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 /* clang-format off */
145 #define ACCESS_FILE ( \
146 LANDLOCK_ACCESS_FS_EXECUTE | \
147 LANDLOCK_ACCESS_FS_WRITE_FILE | \
148 LANDLOCK_ACCESS_FS_READ_FILE)
149 /* clang-format on */
152 * @path: Should have been checked by get_path_from_fd().
154 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
155 const struct path *const path,
156 access_mask_t access_rights)
159 struct landlock_object *object;
161 /* Files only get access rights that make sense. */
162 if (!d_is_dir(path->dentry) &&
163 (access_rights | ACCESS_FILE) != ACCESS_FILE)
165 if (WARN_ON_ONCE(ruleset->num_layers != 1))
168 /* Transforms relative access rights to absolute ones. */
169 access_rights |= LANDLOCK_MASK_ACCESS_FS & ~ruleset->fs_access_masks[0];
170 object = get_inode_object(d_backing_inode(path->dentry));
172 return PTR_ERR(object);
173 mutex_lock(&ruleset->lock);
174 err = landlock_insert_rule(ruleset, object, access_rights);
175 mutex_unlock(&ruleset->lock);
177 * No need to check for an error because landlock_insert_rule()
178 * increments the refcount for the new object if needed.
180 landlock_put_object(object);
184 /* Access-control management */
187 * The lifetime of the returned rule is tied to @domain.
189 * Returns NULL if no rule is found or if @dentry is negative.
191 static inline const struct landlock_rule *
192 find_rule(const struct landlock_ruleset *const domain,
193 const struct dentry *const dentry)
195 const struct landlock_rule *rule;
196 const struct inode *inode;
198 /* Ignores nonexistent leafs. */
199 if (d_is_negative(dentry))
202 inode = d_backing_inode(dentry);
204 rule = landlock_find_rule(
205 domain, rcu_dereference(landlock_inode(inode)->object));
210 static inline layer_mask_t unmask_layers(const struct landlock_rule *const rule,
211 const access_mask_t access_request,
212 layer_mask_t layer_mask)
220 * An access is granted if, for each policy layer, at least one rule
221 * encountered on the pathwalk grants the requested accesses,
222 * regardless of their position in the layer stack. We must then check
223 * the remaining layers for each inode, from the first added layer to
226 for (layer_level = 0; layer_level < rule->num_layers; layer_level++) {
227 const struct landlock_layer *const layer =
228 &rule->layers[layer_level];
229 const layer_mask_t layer_bit = BIT_ULL(layer->level - 1);
231 /* Checks that the layer grants access to the full request. */
232 if ((layer->access & access_request) == access_request) {
233 layer_mask &= ~layer_bit;
242 static int check_access_path(const struct landlock_ruleset *const domain,
243 const struct path *const path,
244 const access_mask_t access_request)
246 bool allowed = false;
247 struct path walker_path;
248 layer_mask_t layer_mask;
253 if (WARN_ON_ONCE(!domain || !path))
256 * Allows access to pseudo filesystems that will never be mountable
257 * (e.g. sockfs, pipefs), but can still be reachable through
258 * /proc/<pid>/fd/<file-descriptor> .
260 if ((path->dentry->d_sb->s_flags & SB_NOUSER) ||
261 (d_is_positive(path->dentry) &&
262 unlikely(IS_PRIVATE(d_backing_inode(path->dentry)))))
264 if (WARN_ON_ONCE(domain->num_layers < 1))
267 /* Saves all layers handling a subset of requested accesses. */
269 for (i = 0; i < domain->num_layers; i++) {
270 if (domain->fs_access_masks[i] & access_request)
271 layer_mask |= BIT_ULL(i);
273 /* An access request not handled by the domain is allowed. */
278 path_get(&walker_path);
280 * We need to walk through all the hierarchy to not miss any relevant
284 struct dentry *parent_dentry;
287 unmask_layers(find_rule(domain, walker_path.dentry),
288 access_request, layer_mask);
289 if (layer_mask == 0) {
290 /* Stops when a rule from each layer grants access. */
296 if (walker_path.dentry == walker_path.mnt->mnt_root) {
297 if (follow_up(&walker_path)) {
298 /* Ignores hidden mount points. */
302 * Stops at the real root. Denies access
303 * because not all layers have granted access.
