s390/iucv: MSG_PEEK causes memory leak in iucv_sock_destruct()

Passing MSG_PEEK flag to skb_recv_datagram() increments skb refcount
(skb->users) and iucv_sock_recvmsg() does not decrement skb refcount
at exit.
This results in skb memory leak in skb_queue_purge() and WARN_ON in
iucv_sock_destruct() during socket close. To fix this decrease
skb refcount by one if MSG_PEEK is set in order to prevent memory
leak and WARN_ON.

WARNING: CPU: 2 PID: 6292 at net/iucv/af_iucv.c:286 iucv_sock_destruct+0x144/0x1a0 [af_iucv]
CPU: 2 PID: 6292 Comm: afiucv_test_msg Kdump: loaded Tainted: G        W          6.10.0-rc7 #1
Hardware name: IBM 3931 A01 704 (z/VM 7.3.0)
Call Trace:
        [<001587c682c4aa98>] iucv_sock_destruct+0x148/0x1a0 [af_iucv]
        [<001587c682c4a9d0>] iucv_sock_destruct+0x80/0x1a0 [af_iucv]
        [<001587c704117a32>] __sk_destruct+0x52/0x550
        [<001587c704104a54>] __sock_release+0xa4/0x230
        [<001587c704104c0c>] sock_close+0x2c/0x40
        [<001587c702c5f5a8>] __fput+0x2e8/0x970
        [<001587c7024148c4>] task_work_run+0x1c4/0x2c0
        [<001587c7023b0716>] do_exit+0x996/0x1050
        [<001587c7023b13aa>] do_group_exit+0x13a/0x360
        [<001587c7023b1626>] __s390x_sys_exit_group+0x56/0x60
        [<001587c7022bccca>] do_syscall+0x27a/0x380
        [<001587c7049a6a0c>] __do_syscall+0x9c/0x160
        [<001587c7049ce8a8>] system_call+0x70/0x98
        Last Breaking-Event-Address:
        [<001587c682c4a9d4>] iucv_sock_destruct+0x84/0x1a0 [af_iucv]

Fixes: eac3731 ("[S390]: Add AF_IUCV socket support")
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Sidraya Jayagond <sidraya@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20241119152219.3712168-1-wintera@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/iucv/af_iucv.c

@@ -1236,7 +1236,9 @@ static int iucv_sock_recvmsg(struct socket *sock, struct msghdr *msg,
 		return -EOPNOTSUPP;
 
 	/* receive/dequeue next skb:
-	 * the function understands MSG_PEEK and, thus, does not dequeue skb */
+	 * the function understands MSG_PEEK and, thus, does not dequeue skb
+	 * only refcount is increased.
+	 */
 	skb = skb_recv_datagram(sk, flags, &err);
 	if (!skb) {
 		if (sk->sk_shutdown & RCV_SHUTDOWN)
@@ -1252,9 +1254,8 @@ static int iucv_sock_recvmsg(struct socket *sock, struct msghdr *msg,
 
 	cskb = skb;
 	if (skb_copy_datagram_msg(cskb, offset, msg, copied)) {
-		if (!(flags & MSG_PEEK))
-			skb_queue_head(&sk->sk_receive_queue, skb);
-		return -EFAULT;
+		err = -EFAULT;
+		goto err_out;
 	}
 
 	/* SOCK_SEQPACKET: set MSG_TRUNC if recv buf size is too small */
@@ -1271,11 +1272,8 @@ static int iucv_sock_recvmsg(struct socket *sock, struct msghdr *msg,
 	err = put_cmsg(msg, SOL_IUCV, SCM_IUCV_TRGCLS,
 		       sizeof(IUCV_SKB_CB(skb)->class),
 		       (void *)&IUCV_SKB_CB(skb)->class);
-	if (err) {
-		if (!(flags & MSG_PEEK))
-			skb_queue_head(&sk->sk_receive_queue, skb);
-		return err;
-	}
+	if (err)
+		goto err_out;
 
 	/* Mark read part of skb as used */
 	if (!(flags & MSG_PEEK)) {
@@ -1331,8 +1329,18 @@ static int iucv_sock_recvmsg(struct socket *sock, struct msghdr *msg,
 	/* SOCK_SEQPACKET: return real length if MSG_TRUNC is set */
 	if (sk->sk_type == SOCK_SEQPACKET && (flags & MSG_TRUNC))
 		copied = rlen;
+	if (flags & MSG_PEEK)
+		skb_unref(skb);
 
 	return copied;
+
+err_out:
+	if (!(flags & MSG_PEEK))
+		skb_queue_head(&sk->sk_receive_queue, skb);
+	else
+		skb_unref(skb);
+
+	return err;
 }
 
 static inline __poll_t iucv_accept_poll(struct sock *parent)