[PATCH 3.16 033/233] tcp: eliminate negative reordering in tcp_clean_rtx_queue

[Ben Hutchings]

3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>

commit bafbb9c73241760023d8981191ddd30bb1c6dbac upstream.

tcp_ack() can call tcp_fragment() which may dededuct the
value tp->fackets_out when MSS changes. When prior_fackets
is larger than tp->fackets_out, tcp_clean_rtx_queue() can
invoke tcp_update_reordering() with negative values. This
results in absurd tp->reodering values higher than
sysctl_tcp_max_reordering.

Note that tcp_update_reordering indeeds sets tp->reordering
to min(sysctl_tcp_max_reordering, metric), but because
the comparison is signed, a negative metric always wins.

Fixes: c7caf8d3ed7a ("[TCP]: Fix reord detection due to snd_una covered holes")
Reported-by: Rebecca Isaacs <risaacs@xxxxxxxxxx>
Signed-off-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/ipv4/tcp_input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3135,7 +3135,7 @@ static int tcp_clean_rtx_queue(struct so
 			int delta;
 
 			/* Non-retransmitted hole got filled? That's reordering */
-			if (reord < prior_fackets)
+			if (reord < prior_fackets && reord <= tp->fackets_out)
 				tcp_update_reordering(sk, tp->fackets_out - reord, 0);
 
 			delta = tcp_is_fack(tp) ? pkts_acked :