[PATCH v6 2/5] landlock: Support file truncation

["Günther Noack" <gnoack3000@gmail.com>]

Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation.

This flag hooks into the path_truncate LSM hook and covers file
truncation using truncate(2), ftruncate(2), open(2) with O_TRUNC, as
well as creat().

This change also increments the Landlock ABI version, updates
corresponding selftests, and updates code documentation to document
the flag.

The following operations are restricted:

open(): requires the LANDLOCK_ACCESS_FS_TRUNCATE right if a file gets
implicitly truncated as part of the open() (e.g. using O_TRUNC).

Notable special cases:
* open(..., O_RDONLY|O_TRUNC) can truncate files as well in Linux
* open() with O_TRUNC does *not* need the TRUNCATE right when it
  creates a new file.

truncate() (on a path): requires the LANDLOCK_ACCESS_FS_TRUNCATE
right.

ftruncate() (on a file): requires that the file had the TRUNCATE right
when it was previously opened.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
---
 include/uapi/linux/landlock.h                | 18 ++--
 security/landlock/fs.c                       | 88 +++++++++++++++++++-
 security/landlock/fs.h                       | 18 ++++
 security/landlock/limits.h                   |  2 +-
 security/landlock/setup.c                    |  1 +
 security/landlock/syscalls.c                 |  2 +-
 tools/testing/selftests/landlock/base_test.c |  2 +-
 tools/testing/selftests/landlock/fs_test.c   |  7 +-
 8 files changed, 124 insertions(+), 14 deletions(-)

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 23df4e0e8ace..8c0124c5cbe6 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -95,8 +95,16 @@ struct landlock_path_beneath_attr {
  * A file can only receive these access rights:
  *
  * - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
- * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access.
+ * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that
+ *   you might additionally need the `LANDLOCK_ACCESS_FS_TRUNCATE` right in
+ *   order to overwrite files with :manpage:`open(2)` using `O_TRUNC` or
+ *   :manpage:`creat(2)`.
  * - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
+ * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`,
+ *   :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with
+ *   `O_TRUNC`. The right to truncate a file gets carried along with an opened
+ *   file descriptor for the purpose of :manpage:`ftruncate(2)`. This access
+ *   right is available since the third version of the Landlock ABI.
  *
  * A directory can receive access rights related to files or directories.  The
  * following access right is applied to the directory itself, and the
@@ -139,10 +147,9 @@ struct landlock_path_beneath_attr {
  *
  *   It is currently not possible to restrict some file-related actions
  *   accessible through these syscall families: :manpage:`chdir(2)`,
- *   :manpage:`truncate(2)`, :manpage:`stat(2)`, :manpage:`flock(2)`,
- *   :manpage:`chmod(2)`, :manpage:`chown(2)`, :manpage:`setxattr(2)`,
- *   :manpage:`utime(2)`, :manpage:`ioctl(2)`, :manpage:`fcntl(2)`,
- *   :manpage:`access(2)`.
+ *   :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
+ *   :manpage:`chown(2)`, :manpage:`setxattr(2)`, :manpage:`utime(2)`,
+ *   :manpage:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
  *   Future Landlock evolutions will enable to restrict them.
  */
 /* clang-format off */
@@ -160,6 +167,7 @@ struct landlock_path_beneath_attr {
 #define LANDLOCK_ACCESS_FS_MAKE_BLOCK			(1ULL << 11)
 #define LANDLOCK_ACCESS_FS_MAKE_SYM			(1ULL << 12)
 #define LANDLOCK_ACCESS_FS_REFER			(1ULL << 13)
+#define LANDLOCK_ACCESS_FS_TRUNCATE			(1ULL << 14)
 /* clang-format on */
 
 #endif /* _UAPI_LINUX_LANDLOCK_H */
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index a9dbd99d9ee7..1b546edf69a6 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -146,7 +146,8 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
 #define ACCESS_FILE ( \
 	LANDLOCK_ACCESS_FS_EXECUTE | \
 	LANDLOCK_ACCESS_FS_WRITE_FILE | \
-	LANDLOCK_ACCESS_FS_READ_FILE)
+	LANDLOCK_ACCESS_FS_READ_FILE | \
+	LANDLOCK_ACCESS_FS_TRUNCATE)
 /* clang-format on */
 
