selftests/landlock: Test file truncation support

These tests exercise the following truncation operations:

* truncate() (truncate by path)
* ftruncate() (truncate by file descriptor)
* open with the O_TRUNC flag
* special case: creat(), which is open with O_CREAT|O_WRONLY|O_TRUNC.

in the following scenarios:

* Files with read, write and truncate rights.
* Files with read and truncate rights.
* Files with the truncate right.
* Files without the truncate right.

In particular, the following scenarios are enforced with the test:

* open() with O_TRUNC requires the truncate right, if it truncates a file.
  open() already checks security_path_truncate() in this case,
  and it required no additional check in the Landlock LSM's file_open hook.
* creat() requires the truncate right
  when called with an existing filename.
* creat() does *not* require the truncate right
  when it's creating a new file.
* ftruncate() requires that the file was opened by a thread that had
  the truncate right for the file at the time of open(). (The rights
  are carried along with the opened file.)

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-6-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

tools/testing/selftests/landlock/fs_test.c

@@ -58,6 +58,7 @@ static const char file1_s2d3[] = TMP_DIR "/s2d1/s2d2/s2d3/f1";
 static const char file2_s2d3[] = TMP_DIR "/s2d1/s2d2/s2d3/f2";
 
 static const char dir_s3d1[] = TMP_DIR "/s3d1";
+static const char file1_s3d1[] = TMP_DIR "/s3d1/f1";
 /* dir_s3d2 is a mount point. */
 static const char dir_s3d2[] = TMP_DIR "/s3d1/s3d2";
 static const char dir_s3d3[] = TMP_DIR "/s3d1/s3d2/s3d3";
@@ -83,6 +84,7 @@ static const char dir_s3d3[] = TMP_DIR "/s3d1/s3d2/s3d3";
  * │           ├── f1
  * │           └── f2
  * └── s3d1
+ *     ├── f1
  *     └── s3d2
  *         └── s3d3
  */
@@ -208,6 +210,7 @@ static void create_layout1(struct __test_metadata *const _metadata)
 	create_file(_metadata, file1_s2d3);
 	create_file(_metadata, file2_s2d3);
 
+	create_file(_metadata, file1_s3d1);
 	create_directory(_metadata, dir_s3d2);
 	set_cap(_metadata, CAP_SYS_ADMIN);
 	ASSERT_EQ(0, mount("tmp", dir_s3d2, "tmpfs", 0, "size=4m,mode=700"));
@@ -230,6 +233,7 @@ static void remove_layout1(struct __test_metadata *const _metadata)
 	EXPECT_EQ(0, remove_path(file1_s2d2));
 	EXPECT_EQ(0, remove_path(file1_s2d1));
 
+	EXPECT_EQ(0, remove_path(file1_s3d1));
 	EXPECT_EQ(0, remove_path(dir_s3d3));
 	set_cap(_metadata, CAP_SYS_ADMIN);
 	umount(dir_s3d2);
@@ -3158,6 +3162,289 @@ TEST_F_FORK(layout1, proc_pipe)
 	ASSERT_EQ(0, close(pipe_fds[1]));
 }
 
