Re: [PATCH v1 2/2] landlock: Allow TSYNC with LOG_SUBDOMAINS_OFF and fd=-1

["Günther Noack" <gnoack3000@gmail.com>]

Hello!

On Sat, Apr 04, 2026 at 10:49:58AM +0200, Mickaël Salaün wrote:
> LANDLOCK_RESTRICT_SELF_TSYNC does not allow
> LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF with ruleset_fd=-1, preventing
> a multithreaded process from atomically propagating subdomain log muting
> to all threads without creating a domain layer.  Relax the fd=-1
> condition to accept TSYNC alongside LOG_SUBDOMAINS_OFF, and update the
> documentation accordingly.
> 
> Add flag validation tests for all TSYNC combinations with ruleset_fd=-1,
> and audit tests verifying both transition directions: muting via TSYNC
> (logged to not logged) and override via TSYNC (not logged to logged).
> 
> Cc: Günther Noack <gnoack@google.com>
> Cc: stable@vger.kernel.org
> Fixes: 42fc7e6543f6 ("landlock: Multithreading support for landlock_restrict_self()")
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> ---
>  include/uapi/linux/landlock.h                 |   4 +-
>  security/landlock/syscalls.c                  |  14 +-
>  tools/testing/selftests/landlock/audit_test.c | 233 ++++++++++++++++++
>  tools/testing/selftests/landlock/tsync_test.c |  74 ++++++
>  4 files changed, 319 insertions(+), 6 deletions(-)
> 
> diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
> index f88fa1f68b77..d37603efc273 100644
> --- a/include/uapi/linux/landlock.h
> +++ b/include/uapi/linux/landlock.h
> @@ -116,7 +116,9 @@ struct landlock_ruleset_attr {
>   *     ``LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF``, this flag only affects
>   *     future nested domains, not the one being created. It can also be used
>   *     with a @ruleset_fd value of -1 to mute subdomain logs without creating a
> - *     domain.
> + *     domain.  When combined with %LANDLOCK_RESTRICT_SELF_TSYNC and a
> + *     @ruleset_fd value of -1, this configuration is propagated to all threads
> + *     of the current process.
>   *
>   * The following flag supports policy enforcement in multithreaded processes:
>   *
> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> index 0d66a68677b7..a0bb664e0d31 100644
> --- a/security/landlock/syscalls.c
> +++ b/security/landlock/syscalls.c
> @@ -512,10 +512,13 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
>  
>  	/*
>  	 * It is allowed to set LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF with
> -	 * -1 as ruleset_fd, but no other flag must be set.
> +	 * -1 as ruleset_fd, optionally combined with
> +	 * LANDLOCK_RESTRICT_SELF_TSYNC to propagate this configuration to all
> +	 * threads.  No other flag must be set.
>  	 */
>  	if (!(ruleset_fd == -1 &&
> -	      flags == LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF)) {
> +	      (flags & ~LANDLOCK_RESTRICT_SELF_TSYNC) ==
> +		      LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF)) {

Well spotted, thanks!


