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 * Copyright © 2021-2022 Microsoft Corporation 8 */ 9 10 #include <linux/atomic.h> 11 #include <linux/bitops.h> 12 #include <linux/bits.h> 13 #include <linux/compiler_types.h> 14 #include <linux/dcache.h> 15 #include <linux/err.h> 16 #include <linux/fs.h> 17 #include <linux/init.h> 18 #include <linux/kernel.h> 19 #include <linux/limits.h> 20 #include <linux/list.h> 21 #include <linux/lsm_hooks.h> 22 #include <linux/mount.h> 23 #include <linux/namei.h> 24 #include <linux/path.h> 25 #include <linux/rcupdate.h> 26 #include <linux/spinlock.h> 27 #include <linux/stat.h> 28 #include <linux/types.h> 29 #include <linux/wait_bit.h> 30 #include <linux/workqueue.h> 31 #include <uapi/linux/landlock.h> 32 33 #include "common.h" 34 #include "cred.h" 35 #include "fs.h" 36 #include "limits.h" 37 #include "object.h" 38 #include "ruleset.h" 39 #include "setup.h" 40 41 /* Underlying object management */ 42 43 static void release_inode(struct landlock_object *const object) 44 __releases(object->lock) 45 { 46 struct inode *const inode = object->underobj; 47 struct super_block *sb; 48 49 if (!inode) { 50 spin_unlock(&object->lock); 51 return; 52 } 53 54 /* 55 * Protects against concurrent use by hook_sb_delete() of the reference 56 * to the underlying inode. 57 */ 58 object->underobj = NULL; 59 /* 60 * Makes sure that if the filesystem is concurrently unmounted, 61 * hook_sb_delete() will wait for us to finish iput(). 62 */ 63 sb = inode->i_sb; 64 atomic_long_inc(&landlock_superblock(sb)->inode_refs); 65 spin_unlock(&object->lock); 66 /* 67 * Because object->underobj was not NULL, hook_sb_delete() and 68 * get_inode_object() guarantee that it is safe to reset 69 * landlock_inode(inode)->object while it is not NULL. It is therefore 70 * not necessary to lock inode->i_lock. 71 */ 72 rcu_assign_pointer(landlock_inode(inode)->object, NULL); 73 /* 74 * Now, new rules can safely be tied to @inode with get_inode_object(). 75 */ 76 77 iput(inode); 78 if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs)) 79 wake_up_var(&landlock_superblock(sb)->inode_refs); 80 } 81 82 static const struct landlock_object_underops landlock_fs_underops = { 83 .release = release_inode 84 }; 85 86 /* Ruleset management */ 87 88 static struct landlock_object *get_inode_object(struct inode *const inode) 89 { 90 struct landlock_object *object, *new_object; 91 struct landlock_inode_security *inode_sec = landlock_inode(inode); 92 93 rcu_read_lock(); 94 retry: 95 object = rcu_dereference(inode_sec->object); 96 if (object) { 97 if (likely(refcount_inc_not_zero(&object->usage))) { 98 rcu_read_unlock(); 99 return object; 100 } 101 /* 102 * We are racing with release_inode(), the object is going 103 * away. Wait for release_inode(), then retry. 104 */ 105 spin_lock(&object->lock); 106 spin_unlock(&object->lock); 107 goto retry; 108 } 109 rcu_read_unlock(); 110 111 /* 112 * If there is no object tied to @inode, then create a new one (without 113 * holding any locks). 114 */ 115 new_object = landlock_create_object(&landlock_fs_underops, inode); 116 if (IS_ERR(new_object)) 117 return new_object; 118 119 /* 120 * Protects against concurrent calls to get_inode_object() or 121 * hook_sb_delete(). 122 */ 123 spin_lock(&inode->i_lock); 124 if (unlikely(rcu_access_pointer(inode_sec->object))) { 125 /* Someone else just created the object, bail out and retry. */ 126 spin_unlock(&inode->i_lock); 127 kfree(new_object); 128 129 rcu_read_lock(); 130 goto retry; 131 } 132 133 /* 134 * @inode will be released by hook_sb_delete() on its superblock 135 * shutdown, or by release_inode() when no more ruleset references the 136 * related object. 137 */ 138 ihold(inode); 139 rcu_assign_pointer(inode_sec->object, new_object); 140 spin_unlock(&inode->i_lock); 141 return new_object; 142 } 143 144 /* All access rights that can be tied to files. */ 145 /* clang-format off */ 146 #define ACCESS_FILE ( \ 147 LANDLOCK_ACCESS_FS_EXECUTE | \ 148 LANDLOCK_ACCESS_FS_WRITE_FILE | \ 149 LANDLOCK_ACCESS_FS_READ_FILE) 150 /* clang-format on */ 151 152 /* 153 * All access rights that are denied by default whether they are handled or not 154 * by a ruleset/layer. This must be ORed with all ruleset->fs_access_masks[] 155 * entries when we need to get the absolute handled access masks. 156 */ 157 /* clang-format off */ 158 #define ACCESS_INITIALLY_DENIED ( \ 159 LANDLOCK_ACCESS_FS_REFER) 160 /* clang-format on */ 161 162 /* 163 * @path: Should have been checked by get_path_from_fd(). 164 */ 165 int landlock_append_fs_rule(struct landlock_ruleset *const ruleset, 166 const struct path *const path, 167 access_mask_t access_rights) 168 { 169 int err; 170 struct landlock_object *object; 171 172 /* Files only get access rights that make sense. */ 173 if (!d_is_dir(path->dentry) && 174 (access_rights | ACCESS_FILE) != ACCESS_FILE) 175 return -EINVAL; 176 if (WARN_ON_ONCE(ruleset->num_layers != 1)) 177 return -EINVAL; 178 179 /* Transforms relative access rights to absolute ones. */ 180 access_rights |= 181 LANDLOCK_MASK_ACCESS_FS & 182 ~(ruleset->fs_access_masks[0] | ACCESS_INITIALLY_DENIED); 183 object = get_inode_object(d_backing_inode(path->dentry)); 184 if (IS_ERR(object)) 185 return PTR_ERR(object); 186 mutex_lock(&ruleset->lock); 187 err = landlock_insert_rule(ruleset, object, access_rights); 188 mutex_unlock(&ruleset->lock); 189 /* 190 * No need to check for an error because landlock_insert_rule() 191 * increments the refcount for the new object if needed. 