netdev - [PATCH v3] tcp: fix wrong checksum calculation on MTU probing

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Date:   Thu, 22 Sep 2016 15:52:04 -0300
From:   Douglas Caetano dos Santos <douglascs@...hos.com.br>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        David Miller <davem@...emloft.net>
CC:     <kuznet@....inr.ac.ru>, <jmorris@...ei.org>,
        <yoshfuji@...ux-ipv6.org>, <kaber@...sh.net>,
        <netdev@...r.kernel.org>
Subject: [PATCH v3] tcp: fix wrong checksum calculation on MTU probing

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.

Signed-off-by: Douglas Caetano dos Santos <douglascs@...hos.com.br>
---
 net/ipv4/tcp_output.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f53d0cc..2d32952 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1966,12 +1966,14 @@ 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.
-- 
2.5.0