[net-next PATCH 4/5] udp: Do not copy destructor if one is not present

[Alexander Duyck <alexander.duyck@gmail.com>]

From: Alexander Duyck <alexander.h.duyck@intel.com>

This patch makes it so that if a destructor is not present we avoid trying
to update the skb socket or any reference counting that would be associated
with the NULL socket and/or descriptor. By doing this we can support
traffic coming from another namespace without any issues.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
---
 net/ipv4/udp_offload.c |   19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index fd94bbb369b2..52760660d674 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -195,6 +195,7 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	unsigned int sum_truesize = 0;
 	struct udphdr *uh;
 	unsigned int mss;
+	bool copy_dtor;
 	__sum16 check;
 	__be16 newlen;
 
@@ -208,12 +209,14 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 	skb_pull(gso_skb, sizeof(*uh));
 
 	/* clear destructor to avoid skb_segment assigning it to tail */
-	WARN_ON_ONCE(gso_skb->destructor != sock_wfree);
+	copy_dtor = gso_skb->destructor == sock_wfree;
+	WARN_ON_ONCE(gso_skb->destructor && !copy_dtor);
 	gso_skb->destructor = NULL;
 
 	segs = skb_segment(gso_skb, features);
 	if (unlikely(IS_ERR_OR_NULL(segs))) {
-		gso_skb->destructor = sock_wfree;
+		if (copy_dtor)
+			gso_skb->destructor = sock_wfree;
 		return segs;
 	}
 
@@ -234,9 +237,11 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 					    (__force u32)newlen));
 
 	for (;;) {
-		seg->destructor = sock_wfree;
-		seg->sk = sk;
-		sum_truesize += seg->truesize;
+		if (copy_dtor) {
+			seg->destructor = sock_wfree;
+			seg->sk = sk;
+			sum_truesize += seg->truesize;
+		}
 
 		if (!seg->next)
 			break;
@@ -268,7 +273,9 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb,
 		uh->check = gso_make_checksum(seg, ~check);
 
 	/* update refcount for the packet */
-	refcount_add(sum_truesize - gso_skb->truesize, &sk->sk_wmem_alloc);
+	if (copy_dtor)
+		refcount_add(sum_truesize - gso_skb->truesize,
+			     &sk->sk_wmem_alloc);
 out:
 	return segs;
 }