+/* Invokes truncate(2) and returns its errno or 0. */
+static int test_truncate(const char *const path)
+{
+	if (truncate(path, 10) < 0)
+		return errno;
+	return 0;
+}
+
+/*
+ * Invokes creat(2) and returns its errno or 0.
+ * Closes the opened file descriptor on success.
+ */
+static int test_creat(const char *const path)
+{
+	int fd = creat(path, 0600);
+
+	if (fd < 0)
+		return errno;
+
+	/*
+	 * Mixing error codes from close(2) and creat(2) should not lead to any
+	 * (access type) confusion for this test.
+	 */
+	if (close(fd) < 0)
+		return errno;
+	return 0;
+}
+
+/*
+ * Exercises file truncation when it's not restricted,
+ * as it was the case before LANDLOCK_ACCESS_FS_TRUNCATE existed.
+ */
+TEST_F_FORK(layout1, truncate_unhandled)
+{
+	const char *const file_r = file1_s1d1;
+	const char *const file_w = file2_s1d1;
+	const char *const file_none = file1_s1d2;
+	const struct rule rules[] = {
+		{
+			.path = file_r,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE,
+		},
+		{
+			.path = file_w,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		/* Implicitly: No rights for file_none. */
+		{},
+	};
+
+	const __u64 handled = LANDLOCK_ACCESS_FS_READ_FILE |
+			      LANDLOCK_ACCESS_FS_WRITE_FILE;
+	int ruleset_fd;
+
+	/* Enable Landlock. */
+	ruleset_fd = create_ruleset(_metadata, handled, rules);
+
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	/*
+	 * Checks read right: truncate and open with O_TRUNC work, unless the
+	 * file is attempted to be opened for writing.
+	 */
+	EXPECT_EQ(0, test_truncate(file_r));
+	EXPECT_EQ(0, test_open(file_r, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_r, O_WRONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_creat(file_r));
+
+	/*
+	 * Checks write right: truncate and open with O_TRUNC work, unless the
+	 * file is attempted to be opened for reading.
+	 */
+	EXPECT_EQ(0, test_truncate(file_w));
+	EXPECT_EQ(EACCES, test_open(file_w, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(0, test_open(file_w, O_WRONLY | O_TRUNC));
+	EXPECT_EQ(0, test_creat(file_w));
+
+	/*
+	 * Checks "no rights" case: truncate works but all open attempts fail,
+	 * including creat.
+	 */
+	EXPECT_EQ(0, test_truncate(file_none));
+	EXPECT_EQ(EACCES, test_open(file_none, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_none, O_WRONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_creat(file_none));
+}
+
+TEST_F_FORK(layout1, truncate)
+{
+	const char *const file_rwt = file1_s1d1;
+	const char *const file_rw = file2_s1d1;
+	const char *const file_rt = file1_s1d2;
+	const char *const file_t = file2_s1d2;
+	const char *const file_none = file1_s1d3;
+	const char *const dir_t = dir_s2d1;
+	const char *const file_in_dir_t = file1_s2d1;
+	const char *const dir_w = dir_s3d1;
+	const char *const file_in_dir_w = file1_s3d1;
+	const struct rule rules[] = {
+		{
+			.path = file_rwt,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE |
+				  LANDLOCK_ACCESS_FS_WRITE_FILE |
+				  LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{
+			.path = file_rw,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE |
+				  LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{
+			.path = file_rt,
+			.access = LANDLOCK_ACCESS_FS_READ_FILE |
+				  LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{
+			.path = file_t,
+			.access = LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		/* Implicitly: No access rights for file_none. */
+		{
+			.path = dir_t,
+			.access = LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{
+			.path = dir_w,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{},
+	};
+	const __u64 handled = LANDLOCK_ACCESS_FS_READ_FILE |
+			      LANDLOCK_ACCESS_FS_WRITE_FILE |
+			      LANDLOCK_ACCESS_FS_TRUNCATE;
+	int ruleset_fd;
+
+	/* Enable Landlock. */
+	ruleset_fd = create_ruleset(_metadata, handled, rules);
+
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	/* Checks read, write and truncate rights: truncation works. */
+	EXPECT_EQ(0, test_truncate(file_rwt));
+	EXPECT_EQ(0, test_open(file_rwt, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(0, test_open(file_rwt, O_WRONLY | O_TRUNC));
