landlock: Support file truncation

Introduce the LANDLOCK_ACCESS_FS_TRUNCATE flag for file truncation. This flag hooks into the path_truncate, file_truncate and file_alloc_security LSM hooks 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. In security/security.c, allocate security blobs at pointer-aligned offsets. This fixes the problem where one LSM's security blob can shift another LSM's security blob to an unaligned address (reported by Nathan Chancellor). The following operations are restricted: open(2): 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(2) (on a path): requires the LANDLOCK_ACCESS_FS_TRUNCATE right. ftruncate(2) (on a file): requires that the file had the TRUNCATE right when it was previously opened. File descriptors acquired by other means than open(2) (e.g. memfd_create(2)) continue to support truncation with ftruncate(2).

Günther Noack committed 4 years ago

Commit b9f5ce21 parent 52a13489 files · +153 −21Browse files

Unified · Split

9 files

include/uapi/linux/landlock.h +16 −5
security/landlock/fs.c +99 −5
security/landlock/fs.h +24 −0
security/landlock/limits.h +1 −1
security/landlock/setup.c +1 −0
security/landlock/syscalls.c +1 −1
security/security.c +6 −5
tools/testing/selftests/landlock/base_test.c +1 −1
tools/testing/selftests/landlock/fs_test.c +4 −3

include/uapi/linux/landlock.h+16−5

  * 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``. Whether an opened file can be truncated with
+ *   :manpage:`ftruncate(2)` is determined during :manpage:`open(2)`, in the
+ *   same way as read and write permissions are checked during
+ *   :manpage:`open(2)` using %LANDLOCK_ACCESS_FS_READ_FILE and
+ *   %LANDLOCK_ACCESS_FS_WRITE_FILE. 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
  *
  *   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:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
- *   :manpage:`chmod(2)`, :manpage:`chown(2)`, :manpage:`setxattr(2)`,
+ *   :manpage:`chown(2)`, :manpage:`setxattr(2)`, :manpage:`utime(2)`,
- *   :manpage:`utime(2)`, :manpage:`ioctl(2)`, :manpage:`fcntl(2)`,
+ *   :manpage:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
- *   :manpage:`access(2)`.
  *   Future Landlock evolutions will enable to restrict them.
  */
 /* clang-format off */
 #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 */

