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+7010af67ced6105e5...@syzkaller.appspotmail.com
Signed-off-by: Vasily Averin <v...@virtuozzo.com>
---
v2: drop bogus packets instead change its CHECKSUM_PARTIAL to CHECKSUM_NONE
net/core/skbuff.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index e578544..fbadd93 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2011,6 +2011,12 @@ 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);
}
--
1.8.3.1
