1 // SPDX-License-Identifier: GPL-2.0-only
3 * Landlock LSM - Filesystem management and hooks
5 * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
6 * Copyright © 2018-2020 ANSSI
9 #include <linux/atomic.h>
10 #include <linux/bitops.h>
11 #include <linux/bits.h>
12 #include <linux/compiler_types.h>
13 #include <linux/dcache.h>
14 #include <linux/err.h>
16 #include <linux/init.h>
17 #include <linux/kernel.h>
18 #include <linux/limits.h>
19 #include <linux/list.h>
20 #include <linux/lsm_hooks.h>
21 #include <linux/mount.h>
22 #include <linux/namei.h>
23 #include <linux/path.h>
24 #include <linux/rcupdate.h>
25 #include <linux/spinlock.h>
26 #include <linux/stat.h>
27 #include <linux/types.h>
28 #include <linux/wait_bit.h>
29 #include <linux/workqueue.h>
30 #include <uapi/linux/landlock.h>
40 /* Underlying object management */
42 static void release_inode(struct landlock_object *const object)
43 __releases(object->lock)
45 struct inode *const inode = object->underobj;
46 struct super_block *sb;
49 spin_unlock(&object->lock);
54 * Protects against concurrent use by hook_sb_delete() of the reference
55 * to the underlying inode.
57 object->underobj = NULL;
59 * Makes sure that if the filesystem is concurrently unmounted,
60 * hook_sb_delete() will wait for us to finish iput().
63 atomic_long_inc(&landlock_superblock(sb)->inode_refs);
64 spin_unlock(&object->lock);
66 * Because object->underobj was not NULL, hook_sb_delete() and
67 * get_inode_object() guarantee that it is safe to reset
68 * landlock_inode(inode)->object while it is not NULL. It is therefore
69 * not necessary to lock inode->i_lock.
71 rcu_assign_pointer(landlock_inode(inode)->object, NULL);
73 * Now, new rules can safely be tied to @inode with get_inode_object().
77 if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs))
78 wake_up_var(&landlock_superblock(sb)->inode_refs);
81 static const struct landlock_object_underops landlock_fs_underops = {
82 .release = release_inode
85 /* Ruleset management */
87 static struct landlock_object *get_inode_object(struct inode *const inode)
89 struct landlock_object *object, *new_object;
90 struct landlock_inode_security *inode_sec = landlock_inode(inode);
94 object = rcu_dereference(inode_sec->object);
96 if (likely(refcount_inc_not_zero(&object->usage))) {
101 * We are racing with release_inode(), the object is going
102 * away. Wait for release_inode(), then retry.
104 spin_lock(&object->lock);
105 spin_unlock(&object->lock);
111 * If there is no object tied to @inode, then create a new one (without
112 * holding any locks).
114 new_object = landlock_create_object(&landlock_fs_underops, inode);
115 if (IS_ERR(new_object))
119 * Protects against concurrent calls to get_inode_object() or
122 spin_lock(&inode->i_lock);
123 if (unlikely(rcu_access_pointer(inode_sec->object))) {
124 /* Someone else just created the object, bail out and retry. */
125 spin_unlock(&inode->i_lock);
133 * @inode will be released by hook_sb_delete() on its superblock
134 * shutdown, or by release_inode() when no more ruleset references the
138 rcu_assign_pointer(inode_sec->object, new_object);
139 spin_unlock(&inode->i_lock);
143 /* All access rights that can be tied to files. */
144 #define ACCESS_FILE ( \
145 LANDLOCK_ACCESS_FS_EXECUTE | \
146 LANDLOCK_ACCESS_FS_WRITE_FILE | \
147 LANDLOCK_ACCESS_FS_READ_FILE)
150 * @path: Should have been checked by get_path_from_fd().
152 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
153 const struct path *const path, u32 access_rights)
156 struct landlock_object *object;
158 /* Files only get access rights that make sense. */
159 if (!d_is_dir(path->dentry) && (access_rights | ACCESS_FILE) !=
162 if (WARN_ON_ONCE(ruleset->num_layers != 1))
165 /* Transforms relative access rights to absolute ones. */
166 access_rights |= LANDLOCK_MASK_ACCESS_FS & ~ruleset->fs_access_masks[0];
167 object = get_inode_object(d_backing_inode(path->dentry));
169 return PTR_ERR(object);
170 mutex_lock(&ruleset->lock);
171 err = landlock_insert_rule(ruleset, object, access_rights);
172 mutex_unlock(&ruleset->lock);
174 * No need to check for an error because landlock_insert_rule()
175 * increments the refcount for the new object if needed.