security/landlock/fs.c+99−5

@@ -146,7 +146,8 @@ retry: · 145 unmodified lines
146	#define ACCESS_FILE ( \	146	#define ACCESS_FILE ( \
147	LANDLOCK_ACCESS_FS_EXECUTE \| \	147	LANDLOCK_ACCESS_FS_EXECUTE \| \
148	LANDLOCK_ACCESS_FS_WRITE_FILE \| \	148	LANDLOCK_ACCESS_FS_WRITE_FILE \| \
149	LANDLOCK_ACCESS_FS_READ_FILE)	149	LANDLOCK_ACCESS_FS_READ_FILE \| \
		150	LANDLOCK_ACCESS_FS_TRUNCATE)
150	/* clang-format on */	151	/* clang-format on */
151		152
152	/*	153	/*
@@ -1154,9 +1155,23 @@ static int hook_path_rmdir(const struct path *const dir, · 1001 unmodified lines
1154	return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);	1155	return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
1155	}	1156	}
1156		1157
		1158	static int hook_path_truncate(const struct path *const path)
		1159	{
		1160	return current_check_access_path(path, LANDLOCK_ACCESS_FS_TRUNCATE);
		1161	}
		1162
1157	/* File hooks */	1163	/* File hooks */
1158		1164
1159	static inline access_mask_t get_file_access(const struct file *const file)	1165	/**
		1166	* get_required_file_open_access - Get access needed to open a file
		1167	*
		1168	* @file: File being opened.
		1169	*
		1170	* Returns the access rights that are required for opening the given file,
		1171	* depending on the file type and open mode.
		1172	*/
		1173	static inline access_mask_t
		1174	get_required_file_open_access(const struct file *const file)
1160	{	1175	{
1161	access_mask_t access = 0;	1176	access_mask_t access = 0;
1162		1177
@@ -1174,19 +1189,95 @@ static inline access_mask_t get_file_access(const struct file *const file) · 11 unmodified lines
1174	return access;	1189	return access;
1175	}	1190	}
1176		1191
		1192	static int hook_file_alloc_security(struct file *const file)
		1193	{
		1194	/*
		1195	* Grants all access rights, even if most of them are not checked later
		1196	* on. It is more consistent.
		1197	*
		1198	* Notably, file descriptors for regular files can also be acquired
		1199	* without going through the file_open hook, for example when using
		1200	* memfd_create(2).
		1201	*/
		1202	landlock_file(file)->allowed_access = LANDLOCK_MASK_ACCESS_FS;
		1203	return 0;
		1204	}
		1205
1177	static int hook_file_open(struct file *const file)	1206	static int hook_file_open(struct file *const file)
1178	{	1207	{
		1208	layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
		1209	access_mask_t open_access_request, full_access_request, allowed_access;
		1210	const access_mask_t optional_access = LANDLOCK_ACCESS_FS_TRUNCATE;
1179	const struct landlock_ruleset *const dom =	1211	const struct landlock_ruleset *const dom =
1180	landlock_get_current_domain();	1212	landlock_get_current_domain();
1181		1213
1182	if (!dom)	1214	if (!dom)
1183	return 0;	1215	return 0;
		1216
1184	/*	1217	/*
1185	* Because a file may be opened with O_PATH, get_file_access() may	1218	* Because a file may be opened with O_PATH, get_required_file_open_access()
1186	* return 0. This case will be handled with a future Landlock	1219	* may return 0. This case will be handled with a future Landlock
1187	* evolution.	1220	* evolution.
1188	*/	1221	*/
1189	return check_access_path(dom, &file->f_path, get_file_access(file));	1222	open_access_request = get_required_file_open_access(file);
		1223
		1224	/*
		1225	* We look up more access than what we immediately need for open(), so
		1226	* that we can later authorize operations on opened files.
		1227	*/
		1228	full_access_request = open_access_request \| optional_access;
		1229
		1230	if (is_access_to_paths_allowed(
		1231	dom, &file->f_path,
		1232	init_layer_masks(dom, full_access_request, &layer_masks),
		1233	&layer_masks, NULL, 0, NULL, NULL)) {
		1234	allowed_access = full_access_request;
		1235	} else {
		1236	unsigned long access_bit;
		1237	const unsigned long access_req = full_access_request;
		1238
		1239	/*
		1240	* Calculate the actual allowed access rights from layer_masks.
		1241	* Add each access right to allowed_access which has not been
		1242	* vetoed by any layer.
		1243	*/
		1244	allowed_access = 0;
		1245	for_each_set_bit(access_bit, &access_req,
		1246	ARRAY_SIZE(layer_masks)) {
		1247	if (!layer_masks[access_bit])
		1248	allowed_access \|= BIT_ULL(access_bit);
		1249	}
		1250	}
		1251
		1252	/*
		1253	* For operations on already opened files (i.e. ftruncate()), it is the
		1254	* access rights at the time of open() which decide whether the
		1255	* operation is permitted. Therefore, we record the relevant subset of
		1256	* file access rights in the opened struct file.
		1257	*/
		1258	landlock_file(file)->allowed_access = allowed_access;
		1259
		1260	if ((open_access_request & allowed_access) == open_access_request)
		1261	return 0;
		1262
		1263	return -EACCES;
		1264	}
		1265
		1266	static int hook_file_truncate(struct file *const file)
		1267	{
		1268	/*
		1269	* Allows truncation if the truncate right was available at the time of
		1270	* opening the file, to get a consistent access check as for read, write
		1271	* and execute operations.
		1272	*
		1273	* Note: For checks done based on the file's Landlock allowed access, we
		1274	* enforce them independently of whether the current thread is in a
		1275	* Landlock domain, so that open files passed between independent
		1276	* processes retain their behaviour.
		1277	*/
		1278	if (landlock_file(file)->allowed_access & LANDLOCK_ACCESS_FS_TRUNCATE)
		1279	return 0;
		1280	return -EACCES;
1190	}	1281	}
1191		1282
1192	static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {	1283	static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
@@ -1206,8 +1297,11 @@ static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = { · 13 unmodified lines
1206	LSM_HOOK_INIT(path_symlink, hook_path_symlink),	1297	LSM_HOOK_INIT(path_symlink, hook_path_symlink),
1207	LSM_HOOK_INIT(path_unlink, hook_path_unlink),	1298	LSM_HOOK_INIT(path_unlink, hook_path_unlink),
1208	LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),	1299	LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
		1300	LSM_HOOK_INIT(path_truncate, hook_path_truncate),
1209		1301
		1302	LSM_HOOK_INIT(file_alloc_security, hook_file_alloc_security),
1210	LSM_HOOK_INIT(file_open, hook_file_open),	1303	LSM_HOOK_INIT(file_open, hook_file_open),
		1304	LSM_HOOK_INIT(file_truncate, hook_file_truncate),
1211	};	1305	};
1212		1306
1213	__init void landlock_add_fs_hooks(void)	1307	__init void landlock_add_fs_hooks(void)