192 */ 193 landlock_put_object(object); 194 return err; 195 } 196 197 /* Access-control management */ 198 199 /* 200 * The lifetime of the returned rule is tied to @domain. 201 * 202 * Returns NULL if no rule is found or if @dentry is negative. 203 */ 204 static inline const struct landlock_rule * 205 find_rule(const struct landlock_ruleset *const domain, 206 const struct dentry *const dentry) 207 { 208 const struct landlock_rule *rule; 209 const struct inode *inode; 210 211 /* Ignores nonexistent leafs. */ 212 if (d_is_negative(dentry)) 213 return NULL; 214 215 inode = d_backing_inode(dentry); 216 rcu_read_lock(); 217 rule = landlock_find_rule( 218 domain, rcu_dereference(landlock_inode(inode)->object)); 219 rcu_read_unlock(); 220 return rule; 221 } 222 223 /* 224 * @layer_masks is read and may be updated according to the access request and 225 * the matching rule. 226 * 227 * Returns true if the request is allowed (i.e. relevant layer masks for the 228 * request are empty). 229 */ 230 static inline bool 231 unmask_layers(const struct landlock_rule *const rule, 232 const access_mask_t access_request, 233 layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS]) 234 { 235 size_t layer_level; 236 237 if (!access_request || !layer_masks) 238 return true; 239 if (!rule) 240 return false; 241 242 /* 243 * An access is granted if, for each policy layer, at least one rule 244 * encountered on the pathwalk grants the requested access, 245 * regardless of its position in the layer stack. We must then check 246 * the remaining layers for each inode, from the first added layer to 247 * the last one. When there is multiple requested accesses, for each 248 * policy layer, the full set of requested accesses may not be granted 249 * by only one rule, but by the union (binary OR) of multiple rules. 250 * E.g. /a/b <execute> + /a <read> => /a/b <execute + read> 251 */ 252 for (layer_level = 0; layer_level < rule->num_layers; layer_level++) { 253 const struct landlock_layer *const layer = 254 &rule->layers[layer_level]; 255 const layer_mask_t layer_bit = BIT_ULL(layer->level - 1); 256 const unsigned long access_req = access_request; 257 unsigned long access_bit; 258 bool is_empty; 259 260 /* 261 * Records in @layer_masks which layer grants access to each 262 * requested access. 263 */ 264 is_empty = true; 265 for_each_set_bit(access_bit, &access_req, 266 ARRAY_SIZE(*layer_masks)) { 267 if (layer->access & BIT_ULL(access_bit)) 268 (*layer_masks)[access_bit] &= ~layer_bit; 269 is_empty = is_empty && !(*layer_masks)[access_bit]; 270 } 271 if (is_empty) 272 return true; 273 } 274 return false; 275 } 276 277 /* 278 * Allows access to pseudo filesystems that will never be mountable (e.g. 279 * sockfs, pipefs), but can still be reachable through 280 * /proc/<pid>/fd/<file-descriptor> 281 */ 282 static inline bool is_nouser_or_private(const struct dentry *dentry) 283 { 284 return (dentry->d_sb->s_flags & SB_NOUSER) || 285 (d_is_positive(dentry) && 286 unlikely(IS_PRIVATE(d_backing_inode(dentry)))); 287 } 288 289 static inline access_mask_t 290 get_handled_accesses(const struct landlock_ruleset *const domain) 291 { 292 access_mask_t access_dom = ACCESS_INITIALLY_DENIED; 293 size_t layer_level; 294 295 for (layer_level = 0; layer_level < domain->num_layers; layer_level++) 296 access_dom |= domain->fs_access_masks[layer_level]; 297 return access_dom & LANDLOCK_MASK_ACCESS_FS; 298 } 299 300 /** 301 * init_layer_masks - Initialize layer masks from an access request 302 * 303 * Populates @layer_masks such that for each access right in @access_request, 304 * the bits for all the layers are set where this access right is handled. 305 * 306 * @domain: The domain that defines the current restrictions. 307 * @access_request: The requested access rights to check. 308 * @layer_masks: The layer masks to populate. 309 * 310 * Returns: An access mask where each access right bit is set which is handled 311 * in any of the active layers in @domain. 312 */ 313 static inline access_mask_t 314 init_layer_masks(const struct landlock_ruleset *const domain, 315 const access_mask_t access_request, 316 layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS]) 317 { 318 access_mask_t handled_accesses = 0; 319 size_t layer_level; 320 321 memset(layer_masks, 0, sizeof(*layer_masks)); 322 /* An empty access request can happen because of O_WRONLY | O_RDWR. */ 323 if (!access_request) 324 return 0; 325 326 /* Saves all handled accesses per layer. */ 327 for (layer_level = 0; layer_level < domain->num_layers; layer_level++) { 328 const unsigned long access_req = access_request; 329 unsigned long access_bit; 330 331 for_each_set_bit(access_bit, &access_req, 332 ARRAY_SIZE(*layer_masks)) { 333 /* 334 * Artificially handles all initially denied by default 335 * access rights. 