vsock: fix the race conditions in multi-transport support

There are multiple similar bugs implicitly introduced by the
commit c0cfa2d8 ("vsock: add multi-transports support") and
commit 6a2c0962 ("vsock: prevent transport modules unloading").

The bug pattern:
 [1] vsock_sock.transport pointer is copied to a local variable,
 [2] lock_sock() is called,
 [3] the local variable is used.
VSOCK multi-transport support introduced the race condition:
vsock_sock.transport value may change between [1] and [2].

Let's copy vsock_sock.transport pointer to local variables after
the lock_sock() call.

Fixes: c0cfa2d8 ("vsock: add multi-transports support")
Signed-off-by: NAlexander Popov <alex.popov@linux.com>
Reviewed-by: NStefano Garzarella <sgarzare@redhat.com>
Reviewed-by: NJorgen Hansen <jhansen@vmware.com>
Link: https://lore.kernel.org/r/20210201084719.2257066-1-alex.popov@linux.comSigned-off-by: NJakub Kicinski <kuba@kernel.org>

net/vmw_vsock/af_vsock.c

@@ -1014,9 +1014,12 @@ static __poll_t vsock_poll(struct file *file, struct socket *sock,
 			mask |= EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND;
 
 	} else if (sock->type == SOCK_STREAM) {
-		const struct vsock_transport *transport = vsk->transport;
+		const struct vsock_transport *transport;
+
 		lock_sock(sk);
 
+		transport = vsk->transport;
+
 		/* Listening sockets that have connections in their accept
 		 * queue can be read.
 		 */
@@ -1099,10 +1102,11 @@ static int vsock_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 	err = 0;
 	sk = sock->sk;
 	vsk = vsock_sk(sk);
-	transport = vsk->transport;
 
 	lock_sock(sk);
 
+	transport = vsk->transport;
+
 	err = vsock_auto_bind(vsk);
 	if (err)
 		goto out;
@@ -1561,10 +1565,11 @@ static int vsock_stream_setsockopt(struct socket *sock,
 	err = 0;
 	sk = sock->sk;
 	vsk = vsock_sk(sk);
-	transport = vsk->transport;
 
 	lock_sock(sk);
 
+	transport = vsk->transport;
+
 	switch (optname) {
 	case SO_VM_SOCKETS_BUFFER_SIZE:
 		COPY_IN(val);
@@ -1697,7 +1702,6 @@ static int vsock_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 
 	sk = sock->sk;
 	vsk = vsock_sk(sk);
-	transport = vsk->transport;
 	total_written = 0;
 	err = 0;
 
@@ -1706,6 +1710,8 @@ static int vsock_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 
 	lock_sock(sk);
 
+	transport = vsk->transport;
+
 	/* Callers should not provide a destination with stream sockets. */
 	if (msg->msg_namelen) {
 		err = sk->sk_state == TCP_ESTABLISHED ? -EISCONN : -EOPNOTSUPP;
@@ -1840,11 +1846,12 @@ vsock_stream_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 
 	sk = sock->sk;
 	vsk = vsock_sk(sk);
-	transport = vsk->transport;
 	err = 0;
 
 	lock_sock(sk);
 
+	transport = vsk->transport;
+
 	if (!transport || sk->sk_state != TCP_ESTABLISHED) {
 		/* Recvmsg is supposed to return 0 if a peer performs an
 		 * orderly shutdown. Differentiate between that case and when a