309 if (unlikely(IS_ROOT(walker_path.dentry))) {
311 * Stops at disconnected root directories. Only allows
312 * access to internal filesystems (e.g. nsfs, which is
313 * reachable through /proc/<pid>/ns/<namespace>).
315 allowed = !!(walker_path.mnt->mnt_flags & MNT_INTERNAL);
318 parent_dentry = dget_parent(walker_path.dentry);
319 dput(walker_path.dentry);
320 walker_path.dentry = parent_dentry;
322 path_put(&walker_path);
323 return allowed ? 0 : -EACCES;
326 static inline int current_check_access_path(const struct path *const path,
327 const access_mask_t access_request)
329 const struct landlock_ruleset *const dom =
330 landlock_get_current_domain();
334 return check_access_path(dom, path, access_request);
339 static void hook_inode_free_security(struct inode *const inode)
342 * All inodes must already have been untied from their object by
343 * release_inode() or hook_sb_delete().
345 WARN_ON_ONCE(landlock_inode(inode)->object);
348 /* Super-block hooks */
351 * Release the inodes used in a security policy.
353 * Cf. fsnotify_unmount_inodes() and invalidate_inodes()
355 static void hook_sb_delete(struct super_block *const sb)
357 struct inode *inode, *prev_inode = NULL;
359 if (!landlock_initialized)
362 spin_lock(&sb->s_inode_list_lock);
363 list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
364 struct landlock_object *object;
366 /* Only handles referenced inodes. */
367 if (!atomic_read(&inode->i_count))
371 * Protects against concurrent modification of inode (e.g.
372 * from get_inode_object()).
374 spin_lock(&inode->i_lock);
376 * Checks I_FREEING and I_WILL_FREE to protect against a race
377 * condition when release_inode() just called iput(), which
378 * could lead to a NULL dereference of inode->security or a
379 * second call to iput() for the same Landlock object. Also
380 * checks I_NEW because such inode cannot be tied to an object.
382 if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
383 spin_unlock(&inode->i_lock);
388 object = rcu_dereference(landlock_inode(inode)->object);
391 spin_unlock(&inode->i_lock);
394 /* Keeps a reference to this inode until the next loop walk. */
396 spin_unlock(&inode->i_lock);
399 * If there is no concurrent release_inode() ongoing, then we
400 * are in charge of calling iput() on this inode, otherwise we
401 * will just wait for it to finish.
403 spin_lock(&object->lock);
404 if (object->underobj == inode) {
405 object->underobj = NULL;
406 spin_unlock(&object->lock);
410 * Because object->underobj was not NULL,
411 * release_inode() and get_inode_object() guarantee
412 * that it is safe to reset
413 * landlock_inode(inode)->object while it is not NULL.
414 * It is therefore not necessary to lock inode->i_lock.
416 rcu_assign_pointer(landlock_inode(inode)->object, NULL);
418 * At this point, we own the ihold() reference that was
419 * originally set up by get_inode_object() and the
420 * __iget() reference that we just set in this loop
421 * walk. Therefore the following call to iput() will
422 * not sleep nor drop the inode because there is now at
423 * least two references to it.
427 spin_unlock(&object->lock);
433 * At this point, we still own the __iget() reference
434 * that we just set in this loop walk. Therefore we
435 * can drop the list lock and know that the inode won't
436 * disappear from under us until the next loop walk.
438 spin_unlock(&sb->s_inode_list_lock);
440 * We can now actually put the inode reference from the
441 * previous loop walk, which is not needed anymore.
