selftests/landlock: Add cred_transfer test

Check that keyctl(KEYCTL_SESSION_TO_PARENT) preserves the parent's
restrictions.

Fixes: e1199815 ("selftests/landlock: Add user space tests")
Co-developed-by: Jann Horn <jannh@google.com>
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20240724.Ood5aige9she@digikod.netSigned-off-by: Mickaël Salaün <mic@digikod.net>

tools/testing/selftests/landlock/base_test.c

@@ -9,6 +9,7 @@
 #define _GNU_SOURCE
 #include <errno.h>
 #include <fcntl.h>
+#include <linux/keyctl.h>
 #include <linux/landlock.h>
 #include <string.h>
 #include <sys/prctl.h>
@@ -326,4 +327,77 @@ TEST(ruleset_fd_transfer)
 	ASSERT_EQ(EXIT_SUCCESS, WEXITSTATUS(status));
 }
 
+TEST(cred_transfer)
+{
+	struct landlock_ruleset_attr ruleset_attr = {
+		.handled_access_fs = LANDLOCK_ACCESS_FS_READ_DIR,
+	};
+	int ruleset_fd, dir_fd;
+	pid_t child;
+	int status;
+
+	drop_caps(_metadata);
+
+	dir_fd = open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+	EXPECT_LE(0, dir_fd);
+	EXPECT_EQ(0, close(dir_fd));
+
+	/* Denies opening directories. */
+	ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+	ASSERT_LE(0, ruleset_fd);
+	EXPECT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0));
+	ASSERT_EQ(0, landlock_restrict_self(ruleset_fd, 0));
+	EXPECT_EQ(0, close(ruleset_fd));
+
+	/* Checks ruleset enforcement. */
+	EXPECT_EQ(-1, open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC));
+	EXPECT_EQ(EACCES, errno);
+
+	/* Needed for KEYCTL_SESSION_TO_PARENT permission checks */
+	EXPECT_NE(-1, syscall(__NR_keyctl, KEYCTL_JOIN_SESSION_KEYRING, NULL, 0,
+			      0, 0))
+	{
+		TH_LOG("Failed to join session keyring: %s", strerror(errno));
+	}
+
+	child = fork();
+	ASSERT_LE(0, child);
+	if (child == 0) {
+		/* Checks ruleset enforcement. */
+		EXPECT_EQ(-1, open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC));
+		EXPECT_EQ(EACCES, errno);
+
+		/*
+		 * KEYCTL_SESSION_TO_PARENT is a no-op unless we have a
+		 * different session keyring in the child, so make that happen.
+		 */
+		EXPECT_NE(-1, syscall(__NR_keyctl, KEYCTL_JOIN_SESSION_KEYRING,
+				      NULL, 0, 0, 0));
+
+		/*
+		 * KEYCTL_SESSION_TO_PARENT installs credentials on the parent
+		 * that never go through the cred_prepare hook, this path uses
+		 * cred_transfer instead.
+		 */
+		EXPECT_EQ(0, syscall(__NR_keyctl, KEYCTL_SESSION_TO_PARENT, 0,
+				     0, 0, 0));
+
+		/* Re-checks ruleset enforcement. */
+		EXPECT_EQ(-1, open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC));
+		EXPECT_EQ(EACCES, errno);
+
+		_exit(_metadata->exit_code);
+		return;
+	}
+
+	EXPECT_EQ(child, waitpid(child, &status, 0));
+	EXPECT_EQ(1, WIFEXITED(status));
+	EXPECT_EQ(EXIT_SUCCESS, WEXITSTATUS(status));
+
+	/* Re-checks ruleset enforcement. */
+	EXPECT_EQ(-1, open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC));
+	EXPECT_EQ(EACCES, errno);
+}
+
 TEST_HARNESS_MAIN

tools/testing/selftests/landlock/config

@@ -2,6 +2,7 @@ CONFIG_CGROUPS=y
 CONFIG_CGROUP_SCHED=y
 CONFIG_INET=y
 CONFIG_IPV6=y
+CONFIG_KEYS=y
 CONFIG_NET=y
 CONFIG_NET_NS=y
 CONFIG_OVERLAY_FS=y