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    Subject[PATCH AUTOSEL 4.18 32/33] net: fix pskb_trim_rcsum_slow() with odd trim offset
    Date
    From: Dimitris Michailidis <dmichail@google.com>

    [ Upstream commit d55bef5059dd057bd077155375c581b49d25be7e ]

    We've been getting checksum errors involving small UDP packets, usually
    59B packets with 1 extra non-zero padding byte. netdev_rx_csum_fault()
    has been complaining that HW is providing bad checksums. Turns out the
    problem is in pskb_trim_rcsum_slow(), introduced in commit 88078d98d1bb
    ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends").

    The source of the problem is that when the bytes we are trimming start
    at an odd address, as in the case of the 1 padding byte above,
    skb_checksum() returns a byte-swapped value. We cannot just combine this
    with skb->csum using csum_sub(). We need to use csum_block_sub() here
    that takes into account the parity of the start address and handles the
    swapping.

    Matches existing code in __skb_postpull_rcsum() and esp_remove_trailer().

    Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
    Signed-off-by: Dimitris Michailidis <dmichail@google.com>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/core/skbuff.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/net/core/skbuff.c b/net/core/skbuff.c
    index 3680912f056a..c45916b91a9c 100644
    --- a/net/core/skbuff.c
    +++ b/net/core/skbuff.c
    @@ -1845,8 +1845,9 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
    if (skb->ip_summed == CHECKSUM_COMPLETE) {
    int delta = skb->len - len;

    - skb->csum = csum_sub(skb->csum,
    - skb_checksum(skb, len, delta, 0));
    + skb->csum = csum_block_sub(skb->csum,
    + skb_checksum(skb, len, delta, 0),
    + len);
    }
    return __pskb_trim(skb, len);
    }
    --
    2.17.1
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