selftests/landlock: Add layout1.umount_sandboxer tests

Check that a domain is not tied to the executable file that created it.
For instance, that could happen if a Landlock domain took a reference to
a struct path.

Move global path names to common.h and replace copy_binary() with a more
generic copy_file() helper.

Test coverage for security/landlock is 92.7% of 1133 lines according to
gcc/gcov-14.

Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250108154338.1129069-23-mic@digikod.net
[mic: Update date and add test coverage]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

tools/testing/selftests/landlock/Makefile

@@ -10,7 +10,7 @@ src_test := $(wildcard *_test.c)
 
 TEST_GEN_PROGS := $(src_test:.c=)
 
-TEST_GEN_PROGS_EXTENDED := true
+TEST_GEN_PROGS_EXTENDED := true sandbox-and-launch wait-pipe
 
 # Short targets:
 $(TEST_GEN_PROGS): LDLIBS += -lcap -lpthread

tools/testing/selftests/landlock/common.h

@@ -28,6 +28,9 @@
 /* TEST_F_FORK() should not be used for new tests. */
 #define TEST_F_FORK(fixture_name, test_name) TEST_F(fixture_name, test_name)
 
+static const char bin_sandbox_and_launch[] = "./sandbox-and-launch";
+static const char bin_wait_pipe[] = "./wait-pipe";
+
 static void _init_caps(struct __test_metadata *const _metadata, bool drop_all)
 {
 	cap_t cap_p;

tools/testing/selftests/landlock/fs_test.c

@@ -59,7 +59,7 @@ int open_tree(int dfd, const char *filename, unsigned int flags)
 #define RENAME_EXCHANGE (1 << 1)
 #endif
 
-#define BINARY_PATH "./true"
+static const char bin_true[] = "./true";
 
 /* Paths (sibling number and depth) */
 static const char dir_s1d1[] = TMP_DIR "/s1d1";
@@ -1965,8 +1965,8 @@ TEST_F_FORK(layout1, relative_chroot_chdir)
 	test_relative_path(_metadata, REL_CHROOT_CHDIR);
 }
 
-static void copy_binary(struct __test_metadata *const _metadata,
-			const char *const dst_path)
+static void copy_file(struct __test_metadata *const _metadata,
+		      const char *const src_path, const char *const dst_path)
 {
 	int dst_fd, src_fd;
 	struct stat statbuf;
@@ -1976,11 +1976,10 @@ static void copy_binary(struct __test_metadata *const _metadata,
 	{
 		TH_LOG("Failed to open \"%s\": %s", dst_path, strerror(errno));
 	}
-	src_fd = open(BINARY_PATH, O_RDONLY | O_CLOEXEC);
+	src_fd = open(src_path, O_RDONLY | O_CLOEXEC);
 	ASSERT_LE(0, src_fd)
 	{
-		TH_LOG("Failed to open \"" BINARY_PATH "\": %s",
-		       strerror(errno));
+		TH_LOG("Failed to open \"%s\": %s", src_path, strerror(errno));
 	}
 	ASSERT_EQ(0, fstat(src_fd, &statbuf));
 	ASSERT_EQ(statbuf.st_size,
@@ -2028,9 +2027,9 @@ TEST_F_FORK(layout1, execute)
 		create_ruleset(_metadata, rules[0].access, rules);
 
 	ASSERT_LE(0, ruleset_fd);
-	copy_binary(_metadata, file1_s1d1);
-	copy_binary(_metadata, file1_s1d2);
-	copy_binary(_metadata, file1_s1d3);
+	copy_file(_metadata, bin_true, file1_s1d1);
+	copy_file(_metadata, bin_true, file1_s1d2);
+	copy_file(_metadata, bin_true, file1_s1d3);
 
 	enforce_ruleset(_metadata, ruleset_fd);
 	ASSERT_EQ(0, close(ruleset_fd));
@@ -2048,6 +2047,83 @@ TEST_F_FORK(layout1, execute)
 	test_execute(_metadata, 0, file1_s1d3);
 }
 