336 */ 337 if (BIT_ULL(access_bit) & 338 (domain->fs_access_masks[layer_level] | 339 ACCESS_INITIALLY_DENIED)) { 340 (*layer_masks)[access_bit] |= 341 BIT_ULL(layer_level); 342 handled_accesses |= BIT_ULL(access_bit); 343 } 344 } 345 } 346 return handled_accesses; 347 } 348 349 /* 350 * Check that a destination file hierarchy has more restrictions than a source 351 * file hierarchy. This is only used for link and rename actions. 352 * 353 * @layer_masks_child2: Optional child masks. 354 */ 355 static inline bool no_more_access( 356 const layer_mask_t (*const layer_masks_parent1)[LANDLOCK_NUM_ACCESS_FS], 357 const layer_mask_t (*const layer_masks_child1)[LANDLOCK_NUM_ACCESS_FS], 358 const bool child1_is_directory, 359 const layer_mask_t (*const layer_masks_parent2)[LANDLOCK_NUM_ACCESS_FS], 360 const layer_mask_t (*const layer_masks_child2)[LANDLOCK_NUM_ACCESS_FS], 361 const bool child2_is_directory) 362 { 363 unsigned long access_bit; 364 365 for (access_bit = 0; access_bit < ARRAY_SIZE(*layer_masks_parent2); 366 access_bit++) { 367 /* Ignores accesses that only make sense for directories. */ 368 const bool is_file_access = 369 !!(BIT_ULL(access_bit) & ACCESS_FILE); 370 371 if (child1_is_directory || is_file_access) { 372 /* 373 * Checks if the destination restrictions are a 374 * superset of the source ones (i.e. inherited access 375 * rights without child exceptions): 376 * restrictions(parent2) >= restrictions(child1) 377 */ 378 if ((((*layer_masks_parent1)[access_bit] & 379 (*layer_masks_child1)[access_bit]) | 380 (*layer_masks_parent2)[access_bit]) != 381 (*layer_masks_parent2)[access_bit]) 382 return false; 383 } 384 385 if (!layer_masks_child2) 386 continue; 387 if (child2_is_directory || is_file_access) { 388 /* 389 * Checks inverted restrictions for RENAME_EXCHANGE: 390 * restrictions(parent1) >= restrictions(child2) 391 */ 392 if ((((*layer_masks_parent2)[access_bit] & 393 (*layer_masks_child2)[access_bit]) | 394 (*layer_masks_parent1)[access_bit]) != 395 (*layer_masks_parent1)[access_bit]) 396 return false; 397 } 398 } 399 return true; 400 } 401 402 /* 403 * Removes @layer_masks accesses that are not requested. 404 * 405 * Returns true if the request is allowed, false otherwise. 406 */ 407 static inline bool 408 scope_to_request(const access_mask_t access_request, 409 layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS]) 410 { 411 const unsigned long access_req = access_request; 412 unsigned long access_bit; 413 414 if (WARN_ON_ONCE(!layer_masks)) 415 return true; 416 417 for_each_clear_bit(access_bit, &access_req, ARRAY_SIZE(*layer_masks)) 418 (*layer_masks)[access_bit] = 0; 419 return !memchr_inv(layer_masks, 0, sizeof(*layer_masks)); 420 } 421 422 /* 423 * Returns true if there is at least one access right different than 424 * LANDLOCK_ACCESS_FS_REFER. 425 */ 426 static inline bool 427 is_eacces(const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS], 428 const access_mask_t access_request) 429 { 430 unsigned long access_bit; 431 /* LANDLOCK_ACCESS_FS_REFER alone must return -EXDEV. */ 432 const unsigned long access_check = access_request & 433 ~LANDLOCK_ACCESS_FS_REFER; 434 435 if (!layer_masks) 436 return false; 437 438 for_each_set_bit(access_bit, &access_check, ARRAY_SIZE(*layer_masks)) { 439 if ((*layer_masks)[access_bit]) 440 return true; 441 } 442 return false; 443 } 444 445 /** 446 * is_access_to_paths_allowed - Check accesses for requests with a common path 447 * 448 * @domain: Domain to check against. 449 * @path: File hierarchy to walk through. 450 * @access_request_parent1: Accesses to check, once @layer_masks_parent1 is 451 * equal to @layer_masks_parent2 (if any). This is tied to the unique 452 * requested path for most actions, or the source in case of a refer action 453 * (i.e. rename or link), or the source and destination in case of 454 * RENAME_EXCHANGE. 455 * @layer_masks_parent1: Pointer to a matrix of layer masks per access 456 * masks, identifying the layers that forbid a specific access. Bits from 457 * this matrix can be unset according to the @path walk. An empty matrix 458 * means that @domain allows all possible Landlock accesses (i.e. not only 459 * those identified by @access_request_parent1). This matrix can 460 * initially refer to domain layer masks and, when the accesses for the 461 * destination and source are the same, to requested layer masks. 462 * @dentry_child1: Dentry to the initial child of the parent1 path. This 463 * pointer must be NULL for non-refer actions (i.e. not link nor rename). 464 * @access_request_parent2: Similar to @access_request_parent1 but for a 465 * request involving a source and a destination. This refers to the 466 * destination, except in case of RENAME_EXCHANGE where it also refers to 467 * the source. Must be set to 0 when using a simple path request. 468 * @layer_masks_parent2: Similar to @layer_masks_parent1 but for a refer 469 * action. This must be NULL otherwise. 470 * @dentry_child2: Dentry to the initial child of the parent2 path. This 471 * pointer is only set for RENAME_EXCHANGE actions and must be NULL 472 * otherwise. 473 * 474 * This helper first checks that the destination has a superset of restrictions 475 * compared to the source (if any) for a common path. Because of 476 * RENAME_EXCHANGE actions, source and destinations may be swapped. It then 477 * checks that the collected accesses and the remaining ones are enough to 478 * allow the request. 