177 landlock_put_object(object);
181 /* Access-control management */
183 static inline u64 unmask_layers(
184 const struct landlock_ruleset *const domain,
185 const struct path *const path, const u32 access_request,
188 const struct landlock_rule *rule;
189 const struct inode *inode;
192 if (d_is_negative(path->dentry))
193 /* Ignore nonexistent leafs. */
195 inode = d_backing_inode(path->dentry);
197 rule = landlock_find_rule(domain,
198 rcu_dereference(landlock_inode(inode)->object));
204 * An access is granted if, for each policy layer, at least one rule
205 * encountered on the pathwalk grants the requested accesses,
206 * regardless of their position in the layer stack. We must then check
207 * the remaining layers for each inode, from the first added layer to
210 for (i = 0; i < rule->num_layers; i++) {
211 const struct landlock_layer *const layer = &rule->layers[i];
212 const u64 layer_level = BIT_ULL(layer->level - 1);
214 /* Checks that the layer grants access to the full request. */
215 if ((layer->access & access_request) == access_request) {
216 layer_mask &= ~layer_level;
225 static int check_access_path(const struct landlock_ruleset *const domain,
226 const struct path *const path, u32 access_request)
228 bool allowed = false;
229 struct path walker_path;
233 /* Make sure all layers can be checked. */
234 BUILD_BUG_ON(BITS_PER_TYPE(layer_mask) < LANDLOCK_MAX_NUM_LAYERS);
238 if (WARN_ON_ONCE(!domain || !path))
241 * Allows access to pseudo filesystems that will never be mountable
242 * (e.g. sockfs, pipefs), but can still be reachable through
243 * /proc/<pid>/fd/<file-descriptor> .
245 if ((path->dentry->d_sb->s_flags & SB_NOUSER) ||
246 (d_is_positive(path->dentry) &&
247 unlikely(IS_PRIVATE(d_backing_inode(path->dentry)))))
249 if (WARN_ON_ONCE(domain->num_layers < 1))
252 /* Saves all layers handling a subset of requested accesses. */
254 for (i = 0; i < domain->num_layers; i++) {
255 if (domain->fs_access_masks[i] & access_request)
256 layer_mask |= BIT_ULL(i);
258 /* An access request not handled by the domain is allowed. */
263 path_get(&walker_path);
265 * We need to walk through all the hierarchy to not miss any relevant
269 struct dentry *parent_dentry;
271 layer_mask = unmask_layers(domain, &walker_path,
272 access_request, layer_mask);
273 if (layer_mask == 0) {
274 /* Stops when a rule from each layer grants access. */
280 if (walker_path.dentry == walker_path.mnt->mnt_root) {
281 if (follow_up(&walker_path)) {
282 /* Ignores hidden mount points. */
286 * Stops at the real root. Denies access
287 * because not all layers have granted access.
293 if (unlikely(IS_ROOT(walker_path.dentry))) {
295 * Stops at disconnected root directories. Only allows
296 * access to internal filesystems (e.g. nsfs, which is
297 * reachable through /proc/<pid>/ns/<namespace>).
299 allowed = !!(walker_path.mnt->mnt_flags & MNT_INTERNAL);
302 parent_dentry = dget_parent(walker_path.dentry);
303 dput(walker_path.dentry);
304 walker_path.dentry = parent_dentry;
306 path_put(&walker_path);
307 return allowed ? 0 : -EACCES;
310 static inline int current_check_access_path(const struct path *const path,
311 const u32 access_request)
313 const struct landlock_ruleset *const dom =
314 landlock_get_current_domain();
318 return check_access_path(dom, path, access_request);
323 static void hook_inode_free_security(struct inode *const inode)
326 * All inodes must already have been untied from their object by
327 * release_inode() or hook_sb_delete().
329 WARN_ON_ONCE(landlock_inode(inode)->object);
332 /* Super-block hooks */
335 * Release the inodes used in a security policy.
337 * Cf. fsnotify_unmount_inodes() and invalidate_inodes()
339 static void hook_sb_delete(struct super_block *const sb)
341 struct inode *inode, *prev_inode = NULL;
343 if (!landlock_initialized)
346 spin_lock(&sb->s_inode_list_lock);
347 list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
348 struct landlock_object *object;
350 /* Only handles referenced inodes. */
351 if (!atomic_read(&inode->i_count))
355 * Protects against concurrent modification of inode (e.g.
356 * from get_inode_object()).
358 spin_lock(&inode->i_lock);
360 * Checks I_FREEING and I_WILL_FREE to protect against a race
361 * condition when release_inode() just called iput(), which
362 * could lead to a NULL dereference of inode->security or a
363 * second call to iput() for the same Landlock object. Also
364 * checks I_NEW because such inode cannot be tied to an object.
