selftests/landlock: Add a new test for setuid()

The new signal_scoping_thread_setuid tests check that the libc's
setuid() function works as expected even when a thread is sandboxed with
scoped signal restrictions.

Before the signal scoping fix, this test would have failed with the
setuid() call:

  [pid    65] getpid()                    = 65
  [pid    65] tgkill(65, 66, SIGRT_1)     = -1 EPERM (Operation not permitted)
  [pid    65] futex(0x40a66cdc, FUTEX_WAKE_PRIVATE, 1) = 0
  [pid    65] setuid(1001)                = 0

After the fix, tgkill(2) is successfully leveraged to synchronize
credentials update across threads:

  [pid    65] getpid()                    = 65
  [pid    65] tgkill(65, 66, SIGRT_1)     = 0
  [pid    66] <... read resumed>0x40a65eb7, 1) = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
  [pid    66] --- SIGRT_1 {si_signo=SIGRT_1, si_code=SI_TKILL, si_pid=65, si_uid=1000} ---
  [pid    66] getpid()                    = 65
  [pid    66] setuid(1001)                = 0
  [pid    66] futex(0x40a66cdc, FUTEX_WAKE_PRIVATE, 1) = 0
  [pid    66] rt_sigreturn({mask=[]})     = 0
  [pid    66] read(3,  <unfinished ...>
  [pid    65] setuid(1001)                = 0

Test coverage for security/landlock is 92.9% of 1137 lines according to
gcc/gcov-14.

Fixes: c899496 ("selftests/landlock: Test signal scoping for threads")
Cc: Günther Noack <gnoack@google.com>
Cc: Tahera Fahimi <fahimitahera@gmail.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20250318161443.279194-8-mic@digikod.net
[mic: Update test coverage]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

tools/testing/selftests/landlock/common.h

@@ -41,6 +41,7 @@ static void _init_caps(struct __test_metadata *const _metadata, bool drop_all)
 		CAP_MKNOD,
 		CAP_NET_ADMIN,
 		CAP_NET_BIND_SERVICE,
+		CAP_SETUID,
 		CAP_SYS_ADMIN,
 		CAP_SYS_CHROOT,
 		/* clang-format on */

tools/testing/selftests/landlock/scoped_signal_test.c

@@ -253,6 +253,7 @@ enum thread_return {
 	THREAD_INVALID = 0,
 	THREAD_SUCCESS = 1,
 	THREAD_ERROR = 2,
+	THREAD_TEST_FAILED = 3,
 };
 
 static void *thread_sync(void *arg)
@@ -316,6 +317,64 @@ TEST(signal_scoping_thread_after)
 	EXPECT_EQ(0, close(thread_pipe[1]));
 }
 
+struct thread_setuid_args {
+	int pipe_read, new_uid;
+};
+
+void *thread_setuid(void *ptr)
+{
+	const struct thread_setuid_args *arg = ptr;
+	char buf;
+
+	if (read(arg->pipe_read, &buf, 1) != 1)
+		return (void *)THREAD_ERROR;
+
+	/* libc's setuid() should update all thread's credentials. */
+	if (getuid() != arg->new_uid)
+		return (void *)THREAD_TEST_FAILED;
+
+	return (void *)THREAD_SUCCESS;
+}
+
+TEST(signal_scoping_thread_setuid)
+{
+	struct thread_setuid_args arg;
+	pthread_t no_sandbox_thread;
+	enum thread_return ret = THREAD_INVALID;
+	int pipe_parent[2];
+	int prev_uid;
+
+	disable_caps(_metadata);
+
+	/* This test does not need to be run as root. */
+	prev_uid = getuid();
+	arg.new_uid = prev_uid + 1;
+	EXPECT_LT(0, arg.new_uid);
+
+	ASSERT_EQ(0, pipe2(pipe_parent, O_CLOEXEC));
+	arg.pipe_read = pipe_parent[0];
+
+	/* Capabilities must be set before creating a new thread. */
+	set_cap(_metadata, CAP_SETUID);
+	ASSERT_EQ(0, pthread_create(&no_sandbox_thread, NULL, thread_setuid,
+				    &arg));
+
+	/* Enforces restriction after creating the thread. */
+	create_scoped_domain(_metadata, LANDLOCK_SCOPE_SIGNAL);
+
+	EXPECT_NE(arg.new_uid, getuid());
+	EXPECT_EQ(0, setuid(arg.new_uid));
+	EXPECT_EQ(arg.new_uid, getuid());
+	EXPECT_EQ(1, write(pipe_parent[1], ".", 1));
+
+	EXPECT_EQ(0, pthread_join(no_sandbox_thread, (void **)&ret));
+	EXPECT_EQ(THREAD_SUCCESS, ret);
+
+	clear_cap(_metadata, CAP_SETUID);
+	EXPECT_EQ(0, close(pipe_parent[0]));
+	EXPECT_EQ(0, close(pipe_parent[1]));
+}
+
 const short backlog = 10;
 
 static volatile sig_atomic_t signal_received;