| // SPDX-License-Identifier: GPL-2.0-only |
| /* |
| * Landlock LSM - Filesystem management and hooks |
| * |
| * Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net> |
| * Copyright © 2018-2020 ANSSI |
| * Copyright © 2021-2022 Microsoft Corporation |
| */ |
| |
| #include <linux/atomic.h> |
| #include <linux/bitops.h> |
| #include <linux/bits.h> |
| #include <linux/compiler_types.h> |
| #include <linux/dcache.h> |
| #include <linux/err.h> |
| #include <linux/fs.h> |
| #include <linux/init.h> |
| #include <linux/kernel.h> |
| #include <linux/limits.h> |
| #include <linux/list.h> |
| #include <linux/lsm_hooks.h> |
| #include <linux/mount.h> |
| #include <linux/namei.h> |
| #include <linux/path.h> |
| #include <linux/rcupdate.h> |
| #include <linux/spinlock.h> |
| #include <linux/stat.h> |
| #include <linux/types.h> |
| #include <linux/wait_bit.h> |
| #include <linux/workqueue.h> |
| #include <uapi/linux/landlock.h> |
| |
| #include "common.h" |
| #include "cred.h" |
| #include "fs.h" |
| #include "limits.h" |
| #include "object.h" |
| #include "ruleset.h" |
| #include "setup.h" |
| |
| /* Underlying object management */ |
| |
| static void release_inode(struct landlock_object *const object) |
| __releases(object->lock) |
| { |
| struct inode *const inode = object->underobj; |
| struct super_block *sb; |
| |
| if (!inode) { |
| spin_unlock(&object->lock); |
| return; |
| } |
| |
| /* |
| * Protects against concurrent use by hook_sb_delete() of the reference |
| * to the underlying inode. |
| */ |
| object->underobj = NULL; |
| /* |
| * Makes sure that if the filesystem is concurrently unmounted, |
| * hook_sb_delete() will wait for us to finish iput(). |
| */ |
| sb = inode->i_sb; |
| atomic_long_inc(&landlock_superblock(sb)->inode_refs); |
| spin_unlock(&object->lock); |
| /* |
| * Because object->underobj was not NULL, hook_sb_delete() and |
| * get_inode_object() guarantee that it is safe to reset |
| * landlock_inode(inode)->object while it is not NULL. It is therefore |
| * not necessary to lock inode->i_lock. |
| */ |
| rcu_assign_pointer(landlock_inode(inode)->object, NULL); |
| /* |
| * Now, new rules can safely be tied to @inode with get_inode_object(). |
| */ |
| |
| iput(inode); |
| if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs)) |
| wake_up_var(&landlock_superblock(sb)->inode_refs); |
| } |
| |
| static const struct landlock_object_underops landlock_fs_underops = { |
| .release = release_inode |
| }; |
| |
| /* Ruleset management */ |
| |
| static struct landlock_object *get_inode_object(struct inode *const inode) |
| { |
| struct landlock_object *object, *new_object; |
| struct landlock_inode_security *inode_sec = landlock_inode(inode); |
| |
| rcu_read_lock(); |
| retry: |
| object = rcu_dereference(inode_sec->object); |
| if (object) { |
| if (likely(refcount_inc_not_zero(&object->usage))) { |
| rcu_read_unlock(); |
| return object; |
| } |
| /* |
| * We are racing with release_inode(), the object is going |
| * away. Wait for release_inode(), then retry. |
| */ |
| spin_lock(&object->lock); |
| spin_unlock(&object->lock); |
| goto retry; |
| } |
| rcu_read_unlock(); |
| |
| /* |
| * If there is no object tied to @inode, then create a new one (without |
| * holding any locks). |
| */ |
| new_object = landlock_create_object(&landlock_fs_underops, inode); |
| if (IS_ERR(new_object)) |
| return new_object; |
| |
| /* |
| * Protects against concurrent calls to get_inode_object() or |
| * hook_sb_delete(). |
| */ |
| spin_lock(&inode->i_lock); |
| if (unlikely(rcu_access_pointer(inode_sec->object))) { |
| /* Someone else just created the object, bail out and retry. */ |
| spin_unlock(&inode->i_lock); |
| kfree(new_object); |
| |
| rcu_read_lock(); |
| goto retry; |
| } |
| |
| /* |
| * @inode will be released by hook_sb_delete() on its superblock |
| * shutdown, or by release_inode() when no more ruleset references the |
| * related object. |
| */ |
| ihold(inode); |
| rcu_assign_pointer(inode_sec->object, new_object); |
| spin_unlock(&inode->i_lock); |
| return new_object; |
| } |
| |
| /* All access rights that can be tied to files. */ |
| /* clang-format off */ |
| #define ACCESS_FILE ( \ |
| LANDLOCK_ACCESS_FS_EXECUTE | \ |
| LANDLOCK_ACCESS_FS_WRITE_FILE | \ |
| LANDLOCK_ACCESS_FS_READ_FILE) |
| /* clang-format on */ |
| |
| /* |
| * All access rights that are denied by default whether they are handled or not |
| * by a ruleset/layer. This must be ORed with all ruleset->fs_access_masks[] |
| * entries when we need to get the absolute handled access masks. |
| */ |
| /* clang-format off */ |
| #define ACCESS_INITIALLY_DENIED ( \ |
| LANDLOCK_ACCESS_FS_REFER) |
| /* clang-format on */ |
| |
| /* |
| * @path: Should have been checked by get_path_from_fd(). |