+
+	/* Checks read and write rights: no truncate variant works. */
+	EXPECT_EQ(EACCES, test_truncate(file_rw));
+	EXPECT_EQ(EACCES, test_open(file_rw, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_rw, O_WRONLY | O_TRUNC));
+
+	/*
+	 * Checks read and truncate rights: truncation works.
+	 *
+	 * Note: Files can get truncated using open() even with O_RDONLY.
+	 */
+	EXPECT_EQ(0, test_truncate(file_rt));
+	EXPECT_EQ(0, test_open(file_rt, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_rt, O_WRONLY | O_TRUNC));
+
+	/* Checks truncate right: truncate works, but can't open file. */
+	EXPECT_EQ(0, test_truncate(file_t));
+	EXPECT_EQ(EACCES, test_open(file_t, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_t, O_WRONLY | O_TRUNC));
+
+	/* Checks "no rights" case: No form of truncation works. */
+	EXPECT_EQ(EACCES, test_truncate(file_none));
+	EXPECT_EQ(EACCES, test_open(file_none, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_none, O_WRONLY | O_TRUNC));
+
+	/*
+	 * Checks truncate right on directory: truncate works on contained
+	 * files.
+	 */
+	EXPECT_EQ(0, test_truncate(file_in_dir_t));
+	EXPECT_EQ(EACCES, test_open(file_in_dir_t, O_RDONLY | O_TRUNC));
+	EXPECT_EQ(EACCES, test_open(file_in_dir_t, O_WRONLY | O_TRUNC));
+
+	/*
+	 * Checks creat in dir_w: This requires the truncate right when
+	 * overwriting an existing file, but does not require it when the file
+	 * is new.
+	 */
+	EXPECT_EQ(EACCES, test_creat(file_in_dir_w));
+
+	ASSERT_EQ(0, unlink(file_in_dir_w));
+	EXPECT_EQ(0, test_creat(file_in_dir_w));
+}
+
+/* Invokes ftruncate(2) and returns its errno or 0. */
+static int test_ftruncate(int fd)
+{
+	if (ftruncate(fd, 10) < 0)
+		return errno;
+	return 0;
+}
+
+TEST_F_FORK(layout1, ftruncate)
+{
+	/*
+	 * This test opens a new file descriptor at different stages of
+	 * Landlock restriction:
+	 *
+	 * without restriction:                    ftruncate works
+	 * something else but truncate restricted: ftruncate works
+	 * truncate restricted and permitted:      ftruncate works
+	 * truncate restricted and not permitted:  ftruncate fails
+	 *
+	 * Whether this works or not is expected to depend on the time when the
+	 * FD was opened, not to depend on the time when ftruncate() was
+	 * called.
+	 */
+	const char *const path = file1_s1d1;
+	const __u64 handled1 = LANDLOCK_ACCESS_FS_READ_FILE |
+			       LANDLOCK_ACCESS_FS_WRITE_FILE;
+	const struct rule layer1[] = {
+		{
+			.path = path,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{},
+	};
+	const __u64 handled2 = LANDLOCK_ACCESS_FS_TRUNCATE;
+	const struct rule layer2[] = {
+		{
+			.path = path,
+			.access = LANDLOCK_ACCESS_FS_TRUNCATE,
+		},
+		{},
+	};
+	const __u64 handled3 = LANDLOCK_ACCESS_FS_TRUNCATE |
+			       LANDLOCK_ACCESS_FS_WRITE_FILE;
+	const struct rule layer3[] = {
+		{
+			.path = path,
+			.access = LANDLOCK_ACCESS_FS_WRITE_FILE,
+		},
+		{},
+	};
+	int fd_layer0, fd_layer1, fd_layer2, fd_layer3, ruleset_fd;
+
+	fd_layer0 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+
+	ruleset_fd = create_ruleset(_metadata, handled1, layer1);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd_layer1 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+	EXPECT_EQ(0, test_ftruncate(fd_layer1));
+
+	ruleset_fd = create_ruleset(_metadata, handled2, layer2);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd_layer2 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+	EXPECT_EQ(0, test_ftruncate(fd_layer1));
+	EXPECT_EQ(0, test_ftruncate(fd_layer2));
+
+	ruleset_fd = create_ruleset(_metadata, handled3, layer3);
+	ASSERT_LE(0, ruleset_fd);
+	enforce_ruleset(_metadata, ruleset_fd);
+	ASSERT_EQ(0, close(ruleset_fd));
+
+	fd_layer3 = open(path, O_WRONLY);
+	EXPECT_EQ(0, test_ftruncate(fd_layer0));
+	EXPECT_EQ(0, test_ftruncate(fd_layer1));
+	EXPECT_EQ(0, test_ftruncate(fd_layer2));
+	EXPECT_EQ(EACCES, test_ftruncate(fd_layer3));
+
+	ASSERT_EQ(0, close(fd_layer0));
+	ASSERT_EQ(0, close(fd_layer1));
+	ASSERT_EQ(0, close(fd_layer2));
+	ASSERT_EQ(0, close(fd_layer3));
+}
+
 /* clang-format off */
 FIXTURE(layout1_bind) {};
 /* clang-format on */