Re: [PATCH net] s390/iucv: MSG_PEEK causes memory leak in iucv_sock_destruct()

[David Wei <dw@davidwei.uk>]

On 2024-11-19 07:22, Alexandra Winter wrote:
> From: Sidraya Jayagond <sidraya@linux.ibm.com>
> 
> 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: eac3731bd04c ("[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>
> ---
> The following mailaddresses are no longer active:
> Frank Pavlic <fpavlic@de.ibm.com> (blamed_fixes:1/1=100%)
> Martin Schwidefsky <schwidefsky@de.ibm.com> (blamed_fixes:1/1=100%)
> ---
>  net/iucv/af_iucv.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
> index c00323fa9eb6..7929df08d4e0 100644
> --- a/net/iucv/af_iucv.c
> +++ b/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);

I checked the call graph and `flags` is passed through:

skb_recv_datagram()
  -> __skb_recv_datagram()
    -> __skb_try_recv_datagram()
      -> __skb_try_recv_from_queue()

If MSG_PEEK is set and a valid skb is returned then skb->users is
incremented.

>  	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;

Previous behaviour is unchanged. Now if MSG_PEEK is set then skb->users
is decremented. At this point skb is guaranteed to be valid.

>  	}
>  
>  	/* 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;

Same as above.

>  
>  	/* 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);

I checked that all return paths with MSG_PEEK and a valid skb result in
skb_unref().

The remaining return paths either have !MSG_PEEK or !skb:

(1)
	if (!skb) {
		if (sk->sk_shutdown & RCV_SHUTDOWN)
			return 0;
		return err;
	}

(2)
	if (!(flags & MSG_PEEK)) {
		...
		if (iucv->transport == AF_IUCV_TRANS_HIPER) {
			atomic_inc(&iucv->msg_recv);
			if (atomic_read(&iucv->msg_recv) > iucv->msglimit) {
				WARN_ON(1);
				iucv_sock_close(sk);
				return -EFAULT;
			}
		}
		...
	}

>  
>  	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)

Reviewed-by: David Wei <dw@davidwei.uk>