xfrm_user: return error pointer instead of NULL

When dump_one_state() returns an error, e.g. because of a too small
buffer to dump the whole xfrm state, xfrm_state_netlink() returns NULL
instead of an error pointer. But its callers expect an error pointer
and therefore continue to operate on a NULL skbuff.

This could lead to a privilege escalation (execution of user code in
kernel context) if the attacker has CAP_NET_ADMIN and is able to map
address 0.

Change-Id: I64bda1dd6dac3d743a4111bafaa9965b8de95920
Signed-off-by: Mathias Krause <[email protected]>
Acked-by: Steffen Klassert <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
Signed-off-by: Francisco Franco <[email protected]>

net/xfrm/xfrm_user.c

@@ -888,6 +888,7 @@ static struct sk_buff *xfrm_state_netlink(struct sk_buff *in_skb,
 {
 	struct xfrm_dump_info info;
 	struct sk_buff *skb;
+	int err;
 
 	skb = nlmsg_new(NLMSG_DEFAULT_SIZE, GFP_ATOMIC);
 	if (!skb)
@@ -898,9 +899,10 @@ static struct sk_buff *xfrm_state_netlink(struct sk_buff *in_skb,
 	info.nlmsg_seq = seq;
 	info.nlmsg_flags = 0;
 
-	if (dump_one_state(x, 0, &info)) {
+	err = dump_one_state(x, 0, &info);
+	if (err) {
 		kfree_skb(skb);
-		return NULL;
+		return ERR_PTR(err);
 	}
 
 	return skb;