security/landlock/fs.h+24−0

@@ -36,6 +36,24 @@ struct landlock_inode_security { · 35 unmodified lines
36	struct landlock_object __rcu *object;	36	struct landlock_object __rcu *object;
37	};	37	};
38		38
		39	/**
		40	* struct landlock_file_security - File security blob
		41	*
		42	* This information is populated when opening a file in hook_file_open, and
		43	* tracks the relevant Landlock access rights that were available at the time
		44	* of opening the file. Other LSM hooks use these rights in order to authorize
		45	* operations on already opened files.
		46	*/
		47	struct landlock_file_security {
		48	/**
		49	* @allowed_access: Access rights that were available at the time of
		50	* opening the file. This is not necessarily the full set of access
		51	* rights available at that time, but it's the necessary subset as
		52	* needed to authorize later operations on the open file.
		53	*/
		54	access_mask_t allowed_access;
		55	};
		56
39	/**	57	/**
40	* struct landlock_superblock_security - Superblock security blob	58	* struct landlock_superblock_security - Superblock security blob
41	*	59	*
@@ -50,6 +68,12 @@ struct landlock_superblock_security { · 8 unmodified lines
50	atomic_long_t inode_refs;	68	atomic_long_t inode_refs;
51	};	69	};
52		70
		71	static inline struct landlock_file_security *
		72	landlock_file(const struct file *const file)
		73	{
		74	return file->f_security + landlock_blob_sizes.lbs_file;
		75	}
		76
53	static inline struct landlock_inode_security *	77	static inline struct landlock_inode_security *
54	landlock_inode(const struct inode *const inode)	78	landlock_inode(const struct inode *const inode)
55	{	79	{

security/landlock/limits.h+1−1

@@ -18,7 +18,7 @@ · 17 unmodified lines
18	#define LANDLOCK_MAX_NUM_LAYERS 16	18	#define LANDLOCK_MAX_NUM_LAYERS 16
19	#define LANDLOCK_MAX_NUM_RULES U32_MAX	19	#define LANDLOCK_MAX_NUM_RULES U32_MAX
20		20
21	#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_REFER	21	#define LANDLOCK_LAST_ACCESS_FS LANDLOCK_ACCESS_FS_TRUNCATE
22	#define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)	22	#define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
23	#define LANDLOCK_NUM_ACCESS_FS __const_hweight64(LANDLOCK_MASK_ACCESS_FS)	23	#define LANDLOCK_NUM_ACCESS_FS __const_hweight64(LANDLOCK_MASK_ACCESS_FS)
24		24

security/landlock/setup.c+1−0

@@ -19,6 +19,7 @@ bool landlock_initialized __lsm_ro_after_init = false; · 18 unmodified lines
19		19
20	struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {	20	struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
21	.lbs_cred = sizeof(struct landlock_cred_security),	21	.lbs_cred = sizeof(struct landlock_cred_security),
		22	.lbs_file = sizeof(struct landlock_file_security),
22	.lbs_inode = sizeof(struct landlock_inode_security),	23	.lbs_inode = sizeof(struct landlock_inode_security),
23	.lbs_superblock = sizeof(struct landlock_superblock_security),	24	.lbs_superblock = sizeof(struct landlock_superblock_security),
24	};	25	};

security/landlock/syscalls.c+1−1

@@ -129,7 +129,7 @@ static const struct file_operations ruleset_fops = { · 128 unmodified lines
129	.write = fop_dummy_write,	129	.write = fop_dummy_write,
130	};	130	};
131		131
132	#define LANDLOCK_ABI_VERSION 2	132	#define LANDLOCK_ABI_VERSION 3
133		133
134	/**	134	/**
135	* sys_landlock_create_ruleset - Create a new ruleset	135	* sys_landlock_create_ruleset - Create a new ruleset

security/security.c+6−5

@@ -185,11 +185,12 @@ static void __init lsm_set_blob_size(int need, int lbs) · 184 unmodified lines
185	{	185	{
186	int offset;	186	int offset;
187		187
188	if (*need > 0) {	188	if (*need <= 0)
189	offset = *lbs;	189	return;
190	lbs += need;	190
191	*need = offset;	191	offset = ALIGN(lbs, sizeof(void ));
192	}	192	lbs = offset + need;
		193	*need = offset;
193	}	194	}
194		195
195	static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)	196	static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)

tools/testing/selftests/landlock/base_test.c+1−1

@@ -75,7 +75,7 @@ TEST(abi_version) · 74 unmodified lines
75	const struct landlock_ruleset_attr ruleset_attr = {	75	const struct landlock_ruleset_attr ruleset_attr = {
76	.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,	76	.handled_access_fs = LANDLOCK_ACCESS_FS_READ_FILE,
77	};	77	};
78	ASSERT_EQ(2, landlock_create_ruleset(NULL, 0,	78	ASSERT_EQ(3, landlock_create_ruleset(NULL, 0,
79	LANDLOCK_CREATE_RULESET_VERSION));	79	LANDLOCK_CREATE_RULESET_VERSION));
80		80
81	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,	81	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,

tools/testing/selftests/landlock/fs_test.c+4−3

 #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 | \
 	LANDLOCK_ACCESS_FS_MAKE_FIFO | \
 	LANDLOCK_ACCESS_FS_MAKE_BLOCK | \
 	LANDLOCK_ACCESS_FS_MAKE_SYM | \
-	ACCESS_LAST)
+	LANDLOCK_ACCESS_FS_REFER)
 /* clang-format on */