366 if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
367 spin_unlock(&inode->i_lock);
372 object = rcu_dereference(landlock_inode(inode)->object);
375 spin_unlock(&inode->i_lock);
378 /* Keeps a reference to this inode until the next loop walk. */
380 spin_unlock(&inode->i_lock);
383 * If there is no concurrent release_inode() ongoing, then we
384 * are in charge of calling iput() on this inode, otherwise we
385 * will just wait for it to finish.
387 spin_lock(&object->lock);
388 if (object->underobj == inode) {
389 object->underobj = NULL;
390 spin_unlock(&object->lock);
394 * Because object->underobj was not NULL,
395 * release_inode() and get_inode_object() guarantee
396 * that it is safe to reset
397 * landlock_inode(inode)->object while it is not NULL.
398 * It is therefore not necessary to lock inode->i_lock.
400 rcu_assign_pointer(landlock_inode(inode)->object, NULL);
402 * At this point, we own the ihold() reference that was
403 * originally set up by get_inode_object() and the
404 * __iget() reference that we just set in this loop
405 * walk. Therefore the following call to iput() will
406 * not sleep nor drop the inode because there is now at
407 * least two references to it.
411 spin_unlock(&object->lock);
417 * At this point, we still own the __iget() reference
418 * that we just set in this loop walk. Therefore we
419 * can drop the list lock and know that the inode won't
420 * disappear from under us until the next loop walk.
422 spin_unlock(&sb->s_inode_list_lock);
424 * We can now actually put the inode reference from the
425 * previous loop walk, which is not needed anymore.
429 spin_lock(&sb->s_inode_list_lock);
433 spin_unlock(&sb->s_inode_list_lock);
435 /* Puts the inode reference from the last loop walk, if any. */
438 /* Waits for pending iput() in release_inode(). */
439 wait_var_event(&landlock_superblock(sb)->inode_refs, !atomic_long_read(
440 &landlock_superblock(sb)->inode_refs));
444 * Because a Landlock security policy is defined according to the filesystem
445 * topology (i.e. the mount namespace), changing it may grant access to files
446 * not previously allowed.
448 * To make it simple, deny any filesystem topology modification by landlocked
449 * processes. Non-landlocked processes may still change the namespace of a
450 * landlocked process, but this kind of threat must be handled by a system-wide
451 * access-control security policy.
453 * This could be lifted in the future if Landlock can safely handle mount
454 * namespace updates requested by a landlocked process. Indeed, we could
455 * update the current domain (which is currently read-only) by taking into
456 * account the accesses of the source and the destination of a new mount point.
457 * However, it would also require to make all the child domains dynamically
458 * inherit these new constraints. Anyway, for backward compatibility reasons,
459 * a dedicated user space option would be required (e.g. as a ruleset flag).
461 static int hook_sb_mount(const char *const dev_name,
462 const struct path *const path, const char *const type,
463 const unsigned long flags, void *const data)
465 if (!landlock_get_current_domain())
470 static int hook_move_mount(const struct path *const from_path,
471 const struct path *const to_path)
473 if (!landlock_get_current_domain())
479 * Removing a mount point may reveal a previously hidden file hierarchy, which
480 * may then grant access to files, which may have previously been forbidden.
482 static int hook_sb_umount(struct vfsmount *const mnt, const int flags)
484 if (!landlock_get_current_domain())
489 static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts)
491 if (!landlock_get_current_domain())
497 * pivot_root(2), like mount(2), changes the current mount namespace. It must
498 * then be forbidden for a landlocked process.
500 * However, chroot(2) may be allowed because it only changes the relative root
501 * directory of the current process. Moreover, it can be used to restrict the
502 * view of the filesystem.
