Re: [PATCH net-next v2] vsock/test: Add test for null ptr deref when transport changes

[Michal Luczaj <mhal@rbox.co>]

On 3/14/25 10:27, Luigi Leonardi wrote:
> Add a new test to ensure that when the transport changes a null pointer
> dereference does not occur[1].
> 
> Note that this test does not fail, but it may hang on the client side if
> it triggers a kernel oops.
> 
> This works by creating a socket, trying to connect to a server, and then
> executing a second connect operation on the same socket but to a
> different CID (0). This triggers a transport change. If the connect
> operation is interrupted by a signal, this could cause a null-ptr-deref.

Just to be clear: that's the splat, right?

Oops: general protection fault, probably for non-canonical address 0xdffffc000000000c: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000060-0x0000000000000067]
CPU: 2 UID: 0 PID: 463 Comm: kworker/2:3 Not tainted
Workqueue: vsock-loopback vsock_loopback_work
RIP: 0010:vsock_stream_has_data+0x44/0x70
Call Trace:
 virtio_transport_do_close+0x68/0x1a0
 virtio_transport_recv_pkt+0x1045/0x2ae4
 vsock_loopback_work+0x27d/0x3f0
 process_one_work+0x846/0x1420
 worker_thread+0x5b3/0xf80
 kthread+0x35a/0x700
 ret_from_fork+0x2d/0x70
 ret_from_fork_asm+0x1a/0x30

> ...
> +static void test_stream_transport_change_client(const struct test_opts *opts)
> +{
> +	__sighandler_t old_handler;
> +	pid_t pid = getpid();
> +	pthread_t thread_id;
> +	time_t tout;
> +
> +	old_handler = signal(SIGUSR1, test_transport_change_signal_handler);
> +	if (old_handler == SIG_ERR) {
> +		perror("signal");
> +		exit(EXIT_FAILURE);
> +	}
> +
> +	if (pthread_create(&thread_id, NULL, test_stream_transport_change_thread, &pid)) {
> +		perror("pthread_create");

Does pthread_create() set errno on failure?

> +		exit(EXIT_FAILURE);
> +	}
> +
> +	tout = current_nsec() + TIMEOUT * NSEC_PER_SEC;

Isn't 10 seconds a bit excessive? I see the oops pretty much immediately.

> +	do {
> +		struct sockaddr_vm sa = {
> +			.svm_family = AF_VSOCK,
> +			.svm_cid = opts->peer_cid,
> +			.svm_port = opts->peer_port,
> +		};
> +		int s;
> +
> +		s = socket(AF_VSOCK, SOCK_STREAM, 0);
> +		if (s < 0) {
> +			perror("socket");
> +			exit(EXIT_FAILURE);
> +		}
> +
> +		connect(s, (struct sockaddr *)&sa, sizeof(sa));
> +
> +		/* Set CID to 0 cause a transport change. */
> +		sa.svm_cid = 0;
> +		connect(s, (struct sockaddr *)&sa, sizeof(sa));
> +
> +		close(s);
> +	} while (current_nsec() < tout);
> +
> +	if (pthread_cancel(thread_id)) {
> +		perror("pthread_cancel");

And errno here.

> +		exit(EXIT_FAILURE);
> +	}
> +
> +	/* Wait for the thread to terminate */
> +	if (pthread_join(thread_id, NULL)) {
> +		perror("pthread_join");

And here.
Aaand I've realized I've made exactly the same mistake elsewhere :)

> ...
> +static void test_stream_transport_change_server(const struct test_opts *opts)
> +{
> +	time_t tout = current_nsec() + TIMEOUT * NSEC_PER_SEC;
> +
> +	do {
> +		int s = vsock_stream_listen(VMADDR_CID_ANY, opts->peer_port);
> +
> +		close(s);
> +	} while (current_nsec() < tout);
> +}

I'm not certain you need to re-create the listener or measure the time
here. What about something like

	int s = vsock_stream_listen(VMADDR_CID_ANY, opts->peer_port);
	control_expectln("DONE");
	close(s);

Thanks,
Michal