1 // SPDX-License-Identifier: GPL-2.0-only 2 /* 3 * Landlock LSM - Filesystem management and hooks 4 * 5 * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net> 6 * Copyright © 2018-2020 ANSSI 7 */ 8 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> 15 #include <linux/fs.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> 31 32 #include "common.h" 33 #include "cred.h" 34 #include "fs.h" 35 #include "limits.h" 36 #include "object.h" 37 #include "ruleset.h" 38 #include "setup.h" 39 40 /* Underlying object management */ 41 42 static void release_inode(struct landlock_object *const object) 43 __releases(object->lock) 44 { 45 struct inode *const inode = object->underobj; 46 struct super_block *sb; 47 48 if (!inode) { 49 spin_unlock(&object->lock); 50 return; 51 } 52 53 /* 54 * Protects against concurrent use by hook_sb_delete() of the reference 55 * to the underlying inode. 56 */ 57 object->underobj = NULL; 58 /* 59 * Makes sure that if the filesystem is concurrently unmounted, 60 * hook_sb_delete() will wait for us to finish iput(). 61 */ 62 sb = inode->i_sb; 63 atomic_long_inc(&landlock_superblock(sb)->inode_refs); 64 spin_unlock(&object->lock); 65 /* 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. 70 */ 71 rcu_assign_pointer(landlock_inode(inode)->object, NULL); 72 /* 73 * Now, new rules can safely be tied to @inode with get_inode_object(). 74 */ 75 76 iput(inode); 77 if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs)) 78 wake_up_var(&landlock_superblock(sb)->inode_refs); 79 } 80 81 static const struct landlock_object_underops landlock_fs_underops = { 82 .release = release_inode 83 }; 84 85 /* Ruleset management */ 86 87 static struct landlock_object *get_inode_object(struct inode *const inode) 88 { 89 struct landlock_object *object, *new_object; 90 struct landlock_inode_security *inode_sec = landlock_inode(inode); 91 92 rcu_read_lock(); 93 retry: 94 object = rcu_dereference(inode_sec->object); 95 if (object) { 96 if (likely(refcount_inc_not_zero(&object->usage))) { 97 rcu_read_unlock(); 98 return object; 99 } 100 /* 101 * We are racing with release_inode(), the object is going 102 * away. Wait for release_inode(), then retry. 103 */ 104 spin_lock(&object->lock); 105 spin_unlock(&object->lock); 106 goto retry; 107 } 108 rcu_read_unlock(); 109 110 /* 111 * If there is no object tied to @inode, then create a new one (without 112 * holding any locks). 113 */ 114 new_object = landlock_create_object(&landlock_fs_underops, inode); 115 if (IS_ERR(new_object)) 116 return new_object; 117 118 /* 119 * Protects against concurrent calls to get_inode_object() or 120 * hook_sb_delete(). 121 */ 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); 126 kfree(new_object); 127 128 rcu_read_lock(); 129 goto retry; 130 } 131 132 /* 133 * @inode will be released by hook_sb_delete() on its superblock 134 * shutdown, or by release_inode() when no more ruleset references the 135 * related object. 136 */ 137 ihold(inode); 138 rcu_assign_pointer(inode_sec->object, new_object); 139 spin_unlock(&inode->i_lock); 140 return new_object; 141 } 142 143 /* 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 */ 150 151 /* 152 * @path: Should have been checked by get_path_from_fd(). 153 */ 154 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset, 155 const struct path *const path, 156 access_mask_t access_rights) 157 { 158 int err; 159 struct landlock_object *object; 160 161 /* Files only get access rights that make sense. */ 162 if (!d_is_dir(path->dentry) && 163 (access_rights | ACCESS_FILE) != ACCESS_FILE) 164 return -EINVAL; 165 if (WARN_ON_ONCE(ruleset->num_layers != 1)) 166 return -EINVAL; 167 168 /* 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)); 171 if (IS_ERR(object)) 172 return PTR_ERR(object); 173 mutex_lock(&ruleset->lock); 174 err = landlock_insert_rule(ruleset, object, access_rights); 175 mutex_unlock(&ruleset->lock); 176 /* 177 * No need to check for an error because landlock_insert_rule() 178 * increments the refcount for the new object if needed. 