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@xxxxxxxxx>
---
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
