landlock: Allow TSYNC with LOG_SUBDOMAINS_OFF and fd=-1

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: 42fc7e6 ("landlock: Multithreading support for landlock_restrict_self()")
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260407164107.2012589-2-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

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:
  *

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) {
 		/*

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;

tools/testing/selftests/landlock/tsync_test.c

@@ -247,4 +247,81 @@ 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)
+{
+	disable_caps(_metadata);
+}
+
+FIXTURE_TEARDOWN(tsync_without_ruleset)
+{
+}
+
+TEST_F(tsync_without_ruleset, check)
+{
+	int ret;
+
+	ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
+
+	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