445 spin_lock(&sb->s_inode_list_lock);
449 spin_unlock(&sb->s_inode_list_lock);
451 /* Puts the inode reference from the last loop walk, if any. */
454 /* Waits for pending iput() in release_inode(). */
455 wait_var_event(&landlock_superblock(sb)->inode_refs,
456 !atomic_long_read(&landlock_superblock(sb)->inode_refs));
460 * Because a Landlock security policy is defined according to the filesystem
461 * topology (i.e. the mount namespace), changing it may grant access to files
462 * not previously allowed.
464 * To make it simple, deny any filesystem topology modification by landlocked
465 * processes. Non-landlocked processes may still change the namespace of a
466 * landlocked process, but this kind of threat must be handled by a system-wide
467 * access-control security policy.
469 * This could be lifted in the future if Landlock can safely handle mount
470 * namespace updates requested by a landlocked process. Indeed, we could
471 * update the current domain (which is currently read-only) by taking into
472 * account the accesses of the source and the destination of a new mount point.
473 * However, it would also require to make all the child domains dynamically
474 * inherit these new constraints. Anyway, for backward compatibility reasons,
475 * a dedicated user space option would be required (e.g. as a ruleset flag).
477 static int hook_sb_mount(const char *const dev_name,
478 const struct path *const path, const char *const type,
479 const unsigned long flags, void *const data)
481 if (!landlock_get_current_domain())
486 static int hook_move_mount(const struct path *const from_path,
487 const struct path *const to_path)
489 if (!landlock_get_current_domain())
495 * Removing a mount point may reveal a previously hidden file hierarchy, which
496 * may then grant access to files, which may have previously been forbidden.
498 static int hook_sb_umount(struct vfsmount *const mnt, const int flags)
500 if (!landlock_get_current_domain())
505 static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts)
507 if (!landlock_get_current_domain())
513 * pivot_root(2), like mount(2), changes the current mount namespace. It must
514 * then be forbidden for a landlocked process.
516 * However, chroot(2) may be allowed because it only changes the relative root
517 * directory of the current process. Moreover, it can be used to restrict the
518 * view of the filesystem.
520 static int hook_sb_pivotroot(const struct path *const old_path,
521 const struct path *const new_path)
523 if (!landlock_get_current_domain())
530 static inline access_mask_t get_mode_access(const umode_t mode)
532 switch (mode & S_IFMT) {
534 return LANDLOCK_ACCESS_FS_MAKE_SYM;
536 /* A zero mode translates to S_IFREG. */
538 return LANDLOCK_ACCESS_FS_MAKE_REG;
540 return LANDLOCK_ACCESS_FS_MAKE_DIR;
542 return LANDLOCK_ACCESS_FS_MAKE_CHAR;
544 return LANDLOCK_ACCESS_FS_MAKE_BLOCK;
546 return LANDLOCK_ACCESS_FS_MAKE_FIFO;
548 return LANDLOCK_ACCESS_FS_MAKE_SOCK;
556 * Creating multiple links or renaming may lead to privilege escalations if not
557 * handled properly. Indeed, we must be sure that the source doesn't gain more
558 * privileges by being accessible from the destination. This is getting more
559 * complex when dealing with multiple layers. The whole picture can be seen as
560 * a multilayer partial ordering problem. A future version of Landlock will
563 static int hook_path_link(struct dentry *const old_dentry,
564 const struct path *const new_dir,
565 struct dentry *const new_dentry)
567 const struct landlock_ruleset *const dom =
568 landlock_get_current_domain();
572 /* The mount points are the same for old and new paths, cf. EXDEV. */
573 if (old_dentry->d_parent != new_dir->dentry)
574 /* Gracefully forbids reparenting. */