| */ |
| int landlock_append_fs_rule(struct landlock_ruleset *const ruleset, |
| const struct path *const path, |
| access_mask_t access_rights) |
| { |
| int err; |
| struct landlock_object *object; |
| |
| /* Files only get access rights that make sense. */ |
| if (!d_is_dir(path->dentry) && |
| (access_rights | ACCESS_FILE) != ACCESS_FILE) |
| return -EINVAL; |
| if (WARN_ON_ONCE(ruleset->num_layers != 1)) |
| return -EINVAL; |
| |
| /* Transforms relative access rights to absolute ones. */ |
| access_rights |= |
| LANDLOCK_MASK_ACCESS_FS & |
| ~(ruleset->fs_access_masks[0] | ACCESS_INITIALLY_DENIED); |
| object = get_inode_object(d_backing_inode(path->dentry)); |
| if (IS_ERR(object)) |
| return PTR_ERR(object); |
| mutex_lock(&ruleset->lock); |
| err = landlock_insert_rule(ruleset, object, access_rights); |
| mutex_unlock(&ruleset->lock); |
| /* |
| * No need to check for an error because landlock_insert_rule() |
| * increments the refcount for the new object if needed. |
| */ |
| landlock_put_object(object); |
| return err; |
| } |
| |
| /* Access-control management */ |
| |
| /* |
| * The lifetime of the returned rule is tied to @domain. |
| * |
| * Returns NULL if no rule is found or if @dentry is negative. |
| */ |
| static inline const struct landlock_rule * |
| find_rule(const struct landlock_ruleset *const domain, |
| const struct dentry *const dentry) |
| { |
| const struct landlock_rule *rule; |
| const struct inode *inode; |
| |
| /* Ignores nonexistent leafs. */ |
| if (d_is_negative(dentry)) |
| return NULL; |
| |
| inode = d_backing_inode(dentry); |
| rcu_read_lock(); |
| rule = landlock_find_rule( |
| domain, rcu_dereference(landlock_inode(inode)->object)); |
| rcu_read_unlock(); |
| return rule; |
| } |
| |
| /* |
| * @layer_masks is read and may be updated according to the access request and |
| * the matching rule. |
| * |
| * Returns true if the request is allowed (i.e. relevant layer masks for the |
| * request are empty). |
| */ |
| static inline bool |
| unmask_layers(const struct landlock_rule *const rule, |
| const access_mask_t access_request, |
| layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS]) |
| { |
| size_t layer_level; |
| |
| if (!access_request || !layer_masks) |
| return true; |
| if (!rule) |
| return false; |
| |
| /* |
| * An access is granted if, for each policy layer, at least one rule |
| * encountered on the pathwalk grants the requested access, |
| * regardless of its position in the layer stack. We must then check |
| * the remaining layers for each inode, from the first added layer to |
| * the last one. When there is multiple requested accesses, for each |
| * policy layer, the full set of requested accesses may not be granted |
| * by only one rule, but by the union (binary OR) of multiple rules. |
| * E.g. /a/b <execute> + /a <read> => /a/b <execute + read> |
| */ |
| for (layer_level = 0; layer_level < rule->num_layers; layer_level++) { |
| const struct landlock_layer *const layer = |
| &rule->layers[layer_level]; |
| const layer_mask_t layer_bit = BIT_ULL(layer->level - 1); |
| const unsigned long access_req = access_request; |
| unsigned long access_bit; |
| bool is_empty; |
| |
| /* |
| * Records in @layer_masks which layer grants access to each |
| * requested access. |
| */ |
| is_empty = true; |
| for_each_set_bit(access_bit, &access_req, |
| ARRAY_SIZE(*layer_masks)) { |
| if (layer->access & BIT_ULL(access_bit)) |
| (*layer_masks)[access_bit] &= ~layer_bit; |
| is_empty = is_empty && !(*layer_masks)[access_bit]; |
| } |
| if (is_empty) |
| return true; |
| } |
| return false; |
| } |
| |
| /* |
| * Allows access to pseudo filesystems that will never be mountable (e.g. |
| * sockfs, pipefs), but can still be reachable through |
| * /proc/<pid>/fd/<file-descriptor> |
| */ |
| static inline bool is_nouser_or_private(const struct dentry *dentry) |
| { |
| return (dentry->d_sb->s_flags & SB_NOUSER) || |
| (d_is_positive(dentry) && |
| unlikely(IS_PRIVATE(d_backing_inode(dentry)))); |
| } |
| |
| static inline access_mask_t |
| get_handled_accesses(const struct landlock_ruleset *const domain) |
| { |
| access_mask_t access_dom = ACCESS_INITIALLY_DENIED; |
| size_t layer_level; |
| |
| for (layer_level = 0; layer_level < domain->num_layers; layer_level++) |
| access_dom |= domain->fs_access_masks[layer_level]; |
| return access_dom & LANDLOCK_MASK_ACCESS_FS; |
| } |
| |
| static inline access_mask_t |
| init_layer_masks(const struct landlock_ruleset *const domain, |
| const access_mask_t access_request, |
| layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS]) |
| { |
| access_mask_t handled_accesses = 0; |
| size_t layer_level; |
| |
| memset(layer_masks, 0, sizeof(*layer_masks)); |