479 * 480 * Returns: 481 * - true if the access request is granted; 482 * - false otherwise. 483 */ 484 static bool is_access_to_paths_allowed( 485 const struct landlock_ruleset *const domain, 486 const struct path *const path, 487 const access_mask_t access_request_parent1, 488 layer_mask_t (*const layer_masks_parent1)[LANDLOCK_NUM_ACCESS_FS], 489 const struct dentry *const dentry_child1, 490 const access_mask_t access_request_parent2, 491 layer_mask_t (*const layer_masks_parent2)[LANDLOCK_NUM_ACCESS_FS], 492 const struct dentry *const dentry_child2) 493 { 494 bool allowed_parent1 = false, allowed_parent2 = false, is_dom_check, 495 child1_is_directory = true, child2_is_directory = true; 496 struct path walker_path; 497 access_mask_t access_masked_parent1, access_masked_parent2; 498 layer_mask_t _layer_masks_child1[LANDLOCK_NUM_ACCESS_FS], 499 _layer_masks_child2[LANDLOCK_NUM_ACCESS_FS]; 500 layer_mask_t(*layer_masks_child1)[LANDLOCK_NUM_ACCESS_FS] = NULL, 501 (*layer_masks_child2)[LANDLOCK_NUM_ACCESS_FS] = NULL; 502 503 if (!access_request_parent1 && !access_request_parent2) 504 return true; 505 if (WARN_ON_ONCE(!domain || !path)) 506 return true; 507 if (is_nouser_or_private(path->dentry)) 508 return true; 509 if (WARN_ON_ONCE(domain->num_layers < 1 || !layer_masks_parent1)) 510 return false; 511 512 if (unlikely(layer_masks_parent2)) { 513 if (WARN_ON_ONCE(!dentry_child1)) 514 return false; 515 /* 516 * For a double request, first check for potential privilege 517 * escalation by looking at domain handled accesses (which are 518 * a superset of the meaningful requested accesses). 519 */ 520 access_masked_parent1 = access_masked_parent2 = 521 get_handled_accesses(domain); 522 is_dom_check = true; 523 } else { 524 if (WARN_ON_ONCE(dentry_child1 || dentry_child2)) 525 return false; 526 /* For a simple request, only check for requested accesses. */ 527 access_masked_parent1 = access_request_parent1; 528 access_masked_parent2 = access_request_parent2; 529 is_dom_check = false; 530 } 531 532 if (unlikely(dentry_child1)) { 533 unmask_layers(find_rule(domain, dentry_child1), 534 init_layer_masks(domain, LANDLOCK_MASK_ACCESS_FS, 535 &_layer_masks_child1), 536 &_layer_masks_child1); 537 layer_masks_child1 = &_layer_masks_child1; 538 child1_is_directory = d_is_dir(dentry_child1); 539 } 540 if (unlikely(dentry_child2)) { 541 unmask_layers(find_rule(domain, dentry_child2), 542 init_layer_masks(domain, LANDLOCK_MASK_ACCESS_FS, 543 &_layer_masks_child2), 544 &_layer_masks_child2); 545 layer_masks_child2 = &_layer_masks_child2; 546 child2_is_directory = d_is_dir(dentry_child2); 547 } 548 549 walker_path = *path; 550 path_get(&walker_path); 551 /* 552 * We need to walk through all the hierarchy to not miss any relevant 553 * restriction. 554 */ 555 while (true) { 556 struct dentry *parent_dentry; 557 const struct landlock_rule *rule; 558 559 /* 560 * If at least all accesses allowed on the destination are 561 * already allowed on the source, respectively if there is at 562 * least as much as restrictions on the destination than on the 563 * source, then we can safely refer files from the source to 564 * the destination without risking a privilege escalation. 565 * This also applies in the case of RENAME_EXCHANGE, which 566 * implies checks on both direction. This is crucial for 567 * standalone multilayered security policies. Furthermore, 568 * this helps avoid policy writers to shoot themselves in the 569 * foot. 570 */ 571 if (unlikely(is_dom_check && 572 no_more_access( 573 layer_masks_parent1, layer_masks_child1, 574 child1_is_directory, layer_masks_parent2, 575 layer_masks_child2, 576 child2_is_directory))) { 577 allowed_parent1 = scope_to_request( 578 access_request_parent1, layer_masks_parent1); 579 allowed_parent2 = scope_to_request( 580 access_request_parent2, layer_masks_parent2); 581 582 /* Stops when all accesses are granted. */ 583 if (allowed_parent1 && allowed_parent2) 584 break; 585 586 /* 587 * Now, downgrades the remaining checks from domain 588 * handled accesses to requested accesses. 589 */ 590 is_dom_check = false; 591 access_masked_parent1 = access_request_parent1; 592 access_masked_parent2 = access_request_parent2; 593 } 594 595 rule = find_rule(domain, walker_path.dentry); 596 allowed_parent1 = unmask_layers(rule, access_masked_parent1, 597 layer_masks_parent1); 598 allowed_parent2 = unmask_layers(rule, access_masked_parent2, 599 layer_masks_parent2); 600 601 /* Stops when a rule from each layer grants access. */ 602 if (allowed_parent1 && allowed_parent2) 603 break; 604 605 jump_up: 606 if (walker_path.dentry == walker_path.mnt->mnt_root) { 607 if (follow_up(&walker_path)) { 608 /* Ignores hidden mount points. */ 609 goto jump_up; 610 } else { 611 /* 612 * Stops at the real root. Denies access 613 * because not all layers have granted access. 614 */ 615 break; 616 } 617 } 618 if (unlikely(IS_ROOT(walker_path.dentry))) { 619 /* 620 * Stops at disconnected root directories. Only allows 621 * access to internal filesystems (e.g. nsfs, which is 622 * reachable through /proc/<pid>/ns/<namespace>). 623 */ 624 allowed_parent1 = allowed_parent2 = 625 !!