Commit 54970a2f authored Dec 14, 2020 by Vasily Averin Committed by Jakub Kicinski Dec 14, 2020

net: drop bogus skb with CHECKSUM_PARTIAL and offset beyond end of trimmed packet

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>

parent 0780b414

net/core/skbuff.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -2018,6 +2018,12 @@ int pskb_trim_rcsum_slow(struct sk_buff *skb, unsigned int len)
		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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