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: ff48b62 ("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]>
josf · Oct 10, 2020 · ed42989 · ed42989
1 parent 923527d
commit ed42989
Showing 1 changed file with 2 additions and 1 deletion.
diff --git a/net/tipc/msg.c b/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;