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).
Cc: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Acked-by: Paul Moore <paul@paul-moore.com> (LSM)
Link: https://lore.kernel.org/r/20221018182216.301684-5-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>
* 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:`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 */
#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 */
#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 */
/*
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)
+/**
+ * get_required_file_open_access - Get access needed to open a file
+ *
+ * @file: File being opened.
+ *
+ * Returns the access rights that are required for opening the given file,
+ * depending on the file type and open mode.
+ */
+static inline access_mask_t
+get_required_file_open_access(const struct file *const file)
{
access_mask_t access = 0;
return access;
}
+static int hook_file_alloc_security(struct file *const file)
+{
+ /*
+ * Grants all access rights, even if most of them are not checked later
+ * on. It is more consistent.
+ *
+ * Notably, file descriptors for regular files can also be acquired
+ * without going through the file_open hook, for example when using
+ * memfd_create(2).
+ */
+ landlock_file(file)->allowed_access = LANDLOCK_MASK_ACCESS_FS;
+ return 0;
+}
+
static int hook_file_open(struct file *const file)
{
+ layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
+ access_mask_t open_access_request, full_access_request, allowed_access;
+ const access_mask_t optional_access = LANDLOCK_ACCESS_FS_TRUNCATE;
const struct landlock_ruleset *const dom =
landlock_get_current_domain();
if (!dom)
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
+ * Because a file may be opened with O_PATH, get_required_file_open_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));
+ open_access_request = get_required_file_open_access(file);
+
+ /*
+ * We look up more access than what we immediately need for open(), so
+ * that we can later authorize operations on opened files.
+ */
+ full_access_request = open_access_request | optional_access;
+
+ if (is_access_to_paths_allowed(
+ dom, &file->f_path,
+ init_layer_masks(dom, full_access_request, &layer_masks),
+ &layer_masks, NULL, 0, NULL, NULL)) {
+ allowed_access = full_access_request;
+ } else {
+ unsigned long access_bit;
+ const unsigned long access_req = full_access_request;
+
+ /*
+ * Calculate the actual allowed access rights from layer_masks.
+ * Add each access right to allowed_access which has not been
+ * vetoed by any layer.
+ */
+ allowed_access = 0;
+ for_each_set_bit(access_bit, &access_req,
+ ARRAY_SIZE(layer_masks)) {
+ if (!layer_masks[access_bit])
+ allowed_access |= BIT_ULL(access_bit);
+ }
+ }
+
+ /*
+ * 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)->allowed_access = allowed_access;
+
+ if ((open_access_request & allowed_access) == open_access_request)
+ return 0;
+
+ return -EACCES;
+}
+
+static int hook_file_truncate(struct file *const file)
+{
+ /*
+ * Allows truncation if the truncate right was available at the time of
+ * opening the file, to get a consistent access check as for read, write
+ * and execute operations.
+ *
+ * Note: For checks done based on the file's Landlock allowed access, we
+ * enforce them independently of whether the current thread is in a
+ * Landlock domain, so that open files passed between independent
+ * processes retain their behaviour.
+ */
+ if (landlock_file(file)->allowed_access & LANDLOCK_ACCESS_FS_TRUNCATE)
+ return 0;
+ return -EACCES;
}
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_alloc_security, hook_file_alloc_security),
LSM_HOOK_INIT(file_open, hook_file_open),
+ LSM_HOOK_INIT(file_truncate, hook_file_truncate),
};
__init void landlock_add_fs_hooks(void)
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 {
+ /**
+ * @allowed_access: Access rights that were available at the time of
+ * opening the file. This is not necessarily the full set of access
+ * rights available at that time, but it's the necessary subset as
+ * needed to authorize later operations on the open file.
+ */
+ access_mask_t allowed_access;
+};
+
/**
* struct landlock_superblock_security - Superblock security blob
*
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)
{
#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)
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),
};
.write = fop_dummy_write,
};
-#define LANDLOCK_ABI_VERSION 2
+#define LANDLOCK_ABI_VERSION 3
/**
* sys_landlock_create_ruleset - Create a new ruleset
{
int offset;
- if (*need > 0) {
- offset = *lbs;
- *lbs += *need;
- *need = offset;
- }
+ if (*need <= 0)
+ return;
+
+ offset = ALIGN(*lbs, sizeof(void *));
+ *lbs = offset + *need;
+ *need = offset;
}
static void __init lsm_set_blob_sizes(struct lsm_blob_sizes *needed)
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,
#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 */
projects / linux-2.6-microblaze.git / commitdiff