| /* An empty access request can happen because of O_WRONLY | O_RDWR. */ |
| if (!access_request) |
| return 0; |
| |
| /* Saves all handled accesses per layer. */ |
| for (layer_level = 0; layer_level < domain->num_layers; layer_level++) { |
| const unsigned long access_req = access_request; |
| unsigned long access_bit; |
| |
| for_each_set_bit(access_bit, &access_req, |
| ARRAY_SIZE(*layer_masks)) { |
| /* |
| * Artificially handles all initially denied by default |
| * access rights. |
| */ |
| if (BIT_ULL(access_bit) & |
| (domain->fs_access_masks[layer_level] | |
| ACCESS_INITIALLY_DENIED)) { |
| (*layer_masks)[access_bit] |= |
| BIT_ULL(layer_level); |
| handled_accesses |= BIT_ULL(access_bit); |
| } |
| } |
| } |
| return handled_accesses; |
| } |
| |
| /* |
| * Check that a destination file hierarchy has more restrictions than a source |
| * file hierarchy. This is only used for link and rename actions. |
| * |
| * @layer_masks_child2: Optional child masks. |
| */ |
| static inline bool no_more_access( |
| const layer_mask_t (*const layer_masks_parent1)[LANDLOCK_NUM_ACCESS_FS], |
| const layer_mask_t (*const layer_masks_child1)[LANDLOCK_NUM_ACCESS_FS], |
| const bool child1_is_directory, |
| const layer_mask_t (*const layer_masks_parent2)[LANDLOCK_NUM_ACCESS_FS], |
| const layer_mask_t (*const layer_masks_child2)[LANDLOCK_NUM_ACCESS_FS], |
| const bool child2_is_directory) |
| { |
| unsigned long access_bit; |
| |
| for (access_bit = 0; access_bit < ARRAY_SIZE(*layer_masks_parent2); |
| access_bit++) { |
| /* Ignores accesses that only make sense for directories. */ |
| const bool is_file_access = |
| !!(BIT_ULL(access_bit) & ACCESS_FILE); |
| |
| if (child1_is_directory || is_file_access) { |
| /* |
| * Checks if the destination restrictions are a |
| * superset of the source ones (i.e. inherited access |
| * rights without child exceptions): |
| * restrictions(parent2) >= restrictions(child1) |
| */ |
| if ((((*layer_masks_parent1)[access_bit] & |
| (*layer_masks_child1)[access_bit]) | |
| (*layer_masks_parent2)[access_bit]) != |
| (*layer_masks_parent2)[access_bit]) |
| return false; |
| } |
| |
| if (!layer_masks_child2) |
| continue; |
| if (child2_is_directory || is_file_access) { |
| /* |
| * Checks inverted restrictions for RENAME_EXCHANGE: |
| * restrictions(parent1) >= restrictions(child2) |
| */ |
| if ((((*layer_masks_parent2)[access_bit] & |
| (*layer_masks_child2)[access_bit]) | |
| (*layer_masks_parent1)[access_bit]) != |
| (*layer_masks_parent1)[access_bit]) |
| return false; |
| } |
| } |
| return true; |
| } |
| |
| /* |
| * Removes @layer_masks accesses that are not requested. |
| * |
| * Returns true if the request is allowed, false otherwise. |
| */ |
| static inline bool |
| scope_to_request(const access_mask_t access_request, |
| layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS]) |
| { |
| const unsigned long access_req = access_request; |
| unsigned long access_bit; |
| |
| if (WARN_ON_ONCE(!layer_masks)) |
| return true; |
| |
| for_each_clear_bit(access_bit, &access_req, ARRAY_SIZE(*layer_masks)) |
| (*layer_masks)[access_bit] = 0; |
| return !memchr_inv(layer_masks, 0, sizeof(*layer_masks)); |
| } |
| |
| /* |
| * Returns true if there is at least one access right different than |
| * LANDLOCK_ACCESS_FS_REFER. |
| */ |
| static inline bool |
| is_eacces(const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS], |
| const access_mask_t access_request) |
| { |
| unsigned long access_bit; |
| /* LANDLOCK_ACCESS_FS_REFER alone must return -EXDEV. */ |
| const unsigned long access_check = access_request & |
| ~LANDLOCK_ACCESS_FS_REFER; |
| |
| if (!layer_masks) |
| return false; |
| |
| for_each_set_bit(access_bit, &access_check, ARRAY_SIZE(*layer_masks)) { |
| if ((*layer_masks)[access_bit]) |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * check_access_path_dual - Check accesses for requests with a common path |
| * |
| * @domain: Domain to check against. |
| * @path: File hierarchy to walk through. |
| * @access_request_parent1: Accesses to check, once @layer_masks_parent1 is |
| * equal to @layer_masks_parent2 (if any). This is tied to the unique |
| * requested path for most actions, or the source in case of a refer action |
| * (i.e. rename or link), or the source and destination in case of |
| * RENAME_EXCHANGE. |
| * @layer_masks_parent1: Pointer to a matrix of layer masks per access |
| * masks, identifying the layers that forbid a specific access. Bits from |
| * this matrix can be unset according to the @path walk. An empty matrix |
| * means that @domain allows all possible Landlock accesses (i.e. not only |
| * those identified by @access_request_parent1). This matrix can |
| * initially refer to domain layer masks and, when the accesses for the |