504 static int hook_sb_pivotroot(const struct path *const old_path,
505 const struct path *const new_path)
507 if (!landlock_get_current_domain())
514 static inline u32 get_mode_access(const umode_t mode)
516 switch (mode & S_IFMT) {
518 return LANDLOCK_ACCESS_FS_MAKE_SYM;
520 /* A zero mode translates to S_IFREG. */
522 return LANDLOCK_ACCESS_FS_MAKE_REG;
524 return LANDLOCK_ACCESS_FS_MAKE_DIR;
526 return LANDLOCK_ACCESS_FS_MAKE_CHAR;
528 return LANDLOCK_ACCESS_FS_MAKE_BLOCK;
530 return LANDLOCK_ACCESS_FS_MAKE_FIFO;
532 return LANDLOCK_ACCESS_FS_MAKE_SOCK;
540 * Creating multiple links or renaming may lead to privilege escalations if not
541 * handled properly. Indeed, we must be sure that the source doesn't gain more
542 * privileges by being accessible from the destination. This is getting more
543 * complex when dealing with multiple layers. The whole picture can be seen as
544 * a multilayer partial ordering problem. A future version of Landlock will
547 static int hook_path_link(struct dentry *const old_dentry,
548 const struct path *const new_dir,
549 struct dentry *const new_dentry)
551 const struct landlock_ruleset *const dom =
552 landlock_get_current_domain();
556 /* The mount points are the same for old and new paths, cf. EXDEV. */
557 if (old_dentry->d_parent != new_dir->dentry)
558 /* Gracefully forbids reparenting. */
560 if (unlikely(d_is_negative(old_dentry)))
562 return check_access_path(dom, new_dir,
563 get_mode_access(d_backing_inode(old_dentry)->i_mode));
566 static inline u32 maybe_remove(const struct dentry *const dentry)
568 if (d_is_negative(dentry))
570 return d_is_dir(dentry) ? LANDLOCK_ACCESS_FS_REMOVE_DIR :
571 LANDLOCK_ACCESS_FS_REMOVE_FILE;
574 static int hook_path_rename(const struct path *const old_dir,
575 struct dentry *const old_dentry,
576 const struct path *const new_dir,
577 struct dentry *const new_dentry)
579 const struct landlock_ruleset *const dom =
580 landlock_get_current_domain();
584 /* The mount points are the same for old and new paths, cf. EXDEV. */
585 if (old_dir->dentry != new_dir->dentry)
586 /* Gracefully forbids reparenting. */
588 if (unlikely(d_is_negative(old_dentry)))
590 /* RENAME_EXCHANGE is handled because directories are the same. */
591 return check_access_path(dom, old_dir, maybe_remove(old_dentry) |
592 maybe_remove(new_dentry) |
593 get_mode_access(d_backing_inode(old_dentry)->i_mode));
596 static int hook_path_mkdir(const struct path *const dir,
597 struct dentry *const dentry, const umode_t mode)
599 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR);
602 static int hook_path_mknod(const struct path *const dir,
603 struct dentry *const dentry, const umode_t mode,
604 const unsigned int dev)
606 const struct landlock_ruleset *const dom =
607 landlock_get_current_domain();
611 return check_access_path(dom, dir, get_mode_access(mode));
614 static int hook_path_symlink(const struct path *const dir,
615 struct dentry *const dentry, const char *const old_name)
617 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM);
620 static int hook_path_unlink(const struct path *const dir,
621 struct dentry *const dentry)
623 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE);
626 static int hook_path_rmdir(const struct path *const dir,
627 struct dentry *const dentry)
629 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
634 static inline u32 get_file_access(const struct file *const file)
638 if (file->f_mode & FMODE_READ) {
639 /* A directory can only be opened in read mode. */
640 if (S_ISDIR(file_inode(file)->i_mode))
641 return LANDLOCK_ACCESS_FS_READ_DIR;
642 access = LANDLOCK_ACCESS_FS_READ_FILE;
644 if (file->f_mode & FMODE_WRITE)
645 access |= LANDLOCK_ACCESS_FS_WRITE_FILE;
646 /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */
647 if (file->f_flags & __FMODE_EXEC)
648 access |= LANDLOCK_ACCESS_FS_EXECUTE;
652 static int hook_file_open(struct file *const file)
654 const struct landlock_ruleset *const dom =
655 landlock_get_current_domain();
660 * Because a file may be opened with O_PATH, get_file_access() may
661 * return 0. This case will be handled with a future Landlock
664 return check_access_path(dom, &file->f_path, get_file_access(file));
667 static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
668 LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),
670 LSM_HOOK_INIT(sb_delete, hook_sb_delete),
671 LSM_HOOK_INIT(sb_mount, hook_sb_mount),
672 LSM_HOOK_INIT(move_mount, hook_move_mount),
673 LSM_HOOK_INIT(sb_umount, hook_sb_umount),
674 LSM_HOOK_INIT(sb_remount, hook_sb_remount),
675 LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot),
677 LSM_HOOK_INIT(path_link, hook_path_link),
678 LSM_HOOK_INIT(path_rename, hook_path_rename),
679 LSM_HOOK_INIT(path_mkdir, hook_path_mkdir),
680 LSM_HOOK_INIT(path_mknod, hook_path_mknod),
681 LSM_HOOK_INIT(path_symlink, hook_path_symlink),
682 LSM_HOOK_INIT(path_unlink, hook_path_unlink),
683 LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
685 LSM_HOOK_INIT(file_open, hook_file_open),
688 __init void landlock_add_fs_hooks(void)
690 security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),