tcp: Reject invalid ack_seq to Fast Open sockets

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 <hkchu@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 12 insertions, 4 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 036f8573814..1db66398358 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5964,7 +5964,7 @@ 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)
@@ -6053,7 +6053,15 @@ 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 fc04711e80c..d47c1b4421a 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -347,8 +347,8 @@ void tcp_retransmit_timer(struct sock *sk)
                 return;
         }
         if (tp->fastopen_rsk) {
-                BUG_ON(sk->sk_state != TCP_SYN_RECV &&
-                    sk->sk_state != TCP_FIN_WAIT1);
+                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
                  * anything else (e.g., data or FIN segments).