[Devel] [PATCH RH7 4/4] ms/net: drop bogus skb with CHECKSUM_PARTIAL and offset beyond end of trimmed packet

[Vasily Averin]

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 at syzkaller.appspotmail.com
Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
Acked-by: Willem de Bruijn <willemb at google.com>
Link: https://lore.kernel.org/r/1b2494af-2c56-8ee2-7bc0-923fcad1cdf8@virtuozzo.com
Signed-off-by: Jakub Kicinski <kuba at kernel.org>

Now it is in net-next:
https://git.kernel.org/netdev/net-next/c/54970a2fbb67
https://jira.sw.ru/browse/PSBM-123062
Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 include/linux/skbuff.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 296e734..f2c66b1 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3108,6 +3108,13 @@ static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
 		return 0;
 	if (skb->ip_summed == CHECKSUM_COMPLETE)
 		skb->ip_summed = CHECKSUM_NONE;
+	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);
 }
 
-- 
1.8.3.1