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    Subject[PATCH 4.4 03/34] tcp: fix wrong checksum calculation on MTU probing
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

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

    From: Douglas Caetano dos Santos <douglascs@taghos.com.br>


    [ 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.

    Signed-off-by: Douglas Caetano dos Santos <douglascs@taghos.com.br>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_output.c | 12 +++++++-----
    1 file changed, 7 insertions(+), 5 deletions(-)

    --- a/net/ipv4/tcp_output.c
    +++ b/net/ipv4/tcp_output.c
    @@ -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.

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