[PATCH v1 1/2] landlock: Fix log_subdomains_off inheritance across fork()

Mickaël Salaün
2026-04-04T08:50:17Z

hook_cred_transfer() only copies the Landlock security blob when the
source credential has a domain. This is inconsistent with
landlock_restrict_self() which can set log_subdomains_off on a
credential without creating a domain (via the ruleset_fd=-1 path): the
field is committed but not preserved across fork() because the child's
prepare_creds() calls hook_cred_transfer() which skips the copy when
domain is NULL.

This breaks the documented use case where a process mutes subdomain logs
before forking sandboxed children: the children lose the muting and
their domains produce unexpected audit records.

Fix this by unconditionally copying the Landlock credential blob.
landlock_get_ruleset(NULL) is already a safe no-op.

Cc: Günther Noack <gnoack@google.com>
Cc: stable@vger.kernel.org
Fixes: ead9079f7569 ("landlock: Add LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
---
security/landlock/cred.c | 6 +-
tools/testing/selftests/landlock/audit_test.c | 88 +++++++++++++++++++
2 files changed, 90 insertions(+), 4 deletions(-)

diff --git a/security/landlock/cred.c b/security/landlock/cred.c
index 0cb3edde4d18..cc419de75cd6 100644
--- a/security/landlock/cred.c
+++ b/security/landlock/cred.c
@@ -22,10 +22,8 @@ static void hook_cred_transfer(struct cred *const new,
const struct landlock_cred_security *const old_llcred =
landlock_cred(old);

- if (old_llcred->domain) {
- landlock_get_ruleset(old_llcred->domain);
- *landlock_cred(new) = *old_llcred;
- }
+ landlock_get_ruleset(old_llcred->domain);
+ *landlock_cred(new) = *old_llcred;
}

static int hook_cred_prepare(struct cred *const new,
diff --git a/tools/testing/selftests/landlock/audit_test.c b/tools/testing/selftests/landlock/audit_test.c
index 46d02d49835a..20099b8667e7 100644
--- a/tools/testing/selftests/landlock/audit_test.c
+++ b/tools/testing/selftests/landlock/audit_test.c
@@ -279,6 +279,94 @@ TEST_F(audit, thread)
&audit_tv_default, sizeof(audit_tv_default)));
}

+/*
+ * Verifies that log_subdomains_off set via the ruleset_fd=-1 path (without
+ * creating a domain) is inherited by children across fork(). This exercises
+ * the hook_cred_transfer() fix: the Landlock credential blob must be copied
+ * even when the source credential has no domain.
+ *
+ * Phase 1 (baseline): a child without muting creates a domain and triggers a
+ * denial that IS logged.
+ *
+ * Phase 2 (after muting): the parent mutes subdomain logs, forks another child
+ * who creates a domain and triggers a denial that is NOT logged.
+ */
+TEST_F(audit, log_subdomains_off_fork)
+{
+ const struct landlock_ruleset_attr ruleset_attr = {
+ .scoped = LANDLOCK_SCOPE_SIGNAL,
+ };
+ struct audit_records records;
+ int ruleset_fd, status;
+ pid_t child;
+
+ ruleset_fd =
+ landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+ ASSERT_LE(0, ruleset_fd);
+
+ ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
+
+ /*
+ * Phase 1: forks a child that creates a domain and triggers a denial
+ * before any muting. This proves the audit path works.
+ */
+ child = fork();
+ ASSERT_LE(0, child);
+ if (child == 0) {
+ ASSERT_EQ(0, landlock_restrict_self(ruleset_fd, 0));
+ ASSERT_EQ(-1, kill(getppid(), 0));
+ ASSERT_EQ(EPERM, errno);
+ _exit(0);
+ return;
+ }
+
+ ASSERT_EQ(child, waitpid(child, &status, 0));
+ ASSERT_EQ(true, WIFEXITED(status));
+ ASSERT_EQ(0, WEXITSTATUS(status));
+
+ /* The denial must be logged (baseline). */
+ EXPECT_EQ(0, matches_log_signal(_metadata, self->audit_fd, getpid(),
+ NULL));
+
+ /* Drains any remaining records (e.g. domain allocation). */
+ EXPECT_EQ(0, audit_count_records(self->audit_fd, &records));
+
+ /*
+ * Mutes subdomain logs without creating a domain. The parent's
+ * credential has domain=NULL and log_subdomains_off=1.
+ */
+ ASSERT_EQ(0, landlock_restrict_self(
+ -1, LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF));
+
+ /*
+ * Phase 2: forks a child that creates a domain and triggers a denial.
+ * Because log_subdomains_off was inherited via fork(), the child's
+ * domain has log_status=LANDLOCK_LOG_DISABLED.
+ */
+ child = fork();
+ ASSERT_LE(0, child);
+ if (child == 0) {
+ ASSERT_EQ(0, landlock_restrict_self(ruleset_fd, 0));
+ ASSERT_EQ(-1, kill(getppid(), 0));
+ ASSERT_EQ(EPERM, errno);
+ _exit(0);
+ return;
+ }
+
+ ASSERT_EQ(child, waitpid(child, &status, 0));
+ ASSERT_EQ(true, WIFEXITED(status));
+ ASSERT_EQ(0, WEXITSTATUS(status));
+
+ /* No denial record should appear. */
+ EXPECT_EQ(-EAGAIN, matches_log_signal(_metadata, self->audit_fd,
+ getpid(), NULL));
+
+ EXPECT_EQ(0, audit_count_records(self->audit_fd, &records));
+ EXPECT_EQ(0, records.access);
+
+ EXPECT_EQ(0, close(ruleset_fd));
+}
+
FIXTURE(audit_flags)
{
struct audit_filter audit_filter;
--
2.53.0