linux-kernel - [PATCH 3.11 036/137] tcp: fix tcp_match_skb_to_sack() for unaligned SACK at end of an skb

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Message-Id: <1408354398-10226-37-git-send-email-luis.henriques@canonical.com>
Date:	Mon, 18 Aug 2014 10:31:37 +0100
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>,
	"David S. Miller" <davem@...emloft.net>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.11 036/137] tcp: fix tcp_match_skb_to_sack() for unaligned SACK at end of an skb

3.11.10.15 -stable review patch.  If anyone has any objections, please let me know.

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

From: Neal Cardwell <ncardwell@...gle.com>

commit 2cd0d743b05e87445c54ca124a9916f22f16742e upstream.

If there is an MSS change (or misbehaving receiver) that causes a SACK
to arrive that covers the end of an skb but is less than one MSS, then
tcp_match_skb_to_sack() was rounding up pkt_len to the full length of
the skb ("Round if necessary..."), then chopping all bytes off the skb
and creating a zero-byte skb in the write queue.

This was visible now because the recently simplified TLP logic in
bef1909ee3ed1c ("tcp: fixing TLP's FIN recovery") could find that 0-byte
skb at the end of the write queue, and now that we do not check that
skb's length we could send it as a TLP probe.

Consider the following example scenario:

 mss: 1000
 skb: seq: 0 end_seq: 4000  len: 4000
 SACK: start_seq: 3999 end_seq: 4000

The tcp_match_skb_to_sack() code will compute:

 in_sack = false
 pkt_len = start_seq - TCP_SKB_CB(skb)->seq = 3999 - 0 = 3999
 new_len = (pkt_len / mss) * mss = (3999/1000)*1000 = 3000
 new_len += mss = 4000

Previously we would find the new_len > skb->len check failing, so we
would fall through and set pkt_len = new_len = 4000 and chop off
pkt_len of 4000 from the 4000-byte skb, leaving a 0-byte segment
afterward in the write queue.

With this new commit, we notice that the new new_len >= skb->len check
succeeds, so that we return without trying to fragment.

Fixes: adb92db857ee ("tcp: Make SACK code to split only at mss boundaries")
Reported-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Ilpo Jarvinen <ilpo.jarvinen@...sinki.fi>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 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 d8e4c81bc114..ebafc8703c7a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1119,7 +1119,7 @@ static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
 			unsigned int new_len = (pkt_len / mss) * mss;
 			if (!in_sack && new_len < pkt_len) {
 				new_len += mss;
-				if (new_len > skb->len)
+				if (new_len >= skb->len)
 					return 0;
 			}
 			pkt_len = new_len;
-- 
1.9.1

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