[PATCH] net: check skb partial checksum offset after trim

[Vasily Averin <vvs@virtuozzo.com>]

syzkaller reproduces BUG_ON in skb_checksum_help():
tun creates skb with big partial checksum area and small ip packet inside,
then ip_rcv() decreases skb size of below length of checksummed area,
then checksum_tg() called via netfilter hook detects incorrect skb:

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

This patch drops CHEKSUM_PARTIAL mark when skb is trimmed below
size of checksummed area.
Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
Reported-by: syzbot+7010af67ced6105e5ab6@syzkaller.appspotmail.com
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
---
 include/linux/skbuff.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a828cf9..0a9545d 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3419,9 +3419,18 @@ static inline void *skb_push_rcsum(struct sk_buff *skb, unsigned int len)
 
 static inline int pskb_trim_rcsum(struct sk_buff *skb, unsigned int len)
 {
+	int ret;
+
 	if (likely(len >= skb->len))
 		return 0;
-	return pskb_trim_rcsum_slow(skb, len);
+	ret = pskb_trim_rcsum_slow(skb, len);
+	if (!ret && (skb->ip_summed == CHECKSUM_PARTIAL)) {
+		int offset = skb_checksum_start_offset(skb) + skb->csum_offset;
+
+		if (offset + sizeof(__sum16) > skb_headlen(skb))
+			skb->ip_summed = CHECKSUM_NONE;
+	}
+	return ret;
 }
 
 static inline int __skb_trim_rcsum(struct sk_buff *skb, unsigned int len)
-- 
1.8.3.1