179 */ 180 landlock_put_object(object); 181 return err; 182 } 183 184 /* Access-control management */ 185 186 static inline layer_mask_t 187 unmask_layers(const struct landlock_ruleset *const domain, 188 const struct path *const path, const access_mask_t access_request, 189 layer_mask_t layer_mask) 190 { 191 const struct landlock_rule *rule; 192 const struct inode *inode; 193 size_t i; 194 195 if (d_is_negative(path->dentry)) 196 /* Ignore nonexistent leafs. */ 197 return layer_mask; 198 inode = d_backing_inode(path->dentry); 199 rcu_read_lock(); 200 rule = landlock_find_rule( 201 domain, rcu_dereference(landlock_inode(inode)->object)); 202 rcu_read_unlock(); 203 if (!rule) 204 return layer_mask; 205 206 /* 207 * An access is granted if, for each policy layer, at least one rule 208 * encountered on the pathwalk grants the requested accesses, 209 * regardless of their position in the layer stack. We must then check 210 * the remaining layers for each inode, from the first added layer to 211 * the last one. 212 */ 213 for (i = 0; i < rule->num_layers; i++) { 214 const struct landlock_layer *const layer = &rule->layers[i]; 215 const layer_mask_t layer_bit = BIT_ULL(layer->level - 1); 216 217 /* Checks that the layer grants access to the full request. */ 218 if ((layer->access & access_request) == access_request) { 219 layer_mask &= ~layer_bit; 220 221 if (layer_mask == 0) 222 return layer_mask; 223 } 224 } 225 return layer_mask; 226 } 227 228 static int check_access_path(const struct landlock_ruleset *const domain, 229 const struct path *const path, 230 const access_mask_t access_request) 231 { 232 bool allowed = false; 233 struct path walker_path; 234 layer_mask_t layer_mask; 235 size_t i; 236 237 if (!access_request) 238 return 0; 239 if (WARN_ON_ONCE(!domain || !path)) 240 return 0; 241 /* 242 * Allows access to pseudo filesystems that will never be mountable 243 * (e.g. sockfs, pipefs), but can still be reachable through 244 * /proc/<pid>/fd/<file-descriptor> . 245 */ 246 if ((path->dentry->d_sb->s_flags & SB_NOUSER) || 247 (d_is_positive(path->dentry) && 248 unlikely(IS_PRIVATE(d_backing_inode(path->dentry))))) 249 return 0; 250 if (WARN_ON_ONCE(domain->num_layers < 1)) 251 return -EACCES; 252 253 /* Saves all layers handling a subset of requested accesses. */ 254 layer_mask = 0; 255 for (i = 0; i < domain->num_layers; i++) { 256 if (domain->fs_access_masks[i] & access_request) 257 layer_mask |= BIT_ULL(i); 258 } 259 /* An access request not handled by the domain is allowed. */ 260 if (layer_mask == 0) 261 return 0; 262 263 walker_path = *path; 264 path_get(&walker_path); 265 /* 266 * We need to walk through all the hierarchy to not miss any relevant 267 * restriction. 268 */ 269 while (true) { 270 struct dentry *parent_dentry; 271 272 layer_mask = unmask_layers(domain, &walker_path, access_request, 273 layer_mask); 274 if (layer_mask == 0) { 275 /* Stops when a rule from each layer grants access. */ 276 allowed = true; 277 break; 278 } 279 280 jump_up: 281 if (walker_path.dentry == walker_path.mnt->mnt_root) { 282 if (follow_up(&walker_path)) { 283 /* Ignores hidden mount points. */ 284 goto jump_up; 285 } else { 286 /* 287 * Stops at the real root. Denies access 288 * because not all layers have granted access. 289 */ 290 allowed = false; 291 break; 292 } 293 } 294 if (unlikely(IS_ROOT(walker_path.dentry))) { 295 /* 296 * Stops at disconnected root directories. Only allows 297 * access to internal filesystems (e.g. nsfs, which is 298 * reachable through /proc/<pid>/ns/<namespace>). 299 */ 300 allowed = !!(walker_path.mnt->mnt_flags & MNT_INTERNAL); 301 break; 302 } 303 parent_dentry = dget_parent(walker_path.dentry); 304 dput(walker_path.dentry); 305 walker_path.dentry = parent_dentry; 306 } 307 path_put(&walker_path); 308 return allowed ? 