[Devel] [PATCH RH7] net: drop CHECKSUM_PARTIAL if skb is trimmed below size of checksummed area

[Vasily Averin]

syzbot 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));

Link: https://syzkaller.appspot.com/bug?id=b419a5ca95062664fe1a60b764621eb4526e2cd0
Reported-by: syzbot+7010af67ced6105e5ab6 at syzkaller.appspotmail.com

Sent to upstream
Link: https://www.spinics.net/lists/netdev/msg707912.html
https://jira.sw.ru/browse/PSBM-123062
---
 include/linux/skbuff.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 296e734..3a4093c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3104,11 +3104,21 @@ 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;
 	if (skb->ip_summed == CHECKSUM_COMPLETE)
 		skb->ip_summed = CHECKSUM_NONE;
-	return __pskb_trim(skb, len);
+
+	ret = __pskb_trim(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;
 }
 
 #define rb_to_skb(rb) rb_entry_safe(rb, struct sk_buff, rbnode)
-- 
1.8.3.1