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

@@ -95,8 +95,19 @@ struct landlock_path_beneath_attr { · 94 unmodified lines
95	95	* A file can only receive these access rights:
96	96	*
97	97	* - %LANDLOCK_ACCESS_FS_EXECUTE: Execute a file.
98		− * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access.
	98	+ * - %LANDLOCK_ACCESS_FS_WRITE_FILE: Open a file with write access. Note that
	99	+ * you might additionally need the %LANDLOCK_ACCESS_FS_TRUNCATE right in order
	100	+ * to overwrite files with :manpage:`open(2)` using ``O_TRUNC`` or
	101	+ * :manpage:`creat(2)`.
99	102	* - %LANDLOCK_ACCESS_FS_READ_FILE: Open a file with read access.
	103	+ * - %LANDLOCK_ACCESS_FS_TRUNCATE: Truncate a file with :manpage:`truncate(2)`,
	104	+ * :manpage:`ftruncate(2)`, :manpage:`creat(2)`, or :manpage:`open(2)` with
	105	+ * ``O_TRUNC``. Whether an opened file can be truncated with
	106	+ * :manpage:`ftruncate(2)` is determined during :manpage:`open(2)`, in the
	107	+ * same way as read and write permissions are checked during
	108	+ * :manpage:`open(2)` using %LANDLOCK_ACCESS_FS_READ_FILE and
	109	+ * %LANDLOCK_ACCESS_FS_WRITE_FILE. This access right is available since the
	110	+ * third version of the Landlock ABI.
100	111	*
101	112	* A directory can receive access rights related to files or directories. The
102	113	* following access right is applied to the directory itself, and the
@@ -139,10 +150,9 @@ struct landlock_path_beneath_attr { · 36 unmodified lines
139	150	*
140	151	* It is currently not possible to restrict some file-related actions
141	152	* accessible through these syscall families: :manpage:`chdir(2)`,
142		− * :manpage:`truncate(2)`, :manpage:`stat(2)`, :manpage:`flock(2)`,
143		− * :manpage:`chmod(2)`, :manpage:`chown(2)`, :manpage:`setxattr(2)`,
144		− * :manpage:`utime(2)`, :manpage:`ioctl(2)`, :manpage:`fcntl(2)`,
145		− * :manpage:`access(2)`.
	153	+ * :manpage:`stat(2)`, :manpage:`flock(2)`, :manpage:`chmod(2)`,
	154	+ * :manpage:`chown(2)`, :manpage:`setxattr(2)`, :manpage:`utime(2)`,
	155	+ * :manpage:`ioctl(2)`, :manpage:`fcntl(2)`, :manpage:`access(2)`.
146	156	* Future Landlock evolutions will enable to restrict them.
147	157	*/
148	158	/* clang-format off */
@@ -160,6 +170,7 @@ struct landlock_path_beneath_attr { · 11 unmodified lines
160	170	#define LANDLOCK_ACCESS_FS_MAKE_BLOCK (1ULL << 11)
161	171	#define LANDLOCK_ACCESS_FS_MAKE_SYM (1ULL << 12)
162	172	#define LANDLOCK_ACCESS_FS_REFER (1ULL << 13)
	173	+#define LANDLOCK_ACCESS_FS_TRUNCATE (1ULL << 14)
163	174	/* clang-format on */
164	175
165	176	#endif /* _UAPI_LINUX_LANDLOCK_H */