(walker_path.mnt->mnt_flags & MNT_INTERNAL); 626 break; 627 } 628 parent_dentry = dget_parent(walker_path.dentry); 629 dput(walker_path.dentry); 630 walker_path.dentry = parent_dentry; 631 } 632 path_put(&walker_path); 633 634 return allowed_parent1 && allowed_parent2; 635 } 636 637 static inline int check_access_path(const struct landlock_ruleset *const domain, 638 const struct path *const path, 639 access_mask_t access_request) 640 { 641 layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {}; 642 643 access_request = init_layer_masks(domain, access_request, &layer_masks); 644 if (is_access_to_paths_allowed(domain, path, access_request, 645 &layer_masks, NULL, 0, NULL, NULL)) 646 return 0; 647 return -EACCES; 648 } 649 650 static inline int current_check_access_path(const struct path *const path, 651 const access_mask_t access_request) 652 { 653 const struct landlock_ruleset *const dom = 654 landlock_get_current_domain(); 655 656 if (!dom) 657 return 0; 658 return check_access_path(dom, path, access_request); 659 } 660 661 static inline access_mask_t get_mode_access(const umode_t mode) 662 { 663 switch (mode & S_IFMT) { 664 case S_IFLNK: 665 return LANDLOCK_ACCESS_FS_MAKE_SYM; 666 case 0: 667 /* A zero mode translates to S_IFREG. */ 668 case S_IFREG: 669 return LANDLOCK_ACCESS_FS_MAKE_REG; 670 case S_IFDIR: 671 return LANDLOCK_ACCESS_FS_MAKE_DIR; 672 case S_IFCHR: 673 return LANDLOCK_ACCESS_FS_MAKE_CHAR; 674 case S_IFBLK: 675 return LANDLOCK_ACCESS_FS_MAKE_BLOCK; 676 case S_IFIFO: 677 return LANDLOCK_ACCESS_FS_MAKE_FIFO; 678 case S_IFSOCK: 679 return LANDLOCK_ACCESS_FS_MAKE_SOCK; 680 default: 681 WARN_ON_ONCE(1); 682 return 0; 683 } 684 } 685 686 static inline access_mask_t maybe_remove(const struct dentry *const dentry) 687 { 688 if (d_is_negative(dentry)) 689 return 0; 690 return d_is_dir(dentry) ? LANDLOCK_ACCESS_FS_REMOVE_DIR : 691 LANDLOCK_ACCESS_FS_REMOVE_FILE; 692 } 693 694 /** 695 * collect_domain_accesses - Walk through a file path and collect accesses 696 * 697 * @domain: Domain to check against. 698 * @mnt_root: Last directory to check. 699 * @dir: Directory to start the walk from. 700 * @layer_masks_dom: Where to store the collected accesses. 701 * 702 * This helper is useful to begin a path walk from the @dir directory to a 703 * @mnt_root directory used as a mount point. This mount point is the common 704 * ancestor between the source and the destination of a renamed and linked 705 * file. While walking from @dir to @mnt_root, we record all the domain's 706 * allowed accesses in @layer_masks_dom. 707 * 708 * This is similar to is_access_to_paths_allowed() but much simpler because it 709 * only handles walking on the same mount point and only checks one set of 710 * accesses. 711 * 712 * Returns: 713 * - true if all the domain access rights are allowed for @dir; 714 * - false if the walk reached @mnt_root. 715 */ 716 static bool collect_domain_accesses( 717 const struct landlock_ruleset *const domain, 718 const struct dentry *const mnt_root, struct dentry *dir, 719 layer_mask_t (*const layer_masks_dom)[LANDLOCK_NUM_ACCESS_FS]) 720 { 721 unsigned long access_dom; 722 bool ret = false; 723 724 if (WARN_ON_ONCE(!domain || !mnt_root || !dir || !layer_masks_dom)) 725 return true; 726 if (is_nouser_or_private(dir)) 727 return true; 728 729 access_dom = init_layer_masks(domain, LANDLOCK_MASK_ACCESS_FS, 730 layer_masks_dom); 731 732 dget(dir); 733 while (true) { 734 struct dentry *parent_dentry; 735 736 /* Gets all layers allowing all domain accesses. */ 737 if (unmask_layers(find_rule(domain, dir), access_dom, 738 layer_masks_dom)) { 739 /* 740 * Stops when all handled accesses are allowed by at 741 * least one rule in each layer. 742 */ 743 ret = true; 744 break; 745 } 746 747 /* We should not reach a root other than @mnt_root. */ 748 if (dir == mnt_root || WARN_ON_ONCE(IS_ROOT(dir))) 749 break; 750 751 parent_dentry = dget_parent(dir); 752 dput(dir); 753 dir = parent_dentry; 754 } 755 dput(dir); 756 return ret; 757 } 758 759 /** 760 * current_check_refer_path - Check if a rename or link action is allowed 761 * 762 * @old_dentry: File or directory requested to be moved or linked. 763 * @new_dir: Destination parent directory. 764 * @new_dentry: Destination file or directory. 765 * @removable: Sets to true if it is a rename operation. 766 * @exchange: Sets to true if it is a rename operation with RENAME_EXCHANGE. 767 * 768 * Because of its unprivileged constraints, Landlock relies on file hierarchies 769 * (and not only inodes) to tie access rights to files. Being able to link or 770 * rename a file hierarchy brings some challenges. Indeed, moving or linking a 771 * file (i.e. creating a new reference to an inode) can have an impact on the 772 * actions allowed for a set of files if it would change its parent directory 773 * (i.e. reparenting). 