Commit 2fe664f1 authored Sep 22, 2016 by Douglas Caetano dos Santos Committed by David S. Miller Sep 23, 2016

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@taghos.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent f44ace4d

net/ipv4/tcp_output.c

+7 −5

Original line number	Diff line number	Diff line
		@@ -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,
		} else {
		__wsum csum = skb_copy_and_csum_bits(skb, 0,
		skb_put(nskb, copy),
		copy, nskb->csum);
		copy, 0);
		nskb->csum = csum_block_add(nskb->csum, csum, len);
		}

		if (skb->len <= copy) {
		/* We've eaten all the data from this skb.

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