| * destination and source are the same, to requested layer masks. |
| * @dentry_child1: Dentry to the initial child of the parent1 path. This |
| * pointer must be NULL for non-refer actions (i.e. not link nor rename). |
| * @access_request_parent2: Similar to @access_request_parent1 but for a |
| * request involving a source and a destination. This refers to the |
| * destination, except in case of RENAME_EXCHANGE where it also refers to |
| * the source. Must be set to 0 when using a simple path request. |
| * @layer_masks_parent2: Similar to @layer_masks_parent1 but for a refer |
| * action. This must be NULL otherwise. |
| * @dentry_child2: Dentry to the initial child of the parent2 path. This |
| * pointer is only set for RENAME_EXCHANGE actions and must be NULL |
| * otherwise. |
| * |
| * This helper first checks that the destination has a superset of restrictions |
| * compared to the source (if any) for a common path. Because of |
| * RENAME_EXCHANGE actions, source and destinations may be swapped. It then |
| * checks that the collected accesses and the remaining ones are enough to |
| * allow the request. |
| * |
| * Returns: |
| * - 0 if the access request is granted; |
| * - -EACCES if it is denied because of access right other than |
| * LANDLOCK_ACCESS_FS_REFER; |
| * - -EXDEV if the renaming or linking would be a privileged escalation |
| * (according to each layered policies), or if LANDLOCK_ACCESS_FS_REFER is |
| * not allowed by the source or the destination. |
| */ |
| static int check_access_path_dual( |
| const struct landlock_ruleset *const domain, |
| const struct path *const path, |
| const access_mask_t access_request_parent1, |
| layer_mask_t (*const layer_masks_parent1)[LANDLOCK_NUM_ACCESS_FS], |
| const struct dentry *const dentry_child1, |
| const access_mask_t access_request_parent2, |
| layer_mask_t (*const layer_masks_parent2)[LANDLOCK_NUM_ACCESS_FS], |
| const struct dentry *const dentry_child2) |
| { |
| bool allowed_parent1 = false, allowed_parent2 = false, is_dom_check, |
| child1_is_directory = true, child2_is_directory = true; |
| struct path walker_path; |
| access_mask_t access_masked_parent1, access_masked_parent2; |
| layer_mask_t _layer_masks_child1[LANDLOCK_NUM_ACCESS_FS], |
| _layer_masks_child2[LANDLOCK_NUM_ACCESS_FS]; |
| layer_mask_t(*layer_masks_child1)[LANDLOCK_NUM_ACCESS_FS] = NULL, |
| (*layer_masks_child2)[LANDLOCK_NUM_ACCESS_FS] = NULL; |
| |
| if (!access_request_parent1 && !access_request_parent2) |
| return 0; |
| if (WARN_ON_ONCE(!domain || !path)) |
| return 0; |
| if (is_nouser_or_private(path->dentry)) |
| return 0; |
| if (WARN_ON_ONCE(domain->num_layers < 1 || !layer_masks_parent1)) |
| return -EACCES; |
| |
| if (unlikely(layer_masks_parent2)) { |
| if (WARN_ON_ONCE(!dentry_child1)) |
| return -EACCES; |
| /* |
| * For a double request, first check for potential privilege |
| * escalation by looking at domain handled accesses (which are |
| * a superset of the meaningful requested accesses). |
| */ |
| access_masked_parent1 = access_masked_parent2 = |
| get_handled_accesses(domain); |
| is_dom_check = true; |
| } else { |
| if (WARN_ON_ONCE(dentry_child1 || dentry_child2)) |
| return -EACCES; |
| /* For a simple request, only check for requested accesses. */ |
| access_masked_parent1 = access_request_parent1; |
| access_masked_parent2 = access_request_parent2; |
| is_dom_check = false; |
| } |
| |
| if (unlikely(dentry_child1)) { |
| unmask_layers(find_rule(domain, dentry_child1), |
| init_layer_masks(domain, LANDLOCK_MASK_ACCESS_FS, |
| &_layer_masks_child1), |
| &_layer_masks_child1); |
| layer_masks_child1 = &_layer_masks_child1; |
| child1_is_directory = d_is_dir(dentry_child1); |
| } |
| if (unlikely(dentry_child2)) { |
| unmask_layers(find_rule(domain, dentry_child2), |
| init_layer_masks(domain, LANDLOCK_MASK_ACCESS_FS, |
| &_layer_masks_child2), |
| &_layer_masks_child2); |
| layer_masks_child2 = &_layer_masks_child2; |
| child2_is_directory = d_is_dir(dentry_child2); |
| } |
| |
| walker_path = *path; |
| path_get(&walker_path); |
| /* |
| * We need to walk through all the hierarchy to not miss any relevant |
| * restriction. |
| */ |
| while (true) { |
| struct dentry *parent_dentry; |
| const struct landlock_rule *rule; |
| |
| /* |
| * If at least all accesses allowed on the destination are |
| * already allowed on the source, respectively if there is at |
| * least as much as restrictions on the destination than on the |
| * source, then we can safely refer files from the source to |
| * the destination without risking a privilege escalation. |
| * This also applies in the case of RENAME_EXCHANGE, which |
| * implies checks on both direction. This is crucial for |
| * standalone multilayered security policies. Furthermore, |