+TEST_F_FORK(layout1, umount_sandboxer)
+{
+	int pipe_child[2], pipe_parent[2];
+	char buf_parent;
+	pid_t child;
+	int status;
+
+	copy_file(_metadata, bin_sandbox_and_launch, file1_s3d3);
+	ASSERT_EQ(0, pipe2(pipe_child, 0));
+	ASSERT_EQ(0, pipe2(pipe_parent, 0));
+
+	child = fork();
+	ASSERT_LE(0, child);
+	if (child == 0) {
+		char pipe_child_str[12], pipe_parent_str[12];
+		char *const argv[] = { (char *)file1_s3d3,
+				       (char *)bin_wait_pipe, pipe_child_str,
+				       pipe_parent_str, NULL };
+
+		/* Passes the pipe FDs to the executed binary and its child. */
+		EXPECT_EQ(0, close(pipe_child[0]));
+		EXPECT_EQ(0, close(pipe_parent[1]));
+		snprintf(pipe_child_str, sizeof(pipe_child_str), "%d",
+			 pipe_child[1]);
+		snprintf(pipe_parent_str, sizeof(pipe_parent_str), "%d",
+			 pipe_parent[0]);
+
+		/*
+		 * We need bin_sandbox_and_launch (copied inside the mount as
+		 * file1_s3d3) to execute bin_wait_pipe (outside the mount) to
+		 * make sure the mount point will not be EBUSY because of
+		 * file1_s3d3 being in use.  This avoids a potential race
+		 * condition between the following read() and umount() calls.
+		 */
+		ASSERT_EQ(0, execve(argv[0], argv, NULL))
+		{
+			TH_LOG("Failed to execute \"%s\": %s", argv[0],
+			       strerror(errno));
+		};
+		_exit(1);
+		return;
+	}
+
+	EXPECT_EQ(0, close(pipe_child[1]));
+	EXPECT_EQ(0, close(pipe_parent[0]));
+
+	/* Waits for the child to sandbox itself. */
+	EXPECT_EQ(1, read(pipe_child[0], &buf_parent, 1));
+
+	/* Tests that the sandboxer is tied to its mount point. */
+	set_cap(_metadata, CAP_SYS_ADMIN);
+	EXPECT_EQ(-1, umount(dir_s3d2));
+	EXPECT_EQ(EBUSY, errno);
+	clear_cap(_metadata, CAP_SYS_ADMIN);
+
+	/* Signals the child to launch a grandchild. */
+	EXPECT_EQ(1, write(pipe_parent[1], ".", 1));
+
+	/* Waits for the grandchild. */
+	EXPECT_EQ(1, read(pipe_child[0], &buf_parent, 1));
+
+	/* Tests that the domain's sandboxer is not tied to its mount point. */
+	set_cap(_metadata, CAP_SYS_ADMIN);
+	EXPECT_EQ(0, umount(dir_s3d2))
+	{
+		TH_LOG("Failed to umount \"%s\": %s", dir_s3d2,
+		       strerror(errno));
+	};
+	clear_cap(_metadata, CAP_SYS_ADMIN);
+
+	/* Signals the grandchild to terminate. */
+	EXPECT_EQ(1, write(pipe_parent[1], ".", 1));
+	ASSERT_EQ(child, waitpid(child, &status, 0));
+	ASSERT_EQ(1, WIFEXITED(status));
+	ASSERT_EQ(0, WEXITSTATUS(status));
+}
+
 TEST_F_FORK(layout1, link)
 {
 	const struct rule layer1[] = {

tools/testing/selftests/landlock/sandbox-and-launch.c

@@ -0,0 +1,82 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Sandbox itself and execute another program (in a different mount point).
+ *
+ * Used by layout1.umount_sandboxer from fs_test.c
+ *
+ * Copyright © 2024-2025 Microsoft Corporation
+ */
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#include "wrappers.h"
+
+int main(int argc, char *argv[])
+{
+	struct landlock_ruleset_attr ruleset_attr = {
+		.scoped = LANDLOCK_SCOPE_SIGNAL,
+	};
+	int pipe_child, pipe_parent, ruleset_fd;
+	char buf;
+
+	/*
+	 * The first argument must be the file descriptor number of a pipe.
+	 * The second argument must be the program to execute.
+	 */
+	if (argc != 4) {
+		fprintf(stderr, "Wrong number of arguments (not three)\n");
+		return 1;
+	}
+
+	pipe_child = atoi(argv[2]);
+	pipe_parent = atoi(argv[3]);
+
+	ruleset_fd =
+		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
+	if (ruleset_fd < 0) {
+		perror("Failed to create ruleset");
+		return 1;
+	}
+
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+		perror("Failed to call prctl()");
+		return 1;
+	}
+
+	if (landlock_restrict_self(ruleset_fd, 0)) {
+		perror("Failed to restrict self");
+		return 1;
+	}
+
+	if (close(ruleset_fd)) {
+		perror("Failed to close ruleset");
+		return 1;
+	}
+
+	/* Signals that we are sandboxed. */
+	errno = 0;
+	if (write(pipe_child, ".", 1) != 1) {
+		perror("Failed to write to the second argument");
+		return 1;
+	}
+
+	/* Waits for the parent to try to umount. */
+	if (read(pipe_parent, &buf, 1) != 1) {
+		perror("Failed to write to the third argument");
+		return 1;
+	}
+
+	/* Shifts arguments. */
+	argv[0] = argv[1];
+	argv[1] = argv[2];
+	argv[2] = argv[3];
+	argv[3] = NULL;
+	execve(argv[0], argv, NULL);
+	perror("Failed to execute the provided binary");
+	return 1;
+}

tools/testing/selftests/landlock/wait-pipe.c

@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Write in a pipe and wait.
+ *
+ * Used by layout1.umount_sandboxer from fs_test.c
+ *
+ * Copyright © 2024-2025 Microsoft Corporation
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int argc, char *argv[])
+{
+	int pipe_child, pipe_parent;
+	char buf;
+
+	/* The first argument must be the file descriptor number of a pipe. */
+	if (argc != 3) {
+		fprintf(stderr, "Wrong number of arguments (not two)\n");
+		return 1;
+	}
+
+	pipe_child = atoi(argv[1]);
+	pipe_parent = atoi(argv[2]);
+
+	/* Signals that we are waiting. */
+	if (write(pipe_child, ".", 1) != 1) {
+		perror("Failed to write to first argument");
+		return 1;
+	}
+
+	/* Waits for the parent do its test. */
+	if (read(pipe_parent, &buf, 1) != 1) {
+		perror("Failed to write to the second argument");
+		return 1;
+	}
+
+	return 0;
+}