Subject: Re: [RFC PATCH v3 14/19] selftests/landlock: Test socketpair(2) restriction

From: "Günther Noack" <[email-masked]>
Date: Wed, 18 Sep 2024 15:47:26 +0200

On Wed, Sep 04, 2024 at 06:48:19PM +0800, Mikhail Ivanov wrote:
> Add test that checks the restriction on socket creation using
> socketpair(2).
> 
> Add `socket_creation` fixture to configure sandboxing in tests in
> which different socket creation actions are tested.
> 
> Signed-off-by: Mikhail Ivanov <[email-masked]>
> ---
>  .../testing/selftests/landlock/socket_test.c  | 101 ++++++++++++++++++
>  1 file changed, 101 insertions(+)
> 
> diff --git a/tools/testing/selftests/landlock/socket_test.c b/tools/testing/selftests/landlock/socket_test.c
> index 8fc507bf902a..67db0e1c1121 100644
> --- a/tools/testing/selftests/landlock/socket_test.c
> +++ b/tools/testing/selftests/landlock/socket_test.c
> @@ -738,4 +738,105 @@ TEST_F(packet_protocol, alias_restriction)
>  	EXPECT_EQ(0, test_socket_variant(&self->prot_tested));
>  }
>  
> +static int test_socketpair(int family, int type, int protocol)
> +{
> +	int fds[2];
> +	int err;
> +
> +	err = socketpair(family, type | SOCK_CLOEXEC, protocol, fds);
> +	if (err)
> +		return errno;
> +	/*
> +	 * Mixing error codes from close(2) and socketpair(2) should not lead to
> +	 * any (access type) confusion for this test.
> +	 */
> +	if (close(fds[0]) != 0)
> +		return errno;
> +	if (close(fds[1]) != 0)
> +		return errno;
> +	return 0;
> +}
> +
> +FIXTURE(socket_creation)
> +{
> +	bool sandboxed;
> +	bool allowed;
> +};
> +
> +FIXTURE_VARIANT(socket_creation)
> +{
> +	bool sandboxed;
> +	bool allowed;
> +};
> +
> +FIXTURE_SETUP(socket_creation)
> +{
> +	self->sandboxed = variant->sandboxed;
> +	self->allowed = variant->allowed;
> +
> +	setup_loopback(_metadata);
> +};
> +
> +FIXTURE_TEARDOWN(socket_creation)
> +{
> +}
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(socket_creation, no_sandbox) {
> +	/* clang-format on */
> +	.sandboxed = false,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(socket_creation, sandbox_allow) {
> +	/* clang-format on */
> +	.sandboxed = true,
> +	.allowed = true,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(socket_creation, sandbox_deny) {
> +	/* clang-format on */
> +	.sandboxed = true,
> +	.allowed = false,
> +};
> +
> +TEST_F(socket_creation, socketpair)
> +{
> +	const struct landlock_ruleset_attr ruleset_attr = {
> +		.handled_access_socket = LANDLOCK_ACCESS_SOCKET_CREATE,
> +	};
> +	struct landlock_socket_attr unix_socket_create = {
> +		.allowed_access = LANDLOCK_ACCESS_SOCKET_CREATE,
> +		.family = AF_UNIX,
> +		.type = SOCK_STREAM,
> +	};
> +	int ruleset_fd;
> +
> +	if (self->sandboxed) {
> +		ruleset_fd = landlock_create_ruleset(&ruleset_attr,
> +						     sizeof(ruleset_attr), 0);
> +		ASSERT_LE(0, ruleset_fd);
> +
> +		if (self->allowed) {
> +			ASSERT_EQ(0, landlock_add_rule(ruleset_fd,
> +						       LANDLOCK_RULE_SOCKET,
> +						       &unix_socket_create, 0));
> +		}
> +		enforce_ruleset(_metadata, ruleset_fd);
> +		ASSERT_EQ(0, close(ruleset_fd));
> +	}
> +
> +	if (!self->sandboxed || self->allowed) {
> +		/*
> +		 * Tries to create sockets when ruleset is not established
> +		 * or protocol is allowed.
> +		 */
> +		EXPECT_EQ(0, test_socketpair(AF_UNIX, SOCK_STREAM, 0));
> +	} else {
> +		/* Tries to create sockets when protocol is restricted. */
> +		EXPECT_EQ(EACCES, test_socketpair(AF_UNIX, SOCK_STREAM, 0));
> +	}

I am torn on whether socketpair() should be denied at all --

  * on one hand, the created sockets are connected to each other
    and the creating process can only talk to itself (or pass one of them on),
    which seems legitimate and harmless.

  * on the other hand, it *does* create two sockets, and
    if they are datagram sockets, it it probably currently possible
    to disassociate them with connect(AF_UNSPEC).

What are your thoughts on that?

Mickaël, I believe we have also discussed similar questions for pipe(2) in the
past, and you had opinions on that?

(On a much more technical note; consider replacing self->allowed with
self->socketpair_error to directly indicate the expected error? It feels that
this could be more straightforward?)

—Günther