774 * 775 * To avoid trivial access right bypasses, Landlock first checks if the file or 776 * directory requested to be moved would gain new access rights inherited from 777 * its new hierarchy. Before returning any error, Landlock then checks that 778 * the parent source hierarchy and the destination hierarchy would allow the 779 * link or rename action. If it is not the case, an error with EACCES is 780 * returned to inform user space that there is no way to remove or create the 781 * requested source file type. If it should be allowed but the new inherited 782 * access rights would be greater than the source access rights, then the 783 * kernel returns an error with EXDEV. Prioritizing EACCES over EXDEV enables 784 * user space to abort the whole operation if there is no way to do it, or to 785 * manually copy the source to the destination if this remains allowed, e.g. 786 * because file creation is allowed on the destination directory but not direct 787 * linking. 788 * 789 * To achieve this goal, the kernel needs to compare two file hierarchies: the 790 * one identifying the source file or directory (including itself), and the 791 * destination one. This can be seen as a multilayer partial ordering problem. 792 * The kernel walks through these paths and collects in a matrix the access 793 * rights that are denied per layer. These matrices are then compared to see 794 * if the destination one has more (or the same) restrictions as the source 795 * one. If this is the case, the requested action will not return EXDEV, which 796 * doesn't mean the action is allowed. The parent hierarchy of the source 797 * (i.e. parent directory), and the destination hierarchy must also be checked 798 * to verify that they explicitly allow such action (i.e. referencing, 799 * creation and potentially removal rights). The kernel implementation is then 800 * required to rely on potentially four matrices of access rights: one for the 801 * source file or directory (i.e. the child), a potentially other one for the 802 * other source/destination (in case of RENAME_EXCHANGE), one for the source 803 * parent hierarchy and a last one for the destination hierarchy. These 804 * ephemeral matrices take some space on the stack, which limits the number of 805 * layers to a deemed reasonable number: 16. 806 * 807 * Returns: 808 * - 0 if access is allowed; 809 * - -EXDEV if @old_dentry would inherit new access rights from @new_dir; 810 * - -EACCES if file removal or creation is denied. 811 */ 812 static int current_check_refer_path(struct dentry *const old_dentry, 813 const struct path *const new_dir, 814 struct dentry *const new_dentry, 815 const bool removable, const bool exchange) 816 { 817 const struct landlock_ruleset *const dom = 818 landlock_get_current_domain(); 819 bool allow_parent1, allow_parent2; 820 access_mask_t access_request_parent1, access_request_parent2; 821 struct path mnt_dir; 822 layer_mask_t layer_masks_parent1[LANDLOCK_NUM_ACCESS_FS], 823 layer_masks_parent2[LANDLOCK_NUM_ACCESS_FS]; 824 825 if (!dom) 826 return 0; 827 if (WARN_ON_ONCE(dom->num_layers < 1)) 828 return -EACCES; 829 if (unlikely(d_is_negative(old_dentry))) 830 return -ENOENT; 831 if (exchange) { 832 if (unlikely(d_is_negative(new_dentry))) 833 return -ENOENT; 834 access_request_parent1 = 835 get_mode_access(d_backing_inode(new_dentry)->i_mode); 836 } else { 837 access_request_parent1 = 0; 838 } 839 access_request_parent2 = 840 get_mode_access(d_backing_inode(old_dentry)->i_mode); 841 if (removable) { 842 access_request_parent1 |= maybe_remove(old_dentry); 843 access_request_parent2 |= maybe_remove(new_dentry); 844 } 845 846 /* The mount points are the same for old and new paths, cf. EXDEV. */ 847 if (old_dentry->d_parent == new_dir->dentry) { 848 /* 849 * The LANDLOCK_ACCESS_FS_REFER access right is not required 850 * for same-directory referer (i.e. no reparenting). 851 */ 852 access_request_parent1 = init_layer_masks( 853 dom, access_request_parent1 | access_request_parent2, 854 &layer_masks_parent1); 855 if (is_access_to_paths_allowed( 856 dom, new_dir, access_request_parent1, 857 &layer_masks_parent1, NULL, 0, NULL, NULL)) 858 return 0; 859 return -EACCES; 860 } 861 862 access_request_parent1 |= LANDLOCK_ACCESS_FS_REFER; 863 access_request_parent2 |= LANDLOCK_ACCESS_FS_REFER; 864 865 /* Saves the common mount point. */ 866 mnt_dir.mnt = new_dir->mnt; 867 mnt_dir.dentry = new_dir->mnt->mnt_root; 868 869 /* new_dir->dentry is equal to new_dentry->d_parent */ 870 allow_parent1 = collect_domain_accesses(dom, mnt_dir.dentry, 871 old_dentry->d_parent, 872 &layer_masks_parent1); 873 allow_parent2 = collect_domain_accesses( 874 dom, mnt_dir.dentry, new_dir->dentry, &layer_masks_parent2); 875 876 if (allow_parent1 && allow_parent2) 877 return 0; 878 879 /* 880 * To be able to compare source and destination domain access rights, 881 * take into account the @old_dentry access rights aggregated with its 882 * parent access rights. This will be useful to compare with the 883 * destination parent access rights. 884 */ 885 if (is_access_to_paths_allowed( 886 dom, &mnt_dir, access_request_parent1, &layer_masks_parent1, 887 old_dentry, access_request_parent2, &layer_masks_parent2, 888 exchange ? new_dentry : NULL)) 889 return 0; 890 891 /* 892 * This prioritizes EACCES over EXDEV for all actions, including 893 * renames with RENAME_EXCHANGE. 894 */ 895 if (likely(is_eacces(&layer_masks_parent1, access_request_parent1) || 896 is_eacces(&layer_masks_parent2, access_request_parent2))) 897 return -EACCES; 898 899 /* 900 * Gracefully forbids reparenting if the destination directory 901 * hierarchy is not a superset of restrictions of the source directory 902 * hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the 903 * source or the destination. 