[PATCH v5 2/2] selftests/landlock: test SCOPE_SIGNAL on the SIGIO/fowner pgid path

[Bryam Vargas <hexlabsecurity@proton.me>]

Add regression tests for the LANDLOCK_SCOPE_SIGNAL handling of the
asynchronous SIGIO delivery path (fcntl(F_SETOWN)) with a process-group
owner.

sigio_to_pgid_members covers the bypass: a sandboxed process at the head
of its process group's PID hlist (the default after fork()) arms
F_SETOWN(-pgrp) + O_ASYNC and triggers the fan-out; the in-domain owner
must be signaled (proving the trigger fired) while the non-sandboxed
member of the group, outside the domain, must not.

sigio_to_pgid_self covers the same-process guarantee: the owner is
registered from a sandboxed non-leader thread, whose domain differs from
the thread-group leader the kernel signals for a process-group owner.
That leader belongs to the owner's own process and must still be signaled.

Without the fix the first test sees the out-of-domain member signaled and
the second sees the owner's own leader denied.

Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
---
 .../selftests/landlock/scoped_signal_test.c   | 183 ++++++++++++++++++
 1 file changed, 183 insertions(+)

diff --git a/tools/testing/selftests/landlock/scoped_signal_test.c b/tools/testing/selftests/landlock/scoped_signal_test.c
index d8bf33417619..4359e0262dcf 100644
--- a/tools/testing/selftests/landlock/scoped_signal_test.c
+++ b/tools/testing/selftests/landlock/scoped_signal_test.c
@@ -559,4 +559,187 @@ TEST_F(fown, sigurg_socket)
 		_metadata->exit_code = KSFT_FAIL;
 }
 
