Re: [PATCH net] vsock: fix locking in vsock_shutdown()

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Feb 08, 2021 at 03:43:07PM +0100, Stefano Garzarella wrote:
>In vsock_shutdown() we touched some socket fields without holding the
>socket lock, such as 'state' and 'sk_flags'.
>
>Also, after the introduction of multi-transport, we are accessing
>'vsk->transport' in vsock_send_shutdown() without holding the lock
>and this call can be made while the connection is in progress, so
>the transport can change in the meantime.
>
>To avoid issues, we hold the socket lock when we enter in
>vsock_shutdown() and release it when we leave.
>
>Among the transports that implement the 'shutdown' callback, only
>hyperv_transport acquired the lock. Since the caller now holds it,
>we no longer take it.
>
>Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
>Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>---
> net/vmw_vsock/af_vsock.c         | 8 +++++---
> net/vmw_vsock/hyperv_transport.c | 2 --
> 2 files changed, 5 insertions(+), 5 deletions(-)
>
>diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
>index 4ea301fc2bf0..5546710d8ac1 100644
>--- a/net/vmw_vsock/af_vsock.c
>+++ b/net/vmw_vsock/af_vsock.c
>@@ -943,10 +943,12 @@ static int vsock_shutdown(struct socket *sock, int mode)
> 	 */
>
> 	sk = sock->sk;
>+
>+	lock_sock(sk);
> 	if (sock->state == SS_UNCONNECTED) {
> 		err = -ENOTCONN;
> 		if (sk->sk_type == SOCK_STREAM)
>-			return err;
>+			goto out;
> 	} else {
> 		sock->state = SS_DISCONNECTING;
> 		err = 0;
>@@ -955,10 +957,8 @@ static int vsock_shutdown(struct socket *sock, int mode)
> 	/* Receive and send shutdowns are treated alike. */
> 	mode = mode & (RCV_SHUTDOWN | SEND_SHUTDOWN);
> 	if (mode) {
>-		lock_sock(sk);
> 		sk->sk_shutdown |= mode;
> 		sk->sk_state_change(sk);
>-		release_sock(sk);
>
> 		if (sk->sk_type == SOCK_STREAM) {
> 			sock_reset_flag(sk, SOCK_DONE);
>@@ -966,6 +966,8 @@ static int vsock_shutdown(struct socket *sock, int mode)
> 		}
> 	}
>
>+out:
>+	release_sock(sk);
> 	return err;
> }
>
>diff --git a/net/vmw_vsock/hyperv_transport.c b/net/vmw_vsock/hyperv_transport.c
>index 630b851f8150..5a3beef73461 100644
>--- a/net/vmw_vsock/hyperv_transport.c
>+++ b/net/vmw_vsock/hyperv_transport.c
>@@ -479,9 +479,7 @@ static int hvs_shutdown(struct vsock_sock *vsk, int mode)
> 	if (!(mode & SEND_SHUTDOWN))
> 		return 0;
>
>-	lock_sock(sk);
> 	hvs_shutdown_lock_held(vsk->trans, mode);
>-	release_sock(sk);

Ooops, removing these lines, 'sk' is not used anymore in hvs_shutdown(), 
I'll send v2 ASAP:

../net/vmw_vsock/hyperv_transport.c: In function ‘hvs_shutdown’:
../net/vmw_vsock/hyperv_transport.c:477:15: warning: unused variable ‘sk’ [-Wunused-variable]
   477 |  struct sock *sk = sk_vsock(vsk);
       |               ^~

Since I'm here, I had a doubt whether to separate this modification or 
leave it in this patch.

What do you suggest?

I did it this way because by modifying only the caller, we would have a 
nested lock.

This way instead we are sure that if we backport this patch, we don't 
forget to touch hvs_shutdown() as well.

Thanks,
Stefano