904 */ 905 return -EXDEV; 906 } 907 908 /* Inode hooks */ 909 910 static void hook_inode_free_security(struct inode *const inode) 911 { 912 /* 913 * All inodes must already have been untied from their object by 914 * release_inode() or hook_sb_delete(). 915 */ 916 WARN_ON_ONCE(landlock_inode(inode)->object); 917 } 918 919 /* Super-block hooks */ 920 921 /* 922 * Release the inodes used in a security policy. 923 * 924 * Cf. fsnotify_unmount_inodes() and invalidate_inodes() 925 */ 926 static void hook_sb_delete(struct super_block *const sb) 927 { 928 struct inode *inode, *prev_inode = NULL; 929 930 if (!landlock_initialized) 931 return; 932 933 spin_lock(&sb->s_inode_list_lock); 934 list_for_each_entry(inode, &sb->s_inodes, i_sb_list) { 935 struct landlock_object *object; 936 937 /* Only handles referenced inodes. */ 938 if (!atomic_read(&inode->i_count)) 939 continue; 940 941 /* 942 * Protects against concurrent modification of inode (e.g. 943 * from get_inode_object()). 944 */ 945 spin_lock(&inode->i_lock); 946 /* 947 * Checks I_FREEING and I_WILL_FREE to protect against a race 948 * condition when release_inode() just called iput(), which 949 * could lead to a NULL dereference of inode->security or a 950 * second call to iput() for the same Landlock object. Also 951 * checks I_NEW because such inode cannot be tied to an object. 952 */ 953 if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) { 954 spin_unlock(&inode->i_lock); 955 continue; 956 } 957 958 rcu_read_lock(); 959 object = rcu_dereference(landlock_inode(inode)->object); 960 if (!object) { 961 rcu_read_unlock(); 962 spin_unlock(&inode->i_lock); 963 continue; 964 } 965 /* Keeps a reference to this inode until the next loop walk. */ 966 __iget(inode); 967 spin_unlock(&inode->i_lock); 968 969 /* 970 * If there is no concurrent release_inode() ongoing, then we 971 * are in charge of calling iput() on this inode, otherwise we 972 * will just wait for it to finish. 973 */ 974 spin_lock(&object->lock); 975 if (object->underobj == inode) { 976 object->underobj = NULL; 977 spin_unlock(&object->lock); 978 rcu_read_unlock(); 979 980 /* 981 * Because object->underobj was not NULL, 982 * release_inode() and get_inode_object() guarantee 983 * that it is safe to reset 984 * landlock_inode(inode)->object while it is not NULL. 985 * It is therefore not necessary to lock inode->i_lock. 986 */ 987 rcu_assign_pointer(landlock_inode(inode)->object, NULL); 988 /* 989 * At this point, we own the ihold() reference that was 990 * originally set up by get_inode_object() and the 991 * __iget() reference that we just set in this loop 992 * walk. Therefore the following call to iput() will 993 * not sleep nor drop the inode because there is now at 994 * least two references to it. 995 */ 996 iput(inode); 997 } else { 998 spin_unlock(&object->lock); 999 rcu_read_unlock(); 1000 } 1001 1002 if (prev_inode) { 1003 /* 1004 * At this point, we still own the __iget() reference 1005 * that we just set in this loop walk. Therefore we 1006 * can drop the list lock and know that the inode won't 1007 * disappear from under us until the next loop walk. 1008 */ 1009 spin_unlock(&sb->s_inode_list_lock); 1010 /* 1011 * We can now actually put the inode reference from the 1012 * previous loop walk, which is not needed anymore. 1013 */ 1014 iput(prev_inode); 1015 cond_resched(); 1016 spin_lock(&sb->s_inode_list_lock); 1017 } 1018 prev_inode = inode; 1019 } 1020 spin_unlock(&sb->s_inode_list_lock); 1021 1022 /* Puts the inode reference from the last loop walk, if any. */ 1023 if (prev_inode) 1024 iput(prev_inode); 1025 /* Waits for pending iput() in release_inode(). */ 1026 wait_var_event(&landlock_superblock(sb)->inode_refs, 1027 !atomic_long_read(&landlock_superblock(sb)->inode_refs)); 1028 } 1029 1030 /* 1031 * Because a Landlock security policy is defined according to the filesystem 1032 * topology (i.e. the mount namespace), changing it may grant access to files 1033 * not previously allowed. 1034 * 1035 * To make it simple, deny any filesystem topology modification by landlocked 1036 * processes. Non-landlocked processes may still change the namespace of a 1037 * landlocked process, but this kind of threat must be handled by a system-wide 1038 * access-control security policy. 1039 * 1040 * This could be lifted in the future if Landlock can safely handle mount 1041 * namespace updates requested by a landlocked process. Indeed, we could 1042 * update the current domain (which is currently read-only) by taking into 1043 * account the accesses of the source and the destination of a new mount point. 1044 * However, it would also require to make all the child domains dynamically 1045 * inherit these new constraints. Anyway, for backward compatibility reasons, 1046 * a dedicated user space option would be required (e.g. as a ruleset flag). 