| * this helps avoid policy writers to shoot themselves in the |
| * foot. |
| */ |
| if (unlikely(is_dom_check && |
| no_more_access( |
| layer_masks_parent1, layer_masks_child1, |
| child1_is_directory, layer_masks_parent2, |
| layer_masks_child2, |
| child2_is_directory))) { |
| allowed_parent1 = scope_to_request( |
| access_request_parent1, layer_masks_parent1); |
| allowed_parent2 = scope_to_request( |
| access_request_parent2, layer_masks_parent2); |
| |
| /* Stops when all accesses are granted. */ |
| if (allowed_parent1 && allowed_parent2) |
| break; |
| |
| /* |
| * Now, downgrades the remaining checks from domain |
| * handled accesses to requested accesses. |
| */ |
| is_dom_check = false; |
| access_masked_parent1 = access_request_parent1; |
| access_masked_parent2 = access_request_parent2; |
| } |
| |
| rule = find_rule(domain, walker_path.dentry); |
| allowed_parent1 = unmask_layers(rule, access_masked_parent1, |
| layer_masks_parent1); |
| allowed_parent2 = unmask_layers(rule, access_masked_parent2, |
| layer_masks_parent2); |
| |
| /* Stops when a rule from each layer grants access. */ |
| if (allowed_parent1 && allowed_parent2) |
| break; |
| |
| jump_up: |
| if (walker_path.dentry == walker_path.mnt->mnt_root) { |
| if (follow_up(&walker_path)) { |
| /* Ignores hidden mount points. */ |
| goto jump_up; |
| } else { |
| /* |
| * Stops at the real root. Denies access |
| * because not all layers have granted access. |
| */ |
| break; |
| } |
| } |
| if (unlikely(IS_ROOT(walker_path.dentry))) { |
| /* |
| * Stops at disconnected root directories. Only allows |
| * access to internal filesystems (e.g. nsfs, which is |
| * reachable through /proc/<pid>/ns/<namespace>). |
| */ |
| allowed_parent1 = allowed_parent2 = |
| !!(walker_path.mnt->mnt_flags & MNT_INTERNAL); |
| break; |
| } |
| parent_dentry = dget_parent(walker_path.dentry); |
| dput(walker_path.dentry); |
| walker_path.dentry = parent_dentry; |
| } |
| path_put(&walker_path); |
| |
| if (allowed_parent1 && allowed_parent2) |
| return 0; |
| |
| /* |
| * This prioritizes EACCES over EXDEV for all actions, including |
| * renames with RENAME_EXCHANGE. |
| */ |
| if (likely(is_eacces(layer_masks_parent1, access_request_parent1) || |
| is_eacces(layer_masks_parent2, access_request_parent2))) |
| return -EACCES; |
| |
| /* |
| * Gracefully forbids reparenting if the destination directory |
| * hierarchy is not a superset of restrictions of the source directory |
| * hierarchy, or if LANDLOCK_ACCESS_FS_REFER is not allowed by the |
| * source or the destination. |
| */ |
| return -EXDEV; |
| } |
| |
| static inline int check_access_path(const struct landlock_ruleset *const domain, |
| const struct path *const path, |
| access_mask_t access_request) |
| { |
| layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {}; |
| |
| access_request = init_layer_masks(domain, access_request, &layer_masks); |
| return check_access_path_dual(domain, path, access_request, |
| &layer_masks, NULL, 0, NULL, NULL); |
| } |
| |
| static inline int current_check_access_path(const struct path *const path, |
| const access_mask_t access_request) |
| { |
| const struct landlock_ruleset *const dom = |
| landlock_get_current_domain(); |
| |
| if (!dom) |
| return 0; |
| return check_access_path(dom, path, access_request); |
| } |
| |
| static inline access_mask_t get_mode_access(const umode_t mode) |
| { |
| switch (mode & S_IFMT) { |
| case S_IFLNK: |
| return LANDLOCK_ACCESS_FS_MAKE_SYM; |
| case 0: |
| /* A zero mode translates to S_IFREG. */ |
| case S_IFREG: |
| return LANDLOCK_ACCESS_FS_MAKE_REG; |
| case S_IFDIR: |
| return LANDLOCK_ACCESS_FS_MAKE_DIR; |
| case S_IFCHR: |
| return LANDLOCK_ACCESS_FS_MAKE_CHAR; |
| case S_IFBLK: |
| return LANDLOCK_ACCESS_FS_MAKE_BLOCK; |
| case S_IFIFO: |
| return LANDLOCK_ACCESS_FS_MAKE_FIFO; |
| case S_IFSOCK: |
| return LANDLOCK_ACCESS_FS_MAKE_SOCK; |
| default: |
| WARN_ON_ONCE(1); |
| return 0; |
| } |
| } |
| |
| static inline access_mask_t maybe_remove(const struct dentry *const dentry) |
| { |
| if (d_is_negative(dentry)) |
| return 0; |
| return d_is_dir(dentry) ? LANDLOCK_ACCESS_FS_REMOVE_DIR : |
| LANDLOCK_ACCESS_FS_REMOVE_FILE; |
| } |
| |
| /** |
| * collect_domain_accesses - Walk through a file path and collect accesses |
| * |
| * @domain: Domain to check against. |
| * @mnt_root: Last directory to check. |
| * @dir: Directory to start the walk from. |
| * @layer_masks_dom: Where to store the collected accesses. |
| * |
| * This helper is useful to begin a path walk from the @dir directory to a |
| * @mnt_root directory used as a mount point. This mount point is the common |
| * ancestor between the source and the destination of a renamed and linked |
