Patch "tcp: fix wrong checksum calculation on MTU probing" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

tcp: fix wrong checksum calculation on MTU probing

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tcp-fix-wrong-checksum-calculation-on-mtu-probing.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Douglas Caetano dos Santos <[email protected]>
Date: Thu, 22 Sep 2016 15:52:04 -0300
Subject: tcp: fix wrong checksum calculation on MTU probing

From: Douglas Caetano dos Santos <[email protected]>

[ Upstream commit 2fe664f1fcf7c4da6891f95708a7a56d3c024354 ]

With TCP MTU probing enabled and offload TX checksumming disabled, tcp_mtu_probe() calculated the wrong checksum when a fragment being copied into the probe's SKB had an odd length. This was caused by the direct use of skb_copy_and_csum_bits() to calculate the checksum, as it pads the fragment being copied, if needed. When this fragment was not the last, a subsequent call used the previous checksum without considering this padding.

The effect was a stale connection in one way, as even retransmissions wouldn't solve the problem, because the checksum was never recalculated for the full SKB length.

---
 net/ipv4/tcp_output.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

@@ -1950,12 +1950,14 @@ static int tcp_mtu_probe(struct sock *sk
 	len = 0;
 	tcp_for_write_queue_from_safe(skb, next, sk) {
 		copy = min_t(int, skb->len, probe_size - len);
-		if (nskb->ip_summed)
+		if (nskb->ip_summed) {
 			skb_copy_bits(skb, 0, skb_put(nskb, copy), copy);
-		else
-			nskb->csum = skb_copy_and_csum_bits(skb, 0,
-							    skb_put(nskb, copy),
-							    copy, nskb->csum);
+		} else {
+			__wsum csum = skb_copy_and_csum_bits(skb, 0,
+							     skb_put(nskb, copy),
+							     copy, 0);
+			nskb->csum = csum_block_add(nskb->csum, csum, len);
+		}
 
 		if (skb->len <= copy) {
 			/* We've eaten all the data from this skb.

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tcp-fix-wrong-checksum-calculation-on-mtu-probing.patch