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

Mickaël Salaün
2026-04-04T08:56:43Z

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)) {
/* 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;
+ 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);
+ }
+}
+
TEST_HARNESS_MAIN
--
2.53.0