| * file. While walking from @dir to @mnt_root, we record all the domain's |
| * allowed accesses in @layer_masks_dom. |
| * |
| * This is similar to check_access_path_dual() but much simpler because it only |
| * handles walking on the same mount point and only checks one set of accesses. |
| * |
| * Returns: |
| * - true if all the domain access rights are allowed for @dir; |
| * - false if the walk reached @mnt_root. |
| */ |
| static bool collect_domain_accesses( |
| const struct landlock_ruleset *const domain, |
| const struct dentry *const mnt_root, struct dentry *dir, |
| layer_mask_t (*const layer_masks_dom)[LANDLOCK_NUM_ACCESS_FS]) |
| { |
| unsigned long access_dom; |
| bool ret = false; |
| |
| if (WARN_ON_ONCE(!domain || !mnt_root || !dir || !layer_masks_dom)) |
| return true; |
| if (is_nouser_or_private(dir)) |
| return true; |
| |
| access_dom = init_layer_masks(domain, LANDLOCK_MASK_ACCESS_FS, |
| layer_masks_dom); |
| |
| dget(dir); |
| while (true) { |
| struct dentry *parent_dentry; |
| |
| /* Gets all layers allowing all domain accesses. */ |
| if (unmask_layers(find_rule(domain, dir), access_dom, |
| layer_masks_dom)) { |
| /* |
| * Stops when all handled accesses are allowed by at |
| * least one rule in each layer. |
| */ |
| ret = true; |
| break; |
| } |
| |
| /* We should not reach a root other than @mnt_root. */ |
| if (dir == mnt_root || WARN_ON_ONCE(IS_ROOT(dir))) |
| break; |
| |
| parent_dentry = dget_parent(dir); |
| dput(dir); |
| dir = parent_dentry; |
| } |
| dput(dir); |
| return ret; |
| } |
| |
| /** |
| * current_check_refer_path - Check if a rename or link action is allowed |
| * |
| * @old_dentry: File or directory requested to be moved or linked. |
| * @new_dir: Destination parent directory. |
| * @new_dentry: Destination file or directory. |
| * @removable: Sets to true if it is a rename operation. |
| * @exchange: Sets to true if it is a rename operation with RENAME_EXCHANGE. |
| * |
| * Because of its unprivileged constraints, Landlock relies on file hierarchies |
| * (and not only inodes) to tie access rights to files. Being able to link or |
| * rename a file hierarchy brings some challenges. Indeed, moving or linking a |
| * file (i.e. creating a new reference to an inode) can have an impact on the |
| * actions allowed for a set of files if it would change its parent directory |
| * (i.e. reparenting). |
| * |
| * To avoid trivial access right bypasses, Landlock first checks if the file or |
| * directory requested to be moved would gain new access rights inherited from |
| * its new hierarchy. Before returning any error, Landlock then checks that |
| * the parent source hierarchy and the destination hierarchy would allow the |
| * link or rename action. If it is not the case, an error with EACCES is |
| * returned to inform user space that there is no way to remove or create the |
| * requested source file type. If it should be allowed but the new inherited |
| * access rights would be greater than the source access rights, then the |
| * kernel returns an error with EXDEV. Prioritizing EACCES over EXDEV enables |
| * user space to abort the whole operation if there is no way to do it, or to |
| * manually copy the source to the destination if this remains allowed, e.g. |
| * because file creation is allowed on the destination directory but not direct |
| * linking. |
| * |
| * To achieve this goal, the kernel needs to compare two file hierarchies: the |
| * one identifying the source file or directory (including itself), and the |
| * destination one. This can be seen as a multilayer partial ordering problem. |
| * The kernel walks through these paths and collects in a matrix the access |
| * rights that are denied per layer. These matrices are then compared to see |
| * if the destination one has more (or the same) restrictions as the source |
| * one. If this is the case, the requested action will not return EXDEV, which |
| * doesn't mean the action is allowed. The parent hierarchy of the source |
| * (i.e. parent directory), and the destination hierarchy must also be checked |
| * to verify that they explicitly allow such action (i.e. referencing, |
| * creation and potentially removal rights). The kernel implementation is then |
| * required to rely on potentially four matrices of access rights: one for the |
| * source file or directory (i.e. the child), a potentially other one for the |
| * other source/destination (in case of RENAME_EXCHANGE), one for the source |
| * parent hierarchy and a last one for the destination hierarchy. These |
| * ephemeral matrices take some space on the stack, which limits the number of |
| * layers to a deemed reasonable number: 16. |
| * |
| * Returns: |
| * - 0 if access is allowed; |