0 : -EACCES; 309 } 310 311 static inline int current_check_access_path(const struct path *const path, 312 const access_mask_t access_request) 313 { 314 const struct landlock_ruleset *const dom = 315 landlock_get_current_domain(); 316 317 if (!dom) 318 return 0; 319 return check_access_path(dom, path, access_request); 320 } 321 322 /* Inode hooks */ 323 324 static void hook_inode_free_security(struct inode *const inode) 325 { 326 /* 327 * All inodes must already have been untied from their object by 328 * release_inode() or hook_sb_delete(). 329 */ 330 WARN_ON_ONCE(landlock_inode(inode)->object); 331 } 332 333 /* Super-block hooks */ 334 335 /* 336 * Release the inodes used in a security policy. 337 * 338 * Cf. fsnotify_unmount_inodes() and invalidate_inodes() 339 */ 340 static void hook_sb_delete(struct super_block *const sb) 341 { 342 struct inode *inode, *prev_inode = NULL; 343 344 if (!landlock_initialized) 345 return; 346 347 spin_lock(&sb->s_inode_list_lock); 348 list_for_each_entry(inode, &sb->s_inodes, i_sb_list) { 349 struct landlock_object *object; 350 351 /* Only handles referenced inodes. */ 352 if (!atomic_read(&inode->i_count)) 353 continue; 354 355 /* 356 * Protects against concurrent modification of inode (e.g. 357 * from get_inode_object()). 358 */ 359 spin_lock(&inode->i_lock); 360 /* 361 * Checks I_FREEING and I_WILL_FREE to protect against a race 362 * condition when release_inode() just called iput(), which 363 * could lead to a NULL dereference of inode->security or a 364 * second call to iput() for the same Landlock object. Also 365 * checks I_NEW because such inode cannot be tied to an object. 366 */ 367 if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) { 368 spin_unlock(&inode->i_lock); 369 continue; 370 } 371 372 rcu_read_lock(); 373 object = rcu_dereference(landlock_inode(inode)->object); 374 if (!object) { 375 rcu_read_unlock(); 376 spin_unlock(&inode->i_lock); 377 continue; 378 } 379 /* Keeps a reference to this inode until the next loop walk. */ 380 __iget(inode); 381 spin_unlock(&inode->i_lock); 382 383 /* 384 * If there is no concurrent release_inode() ongoing, then we 385 * are in charge of calling iput() on this inode, otherwise we 386 * will just wait for it to finish. 387 */ 388 spin_lock(&object->lock); 389 if (object->underobj == inode) { 390 object->underobj = NULL; 391 spin_unlock(&object->lock); 392 rcu_read_unlock(); 393 394 /* 395 * Because object->underobj was not NULL, 396 * release_inode() and get_inode_object() guarantee 397 * that it is safe to reset 398 * landlock_inode(inode)->object while it is not NULL. 399 * It is therefore not necessary to lock inode->i_lock. 400 */ 401 rcu_assign_pointer(landlock_inode(inode)->object, NULL); 402 /* 403 * At this point, we own the ihold() reference that was 404 * originally set up by get_inode_object() and the 405 * __iget() reference that we just set in this loop 406 * walk. Therefore the following call to iput() will 407 * not sleep nor drop the inode because there is now at 408 * least two references to it. 409 */ 410 iput(inode); 411 } else { 412 spin_unlock(&object->lock); 413 rcu_read_unlock(); 414 } 415 416 if (prev_inode) { 417 /* 418 * At this point, we still own the __iget() reference 419 * that we just set in this loop walk. Therefore we 420 * can drop the list lock and know that the inode won't 421 * disappear from under us until the next loop walk. 422 */ 423 spin_unlock(&sb->s_inode_list_lock); 424 /* 425 * We can now actually put the inode reference from the 426 * previous loop walk, which is not needed anymore. 427 */ 428 iput(prev_inode); 429 cond_resched(); 430 spin_lock(&sb->s_inode_list_lock); 431 } 432 prev_inode = inode; 433 } 434 spin_unlock(&sb->s_inode_list_lock); 435 436 /* Puts the inode reference from the last loop walk, if any. */ 437 if (prev_inode) 438 iput(prev_inode); 439 /* Waits for pending iput() in release_inode(). */ 440 wait_var_event(&landlock_superblock(sb)->inode_refs, 441 !atomic_long_read(&landlock_superblock(sb)->inode_refs)); 442 } 443 444 /* 445 * Because a Landlock security policy is defined according to the filesystem 446 * topology (i.e. the mount namespace), changing it may grant access to files 447 * not previously allowed. 