tipc: fix the skb_unshare() in tipc_buf_append()

skb_unshare() drops a reference count on the old skb unconditionally,
so in the failure case, we end up freeing the skb twice here.
And because the skb is allocated in fclone and cloned by caller
tipc_msg_reassemble(), the consequence is actually freeing the
original skb too, thus triggered the UAF by syzbot.

Fix this by replacing this skb_unshare() with skb_cloned()+skb_copy().

Fixes: ff48b6222e ("tipc: use skb_unshare() instead in tipc_buf_append()")
Reported-and-tested-by: [email protected]
Cc: Jon Maloy <[email protected]>
Cc: Ying Xue <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Reviewed-by: Xin Long <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

net/tipc/msg.c

@@ -150,7 +150,8 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
 	if (fragid == FIRST_FRAGMENT) {
 		if (unlikely(head))
 			goto err;
-		frag = skb_unshare(frag, GFP_ATOMIC);
+		if (skb_cloned(frag))
+			frag = skb_copy(frag, GFP_ATOMIC);
 		if (unlikely(!frag))
 			goto err;
 		head = *headbuf = frag;