>  		/* Gets and checks the ruleset. */
>  		ruleset = get_ruleset_from_fd(ruleset_fd, FMODE_CAN_READ);
>  		if (IS_ERR(ruleset))
> @@ -537,9 +540,10 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
>  
>  	/*
>  	 * The only case when a ruleset may not be set is if
> -	 * LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF is set and ruleset_fd is -1.
> -	 * We could optimize this case by not calling commit_creds() if this flag
> -	 * was already set, but it is not worth the complexity.
> +	 * LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF is set (optionally with
> +	 * LANDLOCK_RESTRICT_SELF_TSYNC) and ruleset_fd is -1.  We could
> +	 * optimize this case by not calling commit_creds() if this flag was
> +	 * already set, but it is not worth the complexity.
>  	 */
>  	if (ruleset) {
>  		/*
> diff --git a/tools/testing/selftests/landlock/audit_test.c b/tools/testing/selftests/landlock/audit_test.c
> index 20099b8667e7..a193d8a97560 100644
> --- a/tools/testing/selftests/landlock/audit_test.c
> +++ b/tools/testing/selftests/landlock/audit_test.c
> @@ -162,6 +162,7 @@ TEST_F(audit, layers)
>  struct thread_data {
>  	pid_t parent_pid;
>  	int ruleset_fd, pipe_child, pipe_parent;
> +	bool mute_subdomains;
>  };
>  
>  static void *thread_audit_test(void *arg)
> @@ -367,6 +368,238 @@ TEST_F(audit, log_subdomains_off_fork)
>  	EXPECT_EQ(0, close(ruleset_fd));
>  }
>  
> +/*
> + * Thread function: runs two rounds of (create domain, trigger denial, signal
> + * back), waiting for the main thread before each round.  When mute_subdomains
> + * is set, phase 1 also mutes subdomain logs via the fd=-1 path before creating
> + * the domain.  The ruleset_fd is kept open across both rounds so each
> + * restrict_self call stacks a new domain layer.
> + */
> +static void *thread_sandbox_deny_twice(void *arg)
> +{
> +	const struct thread_data *data = (struct thread_data *)arg;
> +	uintptr_t err = 0;
> +	char buffer;
> +
> +	/* Phase 1: optionally mutes, creates a domain, and triggers a denial. */
> +	if (read(data->pipe_parent, &buffer, 1) != 1) {
> +		err = 1;
> +		goto out;
> +	}
> +
> +	if (data->mute_subdomains &&
> +	    landlock_restrict_self(-1,
> +				   LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF)) {
> +		err = 2;
> +		goto out;
> +	}
> +
> +	if (landlock_restrict_self(data->ruleset_fd, 0)) {
> +		err = 3;
> +		goto out;
> +	}
> +
> +	if (kill(data->parent_pid, 0) != -1 || errno != EPERM) {
> +		err = 4;
> +		goto out;
> +	}
> +
> +	if (write(data->pipe_child, ".", 1) != 1) {
> +		err = 5;
> +		goto out;
> +	}
> +
> +	/* Phase 2: stacks another domain and triggers a denial. */
> +	if (read(data->pipe_parent, &buffer, 1) != 1) {
> +		err = 6;
> +		goto out;
> +	}
> +
> +	if (landlock_restrict_self(data->ruleset_fd, 0)) {
> +		err = 7;
> +		goto out;
> +	}
> +
> +	if (kill(data->parent_pid, 0) != -1 || errno != EPERM) {
> +		err = 8;
> +		goto out;
> +	}
> +
> +	if (write(data->pipe_child, ".", 1) != 1) {
> +		err = 9;
> +		goto out;
> +	}
> +
> +out:
> +	close(data->ruleset_fd);
> +	close(data->pipe_child);
> +	close(data->pipe_parent);
> +	return (void *)err;
> +}
> +
> +/*
> + * Verifies that LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF with
> + * LANDLOCK_RESTRICT_SELF_TSYNC and ruleset_fd=-1 propagates log_subdomains_off
> + * to a sibling thread, suppressing audit logging on domains it subsequently
> + * creates.
> + *
> + * Phase 1 (before TSYNC) acts as an inline baseline: the sibling creates a
> + * domain and triggers a denial that IS logged.
> + *
> + * Phase 2 (after TSYNC) verifies suppression: the sibling stacks another domain
> + * and triggers a denial that is NOT logged.
> + */
> +TEST_F(audit, log_subdomains_off_tsync)
> +{
> +	const struct landlock_ruleset_attr ruleset_attr = {
> +		.scoped = LANDLOCK_SCOPE_SIGNAL,
> +	};
> +	struct audit_records records;
> +	struct thread_data child_data;

The child_data.mute_subdomains field stays uninitialized in this
function (and maybe others).  Please fix.

   struct thread_data child_data = {};


