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    Subject[PATCH 4.9 25/25] net: drop bogus skb with CHECKSUM_PARTIAL and offset beyond end of trimmed packet
    Date
    From: Vasily Averin <vvs@virtuozzo.com>

    commit 54970a2fbb673f090b7f02d7f57b10b2e0707155 upstream.

    syzbot reproduces BUG_ON in skb_checksum_help():
    tun creates (bogus) skb with huge partial-checksummed area and
    small ip packet inside. Then ip_rcv trims the skb based on size
    of internal ip packet, after that csum offset points beyond of
    trimmed skb. Then checksum_tg() called via netfilter hook
    triggers BUG_ON:

    offset = skb_checksum_start_offset(skb);
    BUG_ON(offset >= skb_headlen(skb));

    To work around the problem this patch forces pskb_trim_rcsum_slow()
    to return -EINVAL in described scenario. It allows its callers to
    drop such kind of packets.

    Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
    Reported-by: syzbot+7010af67ced6105e5ab6@syzkaller.appspotmail.com
    Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
    Acked-by: Willem de Bruijn <willemb@google.com>
    Link: https://lore.kernel.org/r/1b2494af-2c56-8ee2-7bc0-923fcad1cdf8@virtuozzo.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

    --- a/net/core/skbuff.c
    +++ b/net/core/skbuff.c
    @@ -1592,6 +1592,12 @@ int pskb_trim_rcsum_slow(struct sk_buff
    skb->csum = csum_block_sub(skb->csum,
    skb_checksum(skb, len, delta, 0),
    len);
    + } else if (skb->ip_summed == CHECKSUM_PARTIAL) {
    + int hdlen = (len > skb_headlen(skb)) ? skb_headlen(skb) : len;
    + int offset = skb_checksum_start_offset(skb) + skb->csum_offset;
    +
    + if (offset + sizeof(__sum16) > hdlen)
    + return -EINVAL;
    }
    return __pskb_trim(skb, len);
    }

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