1047 */ 1048 static int hook_sb_mount(const char *const dev_name, 1049 const struct path *const path, const char *const type, 1050 const unsigned long flags, void *const data) 1051 { 1052 if (!landlock_get_current_domain()) 1053 return 0; 1054 return -EPERM; 1055 } 1056 1057 static int hook_move_mount(const struct path *const from_path, 1058 const struct path *const to_path) 1059 { 1060 if (!landlock_get_current_domain()) 1061 return 0; 1062 return -EPERM; 1063 } 1064 1065 /* 1066 * Removing a mount point may reveal a previously hidden file hierarchy, which 1067 * may then grant access to files, which may have previously been forbidden. 1068 */ 1069 static int hook_sb_umount(struct vfsmount *const mnt, const int flags) 1070 { 1071 if (!landlock_get_current_domain()) 1072 return 0; 1073 return -EPERM; 1074 } 1075 1076 static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts) 1077 { 1078 if (!landlock_get_current_domain()) 1079 return 0; 1080 return -EPERM; 1081 } 1082 1083 /* 1084 * pivot_root(2), like mount(2), changes the current mount namespace. It must 1085 * then be forbidden for a landlocked process. 1086 * 1087 * However, chroot(2) may be allowed because it only changes the relative root 1088 * directory of the current process. Moreover, it can be used to restrict the 1089 * view of the filesystem. 1090 */ 1091 static int hook_sb_pivotroot(const struct path *const old_path, 1092 const struct path *const new_path) 1093 { 1094 if (!landlock_get_current_domain()) 1095 return 0; 1096 return -EPERM; 1097 } 1098 1099 /* Path hooks */ 1100 1101 static int hook_path_link(struct dentry *const old_dentry, 1102 const struct path *const new_dir, 1103 struct dentry *const new_dentry) 1104 { 1105 return current_check_refer_path(old_dentry, new_dir, new_dentry, false, 1106 false); 1107 } 1108 1109 static int hook_path_rename(const struct path *const old_dir, 1110 struct dentry *const old_dentry, 1111 const struct path *const new_dir, 1112 struct dentry *const new_dentry, 1113 const unsigned int flags) 1114 { 1115 /* old_dir refers to old_dentry->d_parent and new_dir->mnt */ 1116 return current_check_refer_path(old_dentry, new_dir, new_dentry, true, 1117 !!(flags & RENAME_EXCHANGE)); 1118 } 1119 1120 static int hook_path_mkdir(const struct path *const dir, 1121 struct dentry *const dentry, const umode_t mode) 1122 { 1123 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR); 1124 } 1125 1126 static int hook_path_mknod(const struct path *const dir, 1127 struct dentry *const dentry, const umode_t mode, 1128 const unsigned int dev) 1129 { 1130 const struct landlock_ruleset *const dom = 1131 landlock_get_current_domain(); 1132 1133 if (!dom) 1134 return 0; 1135 return check_access_path(dom, dir, get_mode_access(mode)); 1136 } 1137 1138 static int hook_path_symlink(const struct path *const dir, 1139 struct dentry *const dentry, 1140 const char *const old_name) 1141 { 1142 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM); 1143 } 1144 1145 static int hook_path_unlink(const struct path *const dir, 1146 struct dentry *const dentry) 1147 { 1148 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE); 1149 } 1150 1151 static int hook_path_rmdir(const struct path *const dir, 1152 struct dentry *const dentry) 1153 { 1154 return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR); 1155 } 1156 1157 /* File hooks */ 1158 1159 static inline access_mask_t get_file_access(const struct file *const file) 1160 { 1161 access_mask_t access = 0; 1162 1163 if (file->f_mode & FMODE_READ) { 1164 /* A directory can only be opened in read mode. */ 1165 if (S_ISDIR(file_inode(file)->i_mode)) 1166 return LANDLOCK_ACCESS_FS_READ_DIR; 1167 access = LANDLOCK_ACCESS_FS_READ_FILE; 1168 } 1169 if (file->f_mode & FMODE_WRITE) 1170 access |= LANDLOCK_ACCESS_FS_WRITE_FILE; 1171 /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */ 1172 if (file->f_flags & __FMODE_EXEC) 1173 access |= LANDLOCK_ACCESS_FS_EXECUTE; 1174 return access; 1175 } 1176 1177 static int hook_file_open(struct file *const file) 1178 { 1179 const struct landlock_ruleset *const dom = 1180 landlock_get_current_domain(); 1181 1182 if (!dom) 1183 return 0; 1184 /* 1185 * Because a file may be opened with O_PATH, get_file_access() may 1186 * return 0. This case will be handled with a future Landlock 1187 * evolution. 1188 */ 1189 return check_access_path(dom, &file->f_path, get_file_access(file)); 1190 } 1191 1192 static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = { 1193 LSM_HOOK_INIT(inode_free_security, hook_inode_free_security), 1194 1195 LSM_HOOK_INIT(sb_delete, hook_sb_delete), 1196 LSM_HOOK_INIT(sb_mount, hook_sb_mount), 1197 LSM_HOOK_INIT(move_mount, hook_move_mount), 1198 LSM_HOOK_INIT(sb_umount, hook_sb_umount), 1199 LSM_HOOK_INIT(sb_remount, hook_sb_remount), 1200 LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot), 1201 1202 LSM_HOOK_INIT(path_link, hook_path_link), 1203 LSM_HOOK_INIT(path_rename, hook_path_rename), 1204 LSM_HOOK_INIT(path_mkdir, hook_path_mkdir), 1205 LSM_HOOK_INIT(path_mknod, hook_path_mknod), 1206 LSM_HOOK_INIT(path_symlink, hook_path_symlink), 1207 LSM_HOOK_INIT(path_unlink, hook_path_unlink), 1208 LSM_HOOK_INIT(path_rmdir, hook_path_rmdir), 1209 1210 LSM_HOOK_INIT(file_open, hook_file_open), 1211 }; 1212 1213 __init void landlock_add_fs_hooks(void) 1214 { 1215 security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks), 1216 LANDLOCK_NAME); 1217 } 1218