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    Subject[011/107] GRO: fix merging a paged skb after non-paged skbs
    2.6.32-longterm review patch.  If anyone has any objections, please let us know.


    From: Michal Schmidt <>

    commit d1dc7abf2fafa34b0ffcd070fd59405aa9c0a4d8 upstream.

    Suppose that several linear skbs of the same flow were received by GRO. They
    were thus merged into one skb with a frag_list. Then a new skb of the same flow
    arrives, but it is a paged skb with data starting in its frags[].

    Before adding the skb to the frag_list skb_gro_receive() will of course adjust
    the skb to throw away the headers. It correctly modifies the page_offset and
    size of the frag, but it leaves incorrect information in the skb:
    ->data_len is not decreased at all.
    ->len is decreased only by headlen, as if no change were done to the frag.
    Later in a receiving process this causes skb_copy_datagram_iovec() to return
    -EFAULT and this is seen in userspace as the result of the recv() syscall.

    In practice the bug can be reproduced with the sfc driver. By default the
    driver uses an adaptive scheme when it switches between using
    napi_gro_receive() (with skbs) and napi_gro_frags() (with pages). The bug is
    reproduced when under rx load with enough successful GRO merging the driver
    decides to switch from the former to the latter.

    Manual control is also possible, so reproducing this is easy with netcat:
    - on machine1 (with sfc): nc -l 12345 > /dev/null
    - on machine2: nc machine1 12345 < /dev/zero
    - on machine1:
    echo 1 > /sys/module/sfc/parameters/rx_alloc_method # use skbs
    echo 2 > /sys/module/sfc/parameters/rx_alloc_method # use pages
    - See that nc has quit suddenly.

    [v2: Modified by Eric Dumazet to avoid advancing skb->data past the end
    and to use a temporary variable.]

    Signed-off-by: Michal Schmidt <>
    Acked-by: Eric Dumazet <>
    Signed-off-by: David S. Miller <>
    Signed-off-by: Greg Kroah-Hartman <>

    net/core/skbuff.c | 8 ++++++--
    1 file changed, 6 insertions(+), 2 deletions(-)

    --- a/net/core/skbuff.c
    +++ b/net/core/skbuff.c
    @@ -2746,8 +2746,12 @@ int skb_gro_receive(struct sk_buff **hea

    if (offset > headlen) {
    - skbinfo->frags[0].page_offset += offset - headlen;
    - skbinfo->frags[0].size -= offset - headlen;
    + unsigned int eat = offset - headlen;
    + skbinfo->frags[0].page_offset += eat;
    + skbinfo->frags[0].size -= eat;
    + skb->data_len -= eat;
    + skb->len -= eat;
    offset = headlen;

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      Last update: 2011-11-03 02:35    [W:0.021 / U:11.200 seconds]
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