| * - -EXDEV if @old_dentry would inherit new access rights from @new_dir; |
| * - -EACCES if file removal or creation is denied. |
| */ |
| static int current_check_refer_path(struct dentry *const old_dentry, |
| const struct path *const new_dir, |
| struct dentry *const new_dentry, |
| const bool removable, const bool exchange) |
| { |
| const struct landlock_ruleset *const dom = |
| landlock_get_current_domain(); |
| bool allow_parent1, allow_parent2; |
| access_mask_t access_request_parent1, access_request_parent2; |
| struct path mnt_dir; |
| layer_mask_t layer_masks_parent1[LANDLOCK_NUM_ACCESS_FS] = {}, |
| layer_masks_parent2[LANDLOCK_NUM_ACCESS_FS] = {}; |
| |
| if (!dom) |
| return 0; |
| if (WARN_ON_ONCE(dom->num_layers < 1)) |
| return -EACCES; |
| if (unlikely(d_is_negative(old_dentry))) |
| return -ENOENT; |
| if (exchange) { |
| if (unlikely(d_is_negative(new_dentry))) |
| return -ENOENT; |
| access_request_parent1 = |
| get_mode_access(d_backing_inode(new_dentry)->i_mode); |
| } else { |
| access_request_parent1 = 0; |
| } |
| access_request_parent2 = |
| get_mode_access(d_backing_inode(old_dentry)->i_mode); |
| if (removable) { |
| access_request_parent1 |= maybe_remove(old_dentry); |
| access_request_parent2 |= maybe_remove(new_dentry); |
| } |
| |
| /* The mount points are the same for old and new paths, cf. EXDEV. */ |
| if (old_dentry->d_parent == new_dir->dentry) { |
| /* |
| * The LANDLOCK_ACCESS_FS_REFER access right is not required |
| * for same-directory referer (i.e. no reparenting). |
| */ |
| access_request_parent1 = init_layer_masks( |
| dom, access_request_parent1 | access_request_parent2, |
| &layer_masks_parent1); |
| return check_access_path_dual(dom, new_dir, |
| access_request_parent1, |
| &layer_masks_parent1, NULL, 0, |
| NULL, NULL); |
| } |
| |
| access_request_parent1 |= LANDLOCK_ACCESS_FS_REFER; |
| access_request_parent2 |= LANDLOCK_ACCESS_FS_REFER; |
| |
| /* Saves the common mount point. */ |
| mnt_dir.mnt = new_dir->mnt; |
| mnt_dir.dentry = new_dir->mnt->mnt_root; |
| |
| /* new_dir->dentry is equal to new_dentry->d_parent */ |
| allow_parent1 = collect_domain_accesses(dom, mnt_dir.dentry, |
| old_dentry->d_parent, |
| &layer_masks_parent1); |
| allow_parent2 = collect_domain_accesses( |
| dom, mnt_dir.dentry, new_dir->dentry, &layer_masks_parent2); |
| |
| if (allow_parent1 && allow_parent2) |
| return 0; |
| |
| /* |
| * To be able to compare source and destination domain access rights, |
| * take into account the @old_dentry access rights aggregated with its |
| * parent access rights. This will be useful to compare with the |
| * destination parent access rights. |
| */ |
| return check_access_path_dual(dom, &mnt_dir, access_request_parent1, |
| &layer_masks_parent1, old_dentry, |
| access_request_parent2, |
| &layer_masks_parent2, |
| exchange ? new_dentry : NULL); |
| } |
| |
| /* Inode hooks */ |
| |
| static void hook_inode_free_security(struct inode *const inode) |
| { |
| /* |
| * All inodes must already have been untied from their object by |
| * release_inode() or hook_sb_delete(). |
| */ |
| WARN_ON_ONCE(landlock_inode(inode)->object); |
| } |
| |
| /* Super-block hooks */ |
| |
| /* |
| * Release the inodes used in a security policy. |
| * |
| * Cf. fsnotify_unmount_inodes() and invalidate_inodes() |
| */ |
| static void hook_sb_delete(struct super_block *const sb) |
| { |
| struct inode *inode, *prev_inode = NULL; |
| |
| if (!landlock_initialized) |
| return; |
| |
| spin_lock(&sb->s_inode_list_lock); |
| list_for_each_entry(inode, &sb->s_inodes, i_sb_list) { |
| struct landlock_object *object; |
| |
| /* Only handles referenced inodes. */ |
| if (!atomic_read(&inode->i_count)) |
| continue; |
| |
| /* |
| * Protects against concurrent modification of inode (e.g. |
| * from get_inode_object()). |
| */ |
| spin_lock(&inode->i_lock); |
| /* |
| * Checks I_FREEING and I_WILL_FREE to protect against a race |
| * condition when release_inode() just called iput(), which |
| * could lead to a NULL dereference of inode->security or a |
| * second call to iput() for the same Landlock object. Also |
| * checks I_NEW because such inode cannot be tied to an object. |
| */ |
| if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) { |
| spin_unlock(&inode->i_lock); |
| continue; |
| } |
| |
| rcu_read_lock(); |
| object = rcu_dereference(landlock_inode(inode)->object); |
| if (!object) { |
| rcu_read_unlock(); |
| spin_unlock(&inode->i_lock); |
| continue; |
| } |
| /* Keeps a reference to this inode until the next loop walk. */ |
| __iget(inode); |
| spin_unlock(&inode->i_lock); |
| |
| /* |
| * If there is no concurrent release_inode() ongoing, then we |
| * are in charge of calling iput() on this inode, otherwise we |
| * will just wait for it to finish. |