 /*
@@ -761,6 +762,47 @@ static bool collect_domain_accesses(
 	return ret;
 }
 
+/**
+ * get_path_access_rights - Returns the subset of rights in access_request
+ * which are permitted for the given path.
+ *
+ * @domain: The domain that defines the current restrictions.
+ * @path: The path to get access rights for.
+ * @access_request: The rights we are interested in.
+ *
+ * Returns: The access mask of the rights that are permitted on the given path,
+ * which are also a subset of access_request (to save some calculation time).
+ */
+static inline access_mask_t
+get_path_access_rights(const struct landlock_ruleset *const domain,
+		       const struct path *const path,
+		       access_mask_t access_request)
+{
+	layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
+	unsigned long access_bit;
+	unsigned long access_req;
+
+	init_layer_masks(domain, access_request, &layer_masks);
+	if (!check_access_path_dual(domain, path, access_request, &layer_masks,
+				    NULL, 0, NULL, NULL)) {
+		/*
+		 * Return immediately for successful accesses and for cases
+		 * where everything is permitted because the path belongs to an
+		 * internal filesystem.
+		 */
+		return access_request;
+	}
+
+	access_req = access_request;
+	for_each_set_bit(access_bit, &access_req, ARRAY_SIZE(layer_masks)) {
+		if (layer_masks[access_bit]) {
+			/* If any layer vetoed the access right, remove it. */
+			access_request &= ~BIT_ULL(access_bit);
+		}
+	}
+	return access_request;
+}
+
 /**
  * current_check_refer_path - Check if a rename or link action is allowed
  *
@@ -1142,6 +1184,11 @@ static int hook_path_rmdir(const struct path *const dir,
 	return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
 }
 
+static int hook_path_truncate(const struct path *const path)
+{
+	return current_check_access_path(path, LANDLOCK_ACCESS_FS_TRUNCATE);
+}
+
 /* File hooks */
 
 static inline access_mask_t get_file_access(const struct file *const file)
@@ -1159,22 +1206,55 @@ static inline access_mask_t get_file_access(const struct file *const file)
 	/* __FMODE_EXEC is indeed part of f_flags, not f_mode. */
 	if (file->f_flags & __FMODE_EXEC)
 		access |= LANDLOCK_ACCESS_FS_EXECUTE;
+
 	return access;
 }
 
 static int hook_file_open(struct file *const file)
 {
+	access_mask_t access_req, access_rights;
+	const access_mask_t optional_rights = LANDLOCK_ACCESS_FS_TRUNCATE;
 	const struct landlock_ruleset *const dom =
 		landlock_get_current_domain();
 
-	if (!dom)
+	if (!dom) {
+		/* Grant all rights. */
+		landlock_file(file)->rights = LANDLOCK_MASK_ACCESS_FS;
 		return 0;
+	}
+
 	/*
 	 * Because a file may be opened with O_PATH, get_file_access() may
 	 * return 0.  This case will be handled with a future Landlock
 	 * evolution.
 	 */
-	return check_access_path(dom, &file->f_path, get_file_access(file));
+	access_req = get_file_access(file);
+	access_rights = get_path_access_rights(dom, &file->f_path,
+					       access_req | optional_rights);
+	if (access_req & ~access_rights)
+		return -EACCES;
+
+	/*
+	 * For operations on already opened files (i.e. ftruncate()), it is the
+	 * access rights at the time of open() which decide whether the
+	 * operation is permitted. Therefore, we record the relevant subset of
+	 * file access rights in the opened struct file.
+	 */
+	landlock_file(file)->rights = access_rights;
+
+	return 0;
+}
+
+static int hook_file_truncate(struct file *const file)
+{
+	/*
+	 * We permit truncation if the truncation right was available at the
+	 * time of opening the file.
+	 */
+	if (!(landlock_file(file)->rights & LANDLOCK_ACCESS_FS_TRUNCATE))
+		return -EACCES;
+
+	return 0;
 }
 
 static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
@@ -1194,6 +1274,8 @@ static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
 	LSM_HOOK_INIT(path_symlink, hook_path_symlink),
 	LSM_HOOK_INIT(path_unlink, hook_path_unlink),
 	LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
+	LSM_HOOK_INIT(path_truncate, hook_path_truncate),
+	LSM_HOOK_INIT(file_truncate, hook_file_truncate),
 
 	LSM_HOOK_INIT(file_open, hook_file_open),
 };
diff --git a/security/landlock/fs.h b/security/landlock/fs.h
index 8db7acf9109b..275ba5375839 100644
--- a/security/landlock/fs.h
+++ b/security/landlock/fs.h
@@ -36,6 +36,18 @@ struct landlock_inode_security {
 	struct landlock_object __rcu *object;
 };
 