> +	int pipe_child[2], pipe_parent[2];
> +	char buffer;
> +	pthread_t thread;
> +	void *thread_ret;
> +
> +	child_data.parent_pid = getppid();
> +	ASSERT_EQ(0, pipe2(pipe_child, O_CLOEXEC));
> +	child_data.pipe_child = pipe_child[1];
> +	ASSERT_EQ(0, pipe2(pipe_parent, O_CLOEXEC));
> +	child_data.pipe_parent = pipe_parent[0];
> +	child_data.ruleset_fd =
> +		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
> +	ASSERT_LE(0, child_data.ruleset_fd);
> +
> +	ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
> +
> +	/* Creates the sibling thread. */
> +	ASSERT_EQ(0, pthread_create(&thread, NULL, thread_sandbox_deny_twice,
> +				    &child_data));
> +
> +	/*
> +	 * Phase 1: the sibling creates a domain and triggers a denial before
> +	 * any log muting.  This proves the audit path works.
> +	 */
> +	ASSERT_EQ(1, write(pipe_parent[1], ".", 1));
> +	ASSERT_EQ(1, read(pipe_child[0], &buffer, 1));
> +
> +	/* The denial must be logged. */
> +	EXPECT_EQ(0, matches_log_signal(_metadata, self->audit_fd,
> +					child_data.parent_pid, NULL));
> +
> +	/* Drains any remaining records (e.g. domain allocation). */
> +	EXPECT_EQ(0, audit_count_records(self->audit_fd, &records));
> +
> +	/*
> +	 * Mutes subdomain logs and propagates to the sibling thread via TSYNC,
> +	 * without creating a domain.
> +	 */
> +	ASSERT_EQ(0, landlock_restrict_self(
> +			     -1, LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF |
> +					 LANDLOCK_RESTRICT_SELF_TSYNC));
> +
> +	/*
> +	 * Phase 2: the sibling stacks another domain and triggers a denial.
> +	 * Because log_subdomains_off was propagated via TSYNC, the new domain
> +	 * has log_status=LANDLOCK_LOG_DISABLED.
> +	 */
> +	ASSERT_EQ(1, write(pipe_parent[1], ".", 1));
> +	ASSERT_EQ(1, read(pipe_child[0], &buffer, 1));
> +
> +	/* No denial record should appear. */
> +	EXPECT_EQ(-EAGAIN, matches_log_signal(_metadata, self->audit_fd,
> +					      child_data.parent_pid, NULL));
> +
> +	EXPECT_EQ(0, audit_count_records(self->audit_fd, &records));
> +	EXPECT_EQ(0, records.access);
> +
> +	EXPECT_EQ(0, close(pipe_child[0]));
> +	EXPECT_EQ(0, close(pipe_parent[1]));
> +	ASSERT_EQ(0, pthread_join(thread, &thread_ret));
> +	EXPECT_EQ(NULL, thread_ret);
> +}
> +
> +/*
> + * Verifies that LANDLOCK_RESTRICT_SELF_TSYNC without
> + * LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF overrides a sibling thread's
> + * log_subdomains_off, re-enabling audit logging on domains the sibling
> + * subsequently creates.
> + *
> + * Phase 1: the sibling sets log_subdomains_off, creates a muted domain, and
> + * triggers a denial that is NOT logged.
> + *
> + * Phase 2 (after TSYNC without LOG_SUBDOMAINS_OFF): the sibling stacks another
> + * domain and triggers a denial that IS logged, proving the muting was
> + * overridden.
> + */
> +TEST_F(audit, tsync_override_log_subdomains_off)
> +{
> +	const struct landlock_ruleset_attr ruleset_attr = {
> +		.scoped = LANDLOCK_SCOPE_SIGNAL,
> +	};
> +	struct audit_records records;
> +	struct thread_data child_data;
> +	int pipe_child[2], pipe_parent[2];
> +	char buffer;
> +	pthread_t thread;
> +	void *thread_ret;
> +
> +	child_data.parent_pid = getppid();
> +	ASSERT_EQ(0, pipe2(pipe_child, O_CLOEXEC));
> +	child_data.pipe_child = pipe_child[1];
> +	ASSERT_EQ(0, pipe2(pipe_parent, O_CLOEXEC));
> +	child_data.pipe_parent = pipe_parent[0];
> +	child_data.ruleset_fd =
> +		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
> +	ASSERT_LE(0, child_data.ruleset_fd);
> +
> +	ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
> +
> +	child_data.mute_subdomains = true;
> +
> +	/* Creates the sibling thread. */
> +	ASSERT_EQ(0, pthread_create(&thread, NULL, thread_sandbox_deny_twice,
> +				    &child_data));
> +
> +	/*
> +	 * Phase 1: the sibling mutes subdomain logs, creates a domain, and
> +	 * triggers a denial.  The denial must not be logged.
> +	 */
> +	ASSERT_EQ(1, write(pipe_parent[1], ".", 1));
> +	ASSERT_EQ(1, read(pipe_child[0], &buffer, 1));
> +
