vsock: fix potential deadlock in transport->release()

Some transports (hyperv, virtio) acquire the sock lock during the
.release() callback.

In the vsock_stream_connect() we call vsock_assign_transport(); if
the socket was previously assigned to another transport, the
vsk->transport->release() is called, but the sock lock is already
held in the vsock_stream_connect(), causing a deadlock reported by
syzbot:

    INFO: task syz-executor280:9768 blocked for more than 143 seconds.
      Not tainted 5.6.0-rc1-syzkaller #0
    "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    syz-executor280 D27912  9768   9766 0x00000000
    Call Trace:
     context_switch kernel/sched/core.c:3386 [inline]
     __schedule+0x934/0x1f90 kernel/sched/core.c:4082
     schedule+0xdc/0x2b0 kernel/sched/core.c:4156
     __lock_sock+0x165/0x290 net/core/sock.c:2413
     lock_sock_nested+0xfe/0x120 net/core/sock.c:2938
     virtio_transport_release+0xc4/0xd60 net/vmw_vsock/virtio_transport_common.c:832
     vsock_assign_transport+0xf3/0x3b0 net/vmw_vsock/af_vsock.c:454
     vsock_stream_connect+0x2b3/0xc70 net/vmw_vsock/af_vsock.c:1288
     __sys_connect_file+0x161/0x1c0 net/socket.c:1857
     __sys_connect+0x174/0x1b0 net/socket.c:1874
     __do_sys_connect net/socket.c:1885 [inline]
     __se_sys_connect net/socket.c:1882 [inline]
     __x64_sys_connect+0x73/0xb0 net/socket.c:1882
     do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
     entry_SYSCALL_64_after_hwframe+0x49/0xbe

To avoid this issue, this patch remove the lock acquiring in the
.release() callback of hyperv and virtio transports, and it holds
the lock when we call vsk->transport->release() in the vsock core.

Reported-by: syzbot+731710996d79d0d58fbc@syzkaller.appspotmail.com
Fixes: 408624af ("vsock: use local transport when it is loaded")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/vmw_vsock/af_vsock.c

@@ -451,6 +451,12 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
 		if (vsk->transport == new_transport)
 			return 0;
 
+		/* transport->release() must be called with sock lock acquired.
+		 * This path can only be taken during vsock_stream_connect(),
+		 * where we have already held the sock lock.
+		 * In the other cases, this function is called on a new socket
+		 * which is not assigned to any transport.
+		 */
 		vsk->transport->release(vsk);
 		vsock_deassign_transport(vsk);
 	}
@@ -753,20 +759,18 @@ static void __vsock_release(struct sock *sk, int level)
 		vsk = vsock_sk(sk);
 		pending = NULL;	/* Compiler warning. */
 
-		/* The release call is supposed to use lock_sock_nested()
-		 * rather than lock_sock(), if a sock lock should be acquired.
-		 */
-		if (vsk->transport)
-			vsk->transport->release(vsk);
-		else if (sk->sk_type == SOCK_STREAM)
-			vsock_remove_sock(vsk);
-
 		/* When "level" is SINGLE_DEPTH_NESTING, use the nested
 		 * version to avoid the warning "possible recursive locking
 		 * detected". When "level" is 0, lock_sock_nested(sk, level)
 		 * is the same as lock_sock(sk).
 		 */
 		lock_sock_nested(sk, level);
+
+		if (vsk->transport)
+			vsk->transport->release(vsk);
+		else if (sk->sk_type == SOCK_STREAM)
+			vsock_remove_sock(vsk);
+
 		sock_orphan(sk);
 		sk->sk_shutdown = SHUTDOWN_MASK;
 

net/vmw_vsock/hyperv_transport.c

@@ -526,12 +526,9 @@ static bool hvs_close_lock_held(struct vsock_sock *vsk)
 
 static void hvs_release(struct vsock_sock *vsk)
 {
-	struct sock *sk = sk_vsock(vsk);
 	bool remove_sock;
 
-	lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
 	remove_sock = hvs_close_lock_held(vsk);
-	release_sock(sk);
 	if (remove_sock)
 		vsock_remove_sock(vsk);
 }

net/vmw_vsock/virtio_transport_common.c

@@ -829,7 +829,6 @@ void virtio_transport_release(struct vsock_sock *vsk)
 	struct sock *sk = &vsk->sk;
 	bool remove_sock = true;
 
-	lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
 	if (sk->sk_type == SOCK_STREAM)
 		remove_sock = virtio_transport_close(vsk);
 
@@ -837,7 +836,6 @@ void virtio_transport_release(struct vsock_sock *vsk)
 		list_del(&pkt->list);
 		virtio_transport_free_pkt(pkt);
 	}
-	release_sock(sk);
 
 	if (remove_sock)
 		vsock_remove_sock(vsk);