448 * 449 * To make it simple, deny any filesystem topology modification by landlocked 450 * processes. Non-landlocked processes may still change the namespace of a 451 * landlocked process, but this kind of threat must be handled by a system-wide 452 * access-control security policy. 453 * 454 * This could be lifted in the future if Landlock can safely handle mount 455 * namespace updates requested by a landlocked process. Indeed, we could 456 * update the current domain (which is currently read-only) by taking into 457 * account the accesses of the source and the destination of a new mount point. 458 * However, it would also require to make all the child domains dynamically 459 * inherit these new constraints. Anyway, for backward compatibility reasons, 460 * a dedicated user space option would be required (e.g. as a ruleset flag). 461 */ 462 static int hook_sb_mount(const char *const dev_name, 463 const struct path *const path, const char *const type, 464 const unsigned long flags, void *const data) 465 { 466 if (!landlock_get_current_domain()) 467 return 0; 468 return -EPERM; 469 } 470 471 static int hook_move_mount(const struct path *const from_path, 472 const struct path *const to_path) 473 { 474 if (!landlock_get_current_domain()) 475 return 0; 476 return -EPERM; 477 } 478 479 /* 480 * Removing a mount point may reveal a previously hidden file hierarchy, which 481 * may then grant access to files, which may have previously been forbidden. 482 */ 483 static int hook_sb_umount(struct vfsmount *const mnt, const int flags) 484 { 485 if (!landlock_get_current_domain()) 486 return 0; 487 return -EPERM; 488 } 489 490 static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts) 491 { 492 if (!landlock_get_current_domain()) 493 return 0; 494 return -EPERM; 495 } 496 497 /* 498 * pivot_root(2), like mount(2), changes the current mount namespace. It must 499 * then be forbidden for a landlocked process. 500 * 501 * However, chroot(2) may be allowed because it only changes the relative root 502 * directory of the current process. Moreover, it can be used to restrict the 503 * view of the filesystem. 504 */ 505 static int hook_sb_pivotroot(const struct path *const old_path, 506 const struct path *const new_path) 507 { 508 if (!landlock_get_current_domain()) 509 return 0; 510 return -EPERM; 511 } 512 513 /* Path hooks */ 514 515 static inline access_mask_t get_mode_access(const umode_t mode) 516 { 517 switch (mode & S_IFMT) { 518 case S_IFLNK: 519 return LANDLOCK_ACCESS_FS_MAKE_SYM; 520 case 0: 521 /* A zero mode translates to S_IFREG. */ 522 case S_IFREG: 523 return LANDLOCK_ACCESS_FS_MAKE_REG; 524 case S_IFDIR: 525 return LANDLOCK_ACCESS_FS_MAKE_DIR; 526 case S_IFCHR: 527 return LANDLOCK_ACCESS_FS_MAKE_CHAR; 528 case S_IFBLK: 529 return LANDLOCK_ACCESS_FS_MAKE_BLOCK; 530 case S_IFIFO: 531 return LANDLOCK_ACCESS_FS_MAKE_FIFO; 532 case S_IFSOCK: 533 return LANDLOCK_ACCESS_FS_MAKE_SOCK; 534 default: 535 WARN_ON_ONCE(1); 536 return 0; 537 } 538 } 539 540 /* 541 * Creating multiple links or renaming may lead to privilege escalations if not 542 * handled properly. Indeed, we must be sure that the source doesn't gain more 543 * privileges by being accessible from the destination. This is getting more 544 * complex when dealing with multiple layers. The whole picture can be seen as 545 * a multilayer partial ordering problem. A future version of Landlock will 546 * deal with that. 547 */ 548 static int hook_path_link(struct dentry *const old_dentry, 549 const struct path *const new_dir, 550 struct dentry *const new_dentry) 551 { 552 const struct landlock_ruleset *const dom = 553 landlock_get_current_domain(); 554 555 if (!dom) 556 return 0; 557 /* The mount points are the same for old and new paths, cf. EXDEV. */ 558 if (old_dentry->d_parent != new_dir->dentry) 559 /* Gracefully forbids reparenting. */ 560 return -EXDEV; 561 if (unlikely(d_is_negative(old_dentry))) 562 return -ENOENT; 563 return check_access_path( 564 dom, new_dir, 565 get_mode_access(d_backing_inode(old_dentry)->i_mode)); 566 } 567 568 static inline access_mask_t maybe_remove(const struct dentry *const