+/*
+ * Checks that LANDLOCK_SCOPE_SIGNAL is enforced on the asynchronous SIGIO
+ * delivery path (fcntl(F_SETOWN)) when the file owner is a process group.
+ *
+ * A sandboxed process sitting at the head of its process group's PID hlist
+ * (the default position right after fork()) used to escape the
+ * fcntl(F_SETOWN, -pgrp) domain recording: pid_task(pgrp, PIDTYPE_PGID)
+ * resolved to the process itself, so the same-thread-group exemption skipped
+ * recording its Landlock domain.  At SIGIO time that domain was then unset and
+ * the signal fanned out to every group member, including non-sandboxed
+ * processes outside the domain.
+ */
+TEST(sigio_to_pgid_members)
+{
+	int trigger[2], sync_child[2];
+	char buf;
+	pid_t child;
+	int status, i;
+
+	drop_caps(_metadata);
+
+	/*
+	 * Isolates the test in its own process group so the SIGIO fan-out stays
+	 * bounded to this parent and the child forked below.
+	 */
+	ASSERT_EQ(0, setpgid(0, 0));
+
+	/* The non-sandboxed parent is the protected (out-of-domain) target. */
+	ASSERT_EQ(0, setup_signal_handler(SIGURG));
+	signal_received = 0;
+
+	ASSERT_EQ(0, pipe2(trigger, O_CLOEXEC));
+	ASSERT_EQ(0, pipe2(sync_child, O_CLOEXEC));
+
+	child = fork();
+	ASSERT_LE(0, child);
+	if (child == 0) {
+		/*
+		 * The child inherits the parent's new process group and, just
+		 * attached with hlist_add_head_rcu(), is now the head of the
+		 * pgid hlist: this is the case that used to skip the recording.
+		 */
+		EXPECT_EQ(0, close(sync_child[0]));
+
+		/* In-domain positive control: the child must be signaled. */
+		ASSERT_EQ(0, setup_signal_handler(SIGURG));
+		signal_received = 0;
+
+		create_scoped_domain(_metadata, LANDLOCK_SCOPE_SIGNAL);
+
+		/* Owns the SIGIO source for the whole process group. */
+		ASSERT_EQ(0, fcntl(trigger[0], F_SETSIG, SIGURG));
+		ASSERT_EQ(0, fcntl(trigger[0], F_SETOWN, -getpgrp()));
+		ASSERT_EQ(0, fcntl(trigger[0], F_SETFL, O_ASYNC));
+
+		/* Fans SIGURG out to every member of the process group. */
+		ASSERT_EQ(1, write(trigger[1], ".", 1));
+
+		/*
+		 * The sandboxed child is in its own domain and must always be
+		 * signaled: this proves the SIGIO actually fired.
+		 */
+		for (i = 0; i < 1000 && !signal_received; i++)
+			usleep(1000);
+		EXPECT_EQ(1, signal_received);
+
+		ASSERT_EQ(1, write(sync_child[1], ".", 1));
+		EXPECT_EQ(0, close(sync_child[1]));
+
+		_exit(_metadata->exit_code);
+		return;
+	}
+	EXPECT_EQ(0, close(sync_child[1]));
+	EXPECT_EQ(0, close(trigger[0]));
+	EXPECT_EQ(0, close(trigger[1]));
+
+	/* Waits for the child to generate the SIGIO. */
+	ASSERT_EQ(1, read(sync_child[0], &buf, 1));
+	EXPECT_EQ(0, close(sync_child[0]));
+
+	/* Lets a delivered-but-pending signal run our handler, if any. */
+	for (i = 0; i < 100 && !signal_received; i++)
+		usleep(1000);
+
+	/*
+	 * SCOPE_SIGNAL must block the fan-out to this non-sandboxed parent,
+	 * which is outside the child's Landlock domain.  Before the fix the
+	 * parent was signaled here.
+	 */
+	EXPECT_EQ(0, signal_received);
+
+	ASSERT_EQ(child, waitpid(child, &status, 0));
+	if (WIFSIGNALED(status) || !WIFEXITED(status) ||
+	    WEXITSTATUS(status) != EXIT_SUCCESS)
+		_metadata->exit_code = KSFT_FAIL;
+}
+
+static void *thread_setown_scoped(void *arg)
+{
+	const int fd = *(int *)arg;
+	int ruleset_fd;
+	const struct landlock_ruleset_attr ruleset_attr = {
+		.scoped = LANDLOCK_SCOPE_SIGNAL,
+	};
+
+	/* Sandboxes only this non-leader thread (no thread syncing). */
+	ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+	if (ruleset_fd < 0)
+		return (void *)THREAD_ERROR;
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) ||
+	    landlock_restrict_self(ruleset_fd, 0)) {
+		close(ruleset_fd);
+		return (void *)THREAD_ERROR;
+	}
+	close(ruleset_fd);
+
+	/* Makes this process group own the SIGIO source. */
+	if (fcntl(fd, F_SETSIG, SIGURG) || fcntl(fd, F_SETOWN, -getpgrp()) ||
+	    fcntl(fd, F_SETFL, O_ASYNC))
+		return (void *)THREAD_ERROR;
+
+	return (void *)THREAD_SUCCESS;
+}
+
+/*
+ * Checks that the SIGIO fan-out is still delivered to the file owner's own
+ * process when fcntl(F_SETOWN, -pgrp) was issued from a sandboxed non-leader
+ * thread.
+ *
+ * The Landlock domain is recorded for a process-group owner (so out-of-domain
+ * members stay blocked, see sigio_to_pgid_members), but the kernel signals a
+ * process group through its members' thread-group leaders.  Here the leader is
+ * not sandboxed and thus has a different domain than the registering thread, so
+ * the registration-time check cannot tell that it belongs to the owner's own
+ * process.  hook_file_send_sigiotask() must recognize it through the recorded
+ * thread group and allow the delivery, matching the same-process guarantee of
+ * commit 18eb75f3af40.  Without that exemption the leader is wrongly denied and
+ * never signaled.
+ */
+TEST(sigio_to_pgid_self)
+{
+	int trigger[2];
+	pthread_t thread;
+	enum thread_return ret = THREAD_INVALID;
+	int i;
+
+	drop_caps(_metadata);
+
+	/* Bounds the SIGIO fan-out to this process. */
+	ASSERT_EQ(0, setpgid(0, 0));
+
+	/* The non-sandboxed thread-group leader is the SIGIO target. */
+	ASSERT_EQ(0, setup_signal_handler(SIGURG));
+	signal_received = 0;
+
+	ASSERT_EQ(0, pipe2(trigger, O_CLOEXEC));
+
+	/*
+	 * Registers the process-group fowner from a sibling thread that
+	 * sandboxes only itself, so its domain differs from the leader's.
+	 */
+	ASSERT_EQ(0, pthread_create(&thread, NULL, thread_setown_scoped,
+				    &trigger[0]));
+	ASSERT_EQ(0, pthread_join(thread, (void **)&ret));
+	ASSERT_EQ(THREAD_SUCCESS, ret);
+
+	/* Fans SIGURG out to the process group. */
+	ASSERT_EQ(1, write(trigger[1], ".", 1));
+
+	for (i = 0; i < 1000 && !signal_received; i++)
+		usleep(1000);
+
+	/*
+	 * Same-process delivery must always be allowed, even though the owner
+	 * was registered from a sandboxed sibling thread.
+	 */
+	EXPECT_EQ(1, signal_received);
+
+	EXPECT_EQ(0, close(trigger[0]));
+	EXPECT_EQ(0, close(trigger[1]));
+}
+
 TEST_HARNESS_MAIN
-- 
2.43.0