> +	EXPECT_EQ(-EAGAIN, matches_log_signal(_metadata, self->audit_fd,
> +					      child_data.parent_pid, NULL));
> +
> +	/* Drains any remaining records. */
> +	EXPECT_EQ(0, audit_count_records(self->audit_fd, &records));
> +	EXPECT_EQ(0, records.access);
> +
> +	/*
> +	 * Overrides the sibling's log_subdomains_off by calling TSYNC without
> +	 * LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF.
> +	 */
> +	ASSERT_EQ(0, landlock_restrict_self(child_data.ruleset_fd,
> +					    LANDLOCK_RESTRICT_SELF_TSYNC));
> +
> +	/*
> +	 * Phase 2: the sibling stacks another domain and triggers a denial.
> +	 * Because TSYNC replaced its log_subdomains_off with 0, the new domain
> +	 * has log_status=LANDLOCK_LOG_PENDING.
> +	 */
> +	ASSERT_EQ(1, write(pipe_parent[1], ".", 1));
> +	ASSERT_EQ(1, read(pipe_child[0], &buffer, 1));
> +
> +	/* The denial must be logged. */
> +	EXPECT_EQ(0, matches_log_signal(_metadata, self->audit_fd,
> +					child_data.parent_pid, NULL));
> +
> +	EXPECT_EQ(0, close(pipe_child[0]));
> +	EXPECT_EQ(0, close(pipe_parent[1]));
> +	ASSERT_EQ(0, pthread_join(thread, &thread_ret));
> +	EXPECT_EQ(NULL, thread_ret);
> +}
> +
>  FIXTURE(audit_flags)
>  {
>  	struct audit_filter audit_filter;
> diff --git a/tools/testing/selftests/landlock/tsync_test.c b/tools/testing/selftests/landlock/tsync_test.c
> index 2b9ad4f154f4..abc290271a1a 100644
> --- a/tools/testing/selftests/landlock/tsync_test.c
> +++ b/tools/testing/selftests/landlock/tsync_test.c
> @@ -247,4 +247,78 @@ TEST(tsync_interrupt)
>  	EXPECT_EQ(0, close(ruleset_fd));
>  }
>  
> +/* clang-format off */
> +FIXTURE(tsync_without_ruleset) {};
> +/* clang-format on */
> +
> +FIXTURE_VARIANT(tsync_without_ruleset)
> +{
> +	const __u32 flags;
> +	const int expected_errno;
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(tsync_without_ruleset, tsync_only) {
> +	/* clang-format on */
> +	.flags = LANDLOCK_RESTRICT_SELF_TSYNC,
> +	.expected_errno = EBADF,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(tsync_without_ruleset, subdomains_off_same_exec_off) {
> +	/* clang-format on */
> +	.flags = LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF |
> +		 LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF |
> +		 LANDLOCK_RESTRICT_SELF_TSYNC,
> +	.expected_errno = EBADF,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(tsync_without_ruleset, subdomains_off_new_exec_on) {
> +	/* clang-format on */
> +	.flags = LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF |
> +		 LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON |
> +		 LANDLOCK_RESTRICT_SELF_TSYNC,
> +	.expected_errno = EBADF,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(tsync_without_ruleset, all_flags) {
> +	/* clang-format on */
> +	.flags = LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF |
> +		 LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON |
> +		 LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF |
> +		 LANDLOCK_RESTRICT_SELF_TSYNC,
> +	.expected_errno = EBADF,
> +};
> +
> +/* clang-format off */
> +FIXTURE_VARIANT_ADD(tsync_without_ruleset, subdomains_off) {
> +	/* clang-format on */
> +	.flags = LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF |
> +		 LANDLOCK_RESTRICT_SELF_TSYNC,
> +	.expected_errno = 0,
> +};
> +
> +FIXTURE_SETUP(tsync_without_ruleset)
> +{
> +}
> +
> +FIXTURE_TEARDOWN(tsync_without_ruleset)
> +{
> +}
> +
> +TEST_F(tsync_without_ruleset, check)
> +{
> +	int ret;
> +
> +	ret = landlock_restrict_self(-1, variant->flags);
> +	if (variant->expected_errno) {
> +		EXPECT_EQ(-1, ret);
> +		EXPECT_EQ(variant->expected_errno, errno);
> +	} else {
> +		EXPECT_EQ(0, ret);
> +	}
> +}

We are not setting the no_new_privs flag in this test, as we do in the
others.

no_new_privs or CAP_SYS_ADMIN are required in the implementation, even
when ruleset_fd == -1 and passing
LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF.

> +
>  TEST_HARNESS_MAIN
> -- 
> 2.53.0
> 

Reviewed-by: Günther Noack <gnoack3000@gmail.com>

But please fix the flaky test.

–Günther