Subject: [PATCH 36/39] net: drop skb on failure in ip_check_defrag()

From: Cong Wang <[email-masked]>
Date: Thu, 1 Nov 2018 12:02:37 -0700

[ Upstream commit 7de414a9dd91426318df7b63da024b2b07e53df5 ]

Most callers of pskb_trim_rcsum() simply drop the skb when
it fails, however, ip_check_defrag() still continues to pass
the skb up to stack. This is suspicious.

In ip_check_defrag(), after we learn the skb is an IP fragment,
passing the skb to callers makes no sense, because callers expect
fragments are defrag'ed on success. So, dropping the skb when we
can't defrag it is reasonable.

Note, prior to commit 88078d98d1bb, this is not a big problem as
checksum will be fixed up anyway. After it, the checksum is not
correct on failure.

Found this during code review.

Fixes: 88078d98d1bb ("net: pskb_trim_rcsum() and CHECKSUM_COMPLETE are friends")
Cc: Eric Dumazet <[email-masked]>
Signed-off-by: Cong Wang <[email-masked]>
Reviewed-by: Eric Dumazet <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/ipv4/ip_fragment.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index d14d741fb05e..9d3bdce1ad8a 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -657,10 +657,14 @@ struct sk_buff *ip_check_defrag(struct net *net, struct sk_buff *skb, u32 user)
 	if (ip_is_fragment(&iph)) {
 		skb = skb_share_check(skb, GFP_ATOMIC);
 		if (skb) {
-			if (!pskb_may_pull(skb, netoff + iph.ihl * 4))
-				return skb;
-			if (pskb_trim_rcsum(skb, netoff + len))
-				return skb;
+			if (!pskb_may_pull(skb, netoff + iph.ihl * 4)) {
+				kfree_skb(skb);
+				return NULL;
+			}
+			if (pskb_trim_rcsum(skb, netoff + len)) {
+				kfree_skb(skb);
+				return NULL;
+			}
 			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
 			if (ip_defrag(net, skb, user))
 				return NULL;
-- 
2.19.1