcp_fastopen_synack_timer(sk);
 		/* Before we receive ACK to our SYN-ACK don't retransmit

-- 
1.7.7.3

On Mon, Oct 22, 2012 at 2:26 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: Yuchung Cheng <redacted>

On Mon, 2012-10-22 at 14:26 -0700, H.K. Jerry Chu 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: Eric Dumazet <redacted>

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>

From: "H.K. Jerry Chu" <redacted>
Date: Mon, 22 Oct 2012 14:26:36 -0700

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>

Applied, thanks.

I fixed up the indentation of the second line of the test when I
applied this.