+/**
+ * struct landlock_file_security - File security blob
+ *
+ * This information is populated when opening a file in hook_file_open, and
+ * tracks the relevant Landlock access rights that were available at the time
+ * of opening the file. Other LSM hooks use these rights in order to authorize
+ * operations on already opened files.
+ */
+struct landlock_file_security {
+	access_mask_t rights;
+};
+
 /**
  * struct landlock_superblock_security - Superblock security blob
  *
@@ -50,6 +62,12 @@ struct landlock_superblock_security {
 	atomic_long_t inode_refs;
 };
 
+static inline struct landlock_file_security *
+landlock_file(const struct file *const file)
+{
+	return file->f_security + landlock_blob_sizes.lbs_file;
+}
+
 static inline struct landlock_inode_security *
 landlock_inode(const struct inode *const inode)
 {
diff --git a/security/landlock/limits.h b/security/landlock/limits.h
index b54184ab9439..82288f0e9e5e 100644
--- a/security/landlock/limits.h
+++ b/security/landlock/limits.h
@@ -18,7 +18,7 @@
 #define LANDLOCK_MAX_NUM_LAYERS		16
 #define LANDLOCK_MAX_NUM_RULES		U32_MAX
 
-#define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_REFER
+#define LANDLOCK_LAST_ACCESS_FS		LANDLOCK_ACCESS_FS_TRUNCATE
 #define LANDLOCK_MASK_ACCESS_FS		((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_FS		__const_hweight64(LANDLOCK_MASK_ACCESS_FS)
 
diff --git a/security/landlock/setup.c b/security/landlock/setup.c
index f8e8e980454c..3f196d2ce4f9 100644
--- a/security/landlock/setup.c
+++ b/security/landlock/setup.c
@@ -19,6 +19,7 @@ bool landlock_initialized __lsm_ro_after_init = false;
 
 struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
 	.lbs_cred = sizeof(struct landlock_cred_security),
+	.lbs_file = sizeof(struct landlock_file_security),
 	.lbs_inode = sizeof(struct landlock_inode_security),
 	.lbs_superblock = sizeof(struct landlock_superblock_security),
 };
diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 735a0865ea11..f4d6fc7ed17f 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -129,7 +129,7 @@ static const struct file_operations ruleset_fops = {
 	.write = fop_dummy_write,
 };
 
-#define LANDLOCK_ABI_VERSION 2
+#define LANDLOCK_ABI_VERSION 3
 
 /**
  * sys_landlock_create_ruleset - Create a new ruleset
diff --git a/tools/testing/selftests/landlock/base_test.c b/tools/testing/selftests/landlock/base_test.c
index da9290817866..72cdae277b02 100644
--- a/tools/testing/selftests/landlock/base_test.c
+++ b/tools/testing/selftests/landlock/base_test.c
@@ -75,7 +75,7 @@ TEST(abi_version)
 	const struct landlock_ruleset_attr ruleset_attr = {
 		.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
 	};
-	ASSERT_EQ(2, landlock_create_ruleset(NULL, 0,
+	ASSERT_EQ(3, landlock_create_ruleset(NULL, 0,
 					     LANDLOCK_CREATE_RULESET_VERSION));
 
 	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,
diff --git a/tools/testing/selftests/landlock/fs_test.c b/tools/testing/selftests/landlock/fs_test.c
index 45de42a027c5..87b28d14a1aa 100644
--- a/tools/testing/selftests/landlock/fs_test.c
+++ b/tools/testing/selftests/landlock/fs_test.c
@@ -406,9 +406,10 @@ TEST_F_FORK(layout1, inval)
 #define ACCESS_FILE ( \
 	LANDLOCK_ACCESS_FS_EXECUTE | \
 	LANDLOCK_ACCESS_FS_WRITE_FILE | \
-	LANDLOCK_ACCESS_FS_READ_FILE)
+	LANDLOCK_ACCESS_FS_READ_FILE | \
+	LANDLOCK_ACCESS_FS_TRUNCATE)
 
-#define ACCESS_LAST LANDLOCK_ACCESS_FS_REFER
+#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE
 
 #define ACCESS_ALL ( \
 	ACCESS_FILE | \
@@ -422,7 +423,7 @@ TEST_F_FORK(layout1, inval)
 	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 	LANDLOCK_ACCESS_FS_MAKE_SYM | \
-	ACCESS_LAST)
+	LANDLOCK_ACCESS_FS_REFER)
 
 /* clang-format on */
 
-- 
2.37.3