security/landlock/fs.c+99−5

@@ -146,7 +146,8 @@ retry: · 145 unmodified lines
146	146	#define ACCESS_FILE ( \
147	147	LANDLOCK_ACCESS_FS_EXECUTE \| \
148	148	LANDLOCK_ACCESS_FS_WRITE_FILE \| \
149		− LANDLOCK_ACCESS_FS_READ_FILE)
	149	+ LANDLOCK_ACCESS_FS_READ_FILE \| \
	150	+ LANDLOCK_ACCESS_FS_TRUNCATE)
150	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	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	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	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	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	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	1211	const struct landlock_ruleset *const dom =
1180	1212	landlock_get_current_domain();
1181	1213
1182	1214	if (!dom)
1183	1215	return 0;
	1216	+
1184	1217	/*
1185		− * Because a file may be opened with O_PATH, get_file_access() may
1186		− * return 0. This case will be handled with a future Landlock
	1218	+ * Because a file may be opened with O_PATH, get_required_file_open_access()
	1219	+ * may return 0. This case will be handled with a future Landlock
1187	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	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	1297	LSM_HOOK_INIT(path_symlink, hook_path_symlink),
1207	1298	LSM_HOOK_INIT(path_unlink, hook_path_unlink),
1208	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	1303	LSM_HOOK_INIT(file_open, hook_file_open),
	1304	+ LSM_HOOK_INIT(file_truncate, hook_file_truncate),
1211	1305	};
1212	1306
1213	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	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	58	* struct landlock_superblock_security - Superblock security blob
41	59	*
@@ -50,6 +68,12 @@ struct landlock_superblock_security { · 8 unmodified lines
50	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	77	static inline struct landlock_inode_security *
54	78	landlock_inode(const struct inode *const inode)
55	79	{

security/landlock/limits.h+1−1

@@ -18,7 +18,7 @@ · 17 unmodified lines
18	18	#define LANDLOCK_MAX_NUM_LAYERS 16
19	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	22	#define LANDLOCK_MASK_ACCESS_FS ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
23	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	20	struct lsm_blob_sizes landlock_blob_sizes __lsm_ro_after_init = {
21	21	.lbs_cred = sizeof(struct landlock_cred_security),
	22	+ .lbs_file = sizeof(struct landlock_file_security),
22	23	.lbs_inode = sizeof(struct landlock_inode_security),
23	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	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	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	186	int offset;
187	187
188		− if (*need > 0) {
189		− offset = *lbs;
190		− lbs += need;
191		− *need = offset;
192		− }
	188	+ if (*need <= 0)
	189	+ return;
	190	+
	191	+ offset = ALIGN(lbs, sizeof(void ));
	192	+ lbs = offset + need;
	193	+ *need = offset;
193	194	}
194	195
195	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	75	const struct landlock_ruleset_attr ruleset_attr = {
76	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	79	LANDLOCK_CREATE_RULESET_VERSION));
80	80
81	81	ASSERT_EQ(-1, landlock_create_ruleset(&ruleset_attr, 0,

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

@@ -406,9 +406,10 @@ TEST_F_FORK(layout1, inval) · 405 unmodified lines
406	406	#define ACCESS_FILE ( \
407	407	LANDLOCK_ACCESS_FS_EXECUTE \| \
408	408	LANDLOCK_ACCESS_FS_WRITE_FILE \| \
409		− LANDLOCK_ACCESS_FS_READ_FILE)
	409	+ LANDLOCK_ACCESS_FS_READ_FILE \| \
	410	+ LANDLOCK_ACCESS_FS_TRUNCATE)
410	411
411		−#define ACCESS_LAST LANDLOCK_ACCESS_FS_REFER
	412	+#define ACCESS_LAST LANDLOCK_ACCESS_FS_TRUNCATE
412	413
413	414	#define ACCESS_ALL ( \
414	415	ACCESS_FILE \| \
@@ -422,7 +423,7 @@ TEST_F_FORK(layout1, inval) · 7 unmodified lines
422	423	LANDLOCK_ACCESS_FS_MAKE_FIFO \| \
423	424	LANDLOCK_ACCESS_FS_MAKE_BLOCK \| \
424	425	LANDLOCK_ACCESS_FS_MAKE_SYM \| \
425		− ACCESS_LAST)
	426	+ LANDLOCK_ACCESS_FS_REFER)
426	427
427	428	/* clang-format on */
428	429