vsock: fix lock inversion in vsock_assign_transport()

Syzbot reported a potential lock inversion deadlock between
vsock_register_mutex and sk_lock-AF_VSOCK when vsock_linger() is called.

The issue was introduced by commit 687aa0c ("vsock: Fix
transport_* TOCTOU") which added vsock_register_mutex locking in
vsock_assign_transport() around the transport->release() call, that can
call vsock_linger(). vsock_assign_transport() can be called with sk_lock
held. vsock_linger() calls sk_wait_event() that temporarily releases and
re-acquires sk_lock. During this window, if another thread hold
vsock_register_mutex while trying to acquire sk_lock, a circular
dependency is created.

Fix this by releasing vsock_register_mutex before calling
transport->release() and vsock_deassign_transport(). This is safe
because we don't need to hold vsock_register_mutex while releasing the
old transport, and we ensure the new transport won't disappear by
obtaining a module reference first via try_module_get().

Reported-by: [email protected]
Tested-by: [email protected]
Fixes: 687aa0c ("vsock: Fix transport_* TOCTOU")
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Stefano Garzarella <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>

Author: stefano-garzarella

net/vmw_vsock/af_vsock.c

@@ -487,12 +487,26 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
 		goto err;
 	}
 
-	if (vsk->transport) {
-		if (vsk->transport == new_transport) {
-			ret = 0;
-			goto err;
-		}
+	if (vsk->transport && vsk->transport == new_transport) {
+		ret = 0;
+		goto err;
+	}
 
+	/* We increase the module refcnt to prevent the transport unloading
+	 * while there are open sockets assigned to it.
+	 */
+	if (!new_transport || !try_module_get(new_transport->module)) {
+		ret = -ENODEV;
+		goto err;
+	}
+
+	/* It's safe to release the mutex after a successful try_module_get().
+	 * Whichever transport `new_transport` points at, it won't go away until
+	 * the last module_put() below or in vsock_deassign_transport().
+	 */
+	mutex_unlock(&vsock_register_mutex);
+
+	if (vsk->transport) {
 		/* transport->release() must be called with sock lock acquired.
 		 * This path can only be taken during vsock_connect(), where we
 		 * have already held the sock lock. In the other cases, this
@@ -512,20 +526,6 @@ int vsock_assign_transport(struct vsock_sock *vsk, struct vsock_sock *psk)
 		vsk->peer_shutdown = 0;
 	}
 
-	/* We increase the module refcnt to prevent the transport unloading
-	 * while there are open sockets assigned to it.
-	 */
-	if (!new_transport || !try_module_get(new_transport->module)) {
-		ret = -ENODEV;
-		goto err;
-	}
-
-	/* It's safe to release the mutex after a successful try_module_get().
-	 * Whichever transport `new_transport` points at, it won't go away until
-	 * the last module_put() below or in vsock_deassign_transport().
-	 */
-	mutex_unlock(&vsock_register_mutex);
-
 	if (sk->sk_type == SOCK_SEQPACKET) {
 		if (!new_transport->seqpacket_allow ||
 		    !new_transport->seqpacket_allow(remote_cid)) {