dentry) 569 { 570 if (d_is_negative(dentry)) 571 return 0; 572 return d_is_dir(dentry) ? LANDLOCK_ACCESS_FS_REMOVE_DIR : 573 LANDLOCK_ACCESS_FS_REMOVE_FILE; 574 } 575 576 static int hook_path_rename(const struct path *const old_dir, 577 struct dentry *const old_dentry, 578 const struct path *const new_dir, 579 struct dentry *const new_dentry) 580 { 581 const struct landlock_ruleset *const dom = 582 landlock_get_current_domain(); 583 584 if (!dom) 585 return 0; 586 /* The mount points are the same for old and new paths, cf. EXDEV. */ 587 if (old_dir->dentry != new_dir->dentry) 588 /* Gracefully forbids reparenting. */ 589 return -EXDEV; 590 if (unlikely(d_is_negative(old_dentry))) 591 return -ENOENT; 592 /* RENAME_EXCHANGE is handled because directories are the same. */ 593 return check_access_path( 594 dom, old_dir, 595 maybe_remove(old_dentry) | maybe_remove(new_dentry) | 596 get_mode_access(d_backing_inode(old_dentry)->i_mode)); 597 } 598 599 static int hook_path_mkdir(const struct path *const dir, 600 struct dentry *const dentry, const umode_t mode) 601 { 602 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR); 603 } 604 605 static int hook_path_mknod(const struct path *const dir, 606 struct dentry *const dentry, const umode_t mode, 607 const unsigned int dev) 608 { 609 const struct landlock_ruleset *const dom = 610 landlock_get_current_domain(); 611 612 if (!dom) 613 return 0; 614 return check_access_path(dom, dir, get_mode_access(mode)); 615 } 616 617 static int hook_path_symlink(const struct path *const dir, 618 struct dentry *const dentry, 619 const char *const old_name) 620 { 621 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM); 622 } 623 624 static int hook_path_unlink(const struct path *const dir, 625 struct dentry *const dentry) 626 { 627 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE); 628 } 629 630 static int hook_path_rmdir(const struct path *const dir, 631 struct dentry *const dentry) 632 { 633 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR); 634 } 635 636 /* File hooks */ 637 638 static inline access_mask_t get_file_access(const struct file *const file) 639 { 640 access_mask_t access = 0; 641 642 if (file->f_mode & FMODE_READ) { 643 /* A directory can only be opened in read mode. */ 644 if (S_ISDIR(file_inode(file)->i_mode)) 645 return LANDLOCK_ACCESS_FS_READ_DIR; 646 access = LANDLOCK_ACCESS_FS_READ_FILE; 647 } 648 if (file->f_mode & FMODE_WRITE) 649 access |= LANDLOCK_ACCESS_FS_WRITE_FILE; 650 /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */ 651 if (file->f_flags & __FMODE_EXEC) 652 access |= LANDLOCK_ACCESS_FS_EXECUTE; 653 return access; 654 } 655 656 static int hook_file_open(struct file *const file) 657 { 658 const struct landlock_ruleset *const dom = 659 landlock_get_current_domain(); 660 661 if (!dom) 662 return 0; 663 /* 664 * Because a file may be opened with O_PATH, get_file_access() may 665 * return 0. This case will be handled with a future Landlock 666 * evolution. 667 */ 668 return check_access_path(dom, &file->f_path, get_file_access(file)); 669 } 670 671 static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = { 672 LSM_HOOK_INIT(inode_free_security, hook_inode_free_security), 673 674 LSM_HOOK_INIT(sb_delete, hook_sb_delete), 675 LSM_HOOK_INIT(sb_mount, hook_sb_mount), 676 LSM_HOOK_INIT(move_mount, hook_move_mount), 677 LSM_HOOK_INIT(sb_umount, hook_sb_umount), 678 LSM_HOOK_INIT(sb_remount, hook_sb_remount), 679 LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot), 680 681 LSM_HOOK_INIT(path_link, hook_path_link), 682 LSM_HOOK_INIT(path_rename, hook_path_rename), 683 LSM_HOOK_INIT(path_mkdir, hook_path_mkdir), 684 LSM_HOOK_INIT(path_mknod, hook_path_mknod), 685 LSM_HOOK_INIT(path_symlink, hook_path_symlink), 686 LSM_HOOK_INIT(path_unlink, hook_path_unlink), 687 LSM_HOOK_INIT(path_rmdir, hook_path_rmdir), 688 689 LSM_HOOK_INIT(file_open, hook_file_open), 690 }; 691 692 __init void landlock_add_fs_hooks(void) 693 { 694 security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks), 695 LANDLOCK_NAME); 696 } 697