netrom: fix double-free in nr_route_frame()

In nr_route_frame(), old_skb is immediately freed without checking if
nr_neigh->ax25 pointer is NULL. Therefore, if nr_neigh->ax25 is NULL,
the caller function will free old_skb again, causing a double-free bug.

Therefore, to prevent this, we need to modify it to check whether
nr_neigh->ax25 is NULL before freeing old_skb.

Cc: <stable@vger.kernel.org>
Reported-by: syzbot+999115c3bf275797dc27@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69694d6f.050a0220.58bed.0029.GAE@google.com/
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Link: https://patch.msgid.link/20260119063359.10604-1-aha310510@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: name2965

net/netrom/nr_route.c

@@ -752,7 +752,7 @@ int nr_route_frame(struct sk_buff *skb, ax25_cb *ax25)
 	unsigned char *dptr;
 	ax25_cb *ax25s;
 	int ret;
-	struct sk_buff *skbn;
+	struct sk_buff *nskb, *oskb;
 
 	/*
 	 * Reject malformed packets early. Check that it contains at least 2
@@ -811,14 +811,16 @@ int nr_route_frame(struct sk_buff *skb, ax25_cb *ax25)
 	/* We are going to change the netrom headers so we should get our
 	   own skb, we also did not know until now how much header space
 	   we had to reserve... - RXQ */
-	if ((skbn=skb_copy_expand(skb, dev->hard_header_len, 0, GFP_ATOMIC)) == NULL) {
+	nskb = skb_copy_expand(skb, dev->hard_header_len, 0, GFP_ATOMIC);
+
+	if (!nskb) {
 		nr_node_unlock(nr_node);
 		nr_node_put(nr_node);
 		dev_put(dev);
 		return 0;
 	}
-	kfree_skb(skb);
-	skb=skbn;
+	oskb = skb;
+	skb = nskb;
 	skb->data[14]--;
 
 	dptr  = skb_push(skb, 1);
@@ -837,6 +839,9 @@ int nr_route_frame(struct sk_buff *skb, ax25_cb *ax25)
 	nr_node_unlock(nr_node);
 	nr_node_put(nr_node);
 
+	if (ret)
+		kfree_skb(oskb);
+
 	return ret;
 }