| */ |
| spin_lock(&object->lock); |
| if (object->underobj == inode) { |
| object->underobj = NULL; |
| spin_unlock(&object->lock); |
| rcu_read_unlock(); |
| |
| /* |
| * Because object->underobj was not NULL, |
| * release_inode() and get_inode_object() guarantee |
| * that it is safe to reset |
| * landlock_inode(inode)->object while it is not NULL. |
| * It is therefore not necessary to lock inode->i_lock. |
| */ |
| rcu_assign_pointer(landlock_inode(inode)->object, NULL); |
| /* |
| * At this point, we own the ihold() reference that was |
| * originally set up by get_inode_object() and the |
| * __iget() reference that we just set in this loop |
| * walk. Therefore the following call to iput() will |
| * not sleep nor drop the inode because there is now at |
| * least two references to it. |
| */ |
| iput(inode); |
| } else { |
| spin_unlock(&object->lock); |
| rcu_read_unlock(); |
| } |
| |
| if (prev_inode) { |
| /* |
| * At this point, we still own the __iget() reference |
| * that we just set in this loop walk. Therefore we |
| * can drop the list lock and know that the inode won't |
| * disappear from under us until the next loop walk. |
| */ |
| spin_unlock(&sb->s_inode_list_lock); |
| /* |
| * We can now actually put the inode reference from the |
| * previous loop walk, which is not needed anymore. |
| */ |
| iput(prev_inode); |
| cond_resched(); |
| spin_lock(&sb->s_inode_list_lock); |
| } |
| prev_inode = inode; |
| } |
| spin_unlock(&sb->s_inode_list_lock); |
| |
| /* Puts the inode reference from the last loop walk, if any. */ |
| if (prev_inode) |
| iput(prev_inode); |
| /* Waits for pending iput() in release_inode(). */ |
| wait_var_event(&landlock_superblock(sb)->inode_refs, |
| !atomic_long_read(&landlock_superblock(sb)->inode_refs)); |
| } |
| |
| /* |
| * Because a Landlock security policy is defined according to the filesystem |
| * topology (i.e. the mount namespace), changing it may grant access to files |
| * not previously allowed. |
| * |
| * To make it simple, deny any filesystem topology modification by landlocked |
| * processes. Non-landlocked processes may still change the namespace of a |
| * landlocked process, but this kind of threat must be handled by a system-wide |
| * access-control security policy. |
| * |
| * This could be lifted in the future if Landlock can safely handle mount |
| * namespace updates requested by a landlocked process. Indeed, we could |
| * update the current domain (which is currently read-only) by taking into |
| * account the accesses of the source and the destination of a new mount point. |
| * However, it would also require to make all the child domains dynamically |
| * inherit these new constraints. Anyway, for backward compatibility reasons, |
| * a dedicated user space option would be required (e.g. as a ruleset flag). |
| */ |
| static int hook_sb_mount(const char *const dev_name, |
| const struct path *const path, const char *const type, |
| const unsigned long flags, void *const data) |
| { |
| if (!landlock_get_current_domain()) |
| return 0; |
| return -EPERM; |
| } |
| |
| static int hook_move_mount(const struct path *const from_path, |
| const struct path *const to_path) |
| { |
| if (!landlock_get_current_domain()) |
| return 0; |
| return -EPERM; |
| } |
| |
| /* |
| * Removing a mount point may reveal a previously hidden file hierarchy, which |
| * may then grant access to files, which may have previously been forbidden. |
| */ |
| static int hook_sb_umount(struct vfsmount *const mnt, const int flags) |
| { |
| if (!landlock_get_current_domain()) |
| return 0; |
| return -EPERM; |
| } |
| |
| static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts) |
| { |
| if (!landlock_get_current_domain()) |
| return 0; |
| return -EPERM; |
| } |
| |
| /* |
| * pivot_root(2), like mount(2), changes the current mount namespace. It must |
| * then be forbidden for a landlocked process. |
| * |
| * However, chroot(2) may be allowed because it only changes the relative root |
| * directory of the current process. Moreover, it can be used to restrict the |
| * view of the filesystem. |
| */ |
| static int hook_sb_pivotroot(const struct path *const old_path, |
| const struct path *const new_path) |
| { |
| if (!landlock_get_current_domain()) |
| return 0; |
| return -EPERM; |
| } |
| |
| /* Path hooks */ |
| |
| static int hook_path_link(struct dentry *const old_dentry, |
| const struct path *const new_dir, |
| struct dentry *const new_dentry) |
| { |
| return current_check_refer_path(old_dentry, new_dir, new_dentry, false, |
| false); |
| } |
| |
| static int hook_path_rename(const struct path *const old_dir, |
| struct dentry *const old_dentry, |
| const struct path *const new_dir, |
| struct dentry *const new_