576 if (unlikely(d_is_negative(old_dentry)))
578 return check_access_path(
580 get_mode_access(d_backing_inode(old_dentry)->i_mode));
583 static inline access_mask_t maybe_remove(const struct dentry *const dentry)
585 if (d_is_negative(dentry))
587 return d_is_dir(dentry) ? LANDLOCK_ACCESS_FS_REMOVE_DIR :
588 LANDLOCK_ACCESS_FS_REMOVE_FILE;
591 static int hook_path_rename(const struct path *const old_dir,
592 struct dentry *const old_dentry,
593 const struct path *const new_dir,
594 struct dentry *const new_dentry)
596 const struct landlock_ruleset *const dom =
597 landlock_get_current_domain();
601 /* The mount points are the same for old and new paths, cf. EXDEV. */
602 if (old_dir->dentry != new_dir->dentry)
603 /* Gracefully forbids reparenting. */
605 if (unlikely(d_is_negative(old_dentry)))
607 /* RENAME_EXCHANGE is handled because directories are the same. */
608 return check_access_path(
610 maybe_remove(old_dentry) | maybe_remove(new_dentry) |
611 get_mode_access(d_backing_inode(old_dentry)->i_mode));
614 static int hook_path_mkdir(const struct path *const dir,
615 struct dentry *const dentry, const umode_t mode)
617 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR);
620 static int hook_path_mknod(const struct path *const dir,
621 struct dentry *const dentry, const umode_t mode,
622 const unsigned int dev)
624 const struct landlock_ruleset *const dom =
625 landlock_get_current_domain();
629 return check_access_path(dom, dir, get_mode_access(mode));
632 static int hook_path_symlink(const struct path *const dir,
633 struct dentry *const dentry,
634 const char *const old_name)
636 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM);
639 static int hook_path_unlink(const struct path *const dir,
640 struct dentry *const dentry)
642 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE);
645 static int hook_path_rmdir(const struct path *const dir,
646 struct dentry *const dentry)
648 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
653 static inline access_mask_t get_file_access(const struct file *const file)
655 access_mask_t access = 0;
657 if (file->f_mode & FMODE_READ) {
658 /* A directory can only be opened in read mode. */
659 if (S_ISDIR(file_inode(file)->i_mode))
660 return LANDLOCK_ACCESS_FS_READ_DIR;
661 access = LANDLOCK_ACCESS_FS_READ_FILE;
663 if (file->f_mode & FMODE_WRITE)
664 access |= LANDLOCK_ACCESS_FS_WRITE_FILE;
665 /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */
666 if (file->f_flags & __FMODE_EXEC)
667 access |= LANDLOCK_ACCESS_FS_EXECUTE;
671 static int hook_file_open(struct file *const file)
673 const struct landlock_ruleset *const dom =
674 landlock_get_current_domain();
679 * Because a file may be opened with O_PATH, get_file_access() may
680 * return 0. This case will be handled with a future Landlock
683 return check_access_path(dom, &file->f_path, get_file_access(file));
686 static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
687 LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),
689 LSM_HOOK_INIT(sb_delete, hook_sb_delete),
690 LSM_HOOK_INIT(sb_mount, hook_sb_mount),
691 LSM_HOOK_INIT(move_mount, hook_move_mount),
692 LSM_HOOK_INIT(sb_umount, hook_sb_umount),
693 LSM_HOOK_INIT(sb_remount, hook_sb_remount),
694 LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot),
696 LSM_HOOK_INIT(path_link, hook_path_link),
697 LSM_HOOK_INIT(path_rename, hook_path_rename),
698 LSM_HOOK_INIT(path_mkdir, hook_path_mkdir),
699 LSM_HOOK_INIT(path_mknod, hook_path_mknod),
700 LSM_HOOK_INIT(path_symlink, hook_path_symlink),
701 LSM_HOOK_INIT(path_unlink, hook_path_unlink),
702 LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
704 LSM_HOOK_INIT(file_open, hook_file_open),
707 __init void landlock_add_fs_hooks(void)
709 security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),