llc: fix data loss when reading from a socket in llc_ui_recvmsg()

For SOCK_STREAM sockets, if user buffer size (len) is less
than skb size (skb->len), the remaining data from skb
will be lost after calling kfree_skb().

To fix this, move the statement for partial reading
above skb deletion.

Found by InfoTeCS on behalf of Linux Verification Center (linuxtesting.org)

Fixes: 30a584d ("[LLX]: SOCK_DGRAM interface fixes")
Cc: stable@vger.kernel.org
Signed-off-by: Ilia Gavrilov <Ilia.Gavrilov@infotecs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Ilia Gavrilov

net/llc/af_llc.c

@@ -887,15 +887,15 @@ static int llc_ui_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		if (sk->sk_type != SOCK_STREAM)
 			goto copy_uaddr;
 
+		/* Partial read */
+		if (used + offset < skb_len)
+			continue;
+
 		if (!(flags & MSG_PEEK)) {
 			skb_unlink(skb, &sk->sk_receive_queue);
 			kfree_skb(skb);
 			*seq = 0;
 		}
-
-		/* Partial read */
-		if (used + offset < skb_len)
-			continue;
 	} while (len > 0);
 
 out: