[PATCH net-next] tcp: refine tcp_sequence() for the FIN exception

[Eric Dumazet <edumazet@google.com>]

Commit 0e24d17bd966 ("tcp: implement RFC 7323 window retraction
receiver requirements") removed the special FIN case that
was added in commit 1e3bb184e941 ("tcp: re-enable acceptance of
FIN packets when RWIN is 0").

If a peer sends a segment containing data and a FIN flag before
it learns about our window retraction and has a buggy TCP stack,
it might place the FIN one byte beyond what it thinks is the
right edge of the window (i.e., max_window_edge + 1).

The data portion (end_seq - th->fin) will end exactly at max_window_edge.
In this case, we will drop the packet if our receive queue is not empty,
even though the data was sent within the window we previously allowed.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Simon Baatz <gmbnomis@gmail.com>
---
 net/ipv4/tcp_input.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index ab7a4e5435a8a2cbb532d42c54af76d8541c903b..8560a9c6d38207c098d673497caf2c7652c36f5c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4812,18 +4812,20 @@ static enum skb_drop_reason tcp_sequence(const struct sock *sk,
 					 const struct tcphdr *th)
 {
 	const struct tcp_sock *tp = tcp_sk(sk);
+	u32 seq_limit;
 
 	if (before(end_seq, tp->rcv_wup))
 		return SKB_DROP_REASON_TCP_OLD_SEQUENCE;
 
-	if (unlikely(after(end_seq, tp->rcv_nxt + tcp_max_receive_window(tp)))) {
+	seq_limit = tp->rcv_nxt + tcp_max_receive_window(tp);
+	if (unlikely(after(end_seq, seq_limit))) {
 		/* Some stacks are known to handle FIN incorrectly; allow the
 		 * FIN to extend beyond the window and check it in detail later.
 		 */
-		if (!after(end_seq - th->fin, tp->rcv_nxt + tcp_receive_window(tp)))
+		if (!after(end_seq - th->fin, seq_limit))
 			return SKB_NOT_DROPPED_YET;
 
-		if (after(seq, tp->rcv_nxt + tcp_max_receive_window(tp)))
+		if (after(seq, seq_limit))
 			return SKB_DROP_REASON_TCP_INVALID_SEQUENCE;
 
 		/* Only accept this packet if receive queue is empty. */
-- 
2.54.0.1032.g2f8565e1d1-goog