[PATCH v2 1/2] tcp: exclude old ACKs from tcp fast path

[Inbal Schussheim]

Exclude old ACKs before SND.UNA from the tcp fast path
as well as ACKs after SND.NXT.

Such ACKs will fall through to the slow path, where tcp_ack()
performs the appropriate validation and challenge ACK handling
according to RFC5961 and Commit 3d501dd326fb1c7 ("tcp: do not
accept ACK of bytes we never sent").

This prevents old ACKs from being accepted
or modifying connection state as part of the fast path before
appropriate ACK validation is applied.

Reported-by: Amit Klein <amit.klein@xxxxxxxxxxxxxxx>
Reported-by: Tamir Shahar <tamir.shahar1@xxxxxxxxxxxxxxx>
Reported-by: Inbal Schussheim <inbal.lipshtat@xxxxxxxxxxxxxxx>
Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Inbal Schussheim <inbal.lipshtat@xxxxxxxxxxxxxxx>
---
 net/ipv4/tcp_input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index daff93d51342..57fec58a4f15 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6539,7 +6539,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
 
 	if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
 	    TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
-	    !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
+	    between(TCP_SKB_CB(skb)->ack_seq, tp->snd_una, tp->snd_nxt)) {
 		int tcp_header_len = tp->tcp_header_len;
 		s32 delta = 0;
 		int flag = 0;
-- 
2.43.0