[Devel] [PATCH RHEL7 COMMIT] ms/net: drop bogus skb with CHECKSUM_PARTIAL and offset beyond end of trimmed packet

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1160.6.1.vz7.171.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.6.1.vz7.171.4
------>
commit dd590cd4c2f3c21197754f40d9f67165a7470dc5
Author: Vasily Averin <vvs at virtuozzo.com>
Date:   Tue Dec 15 12:24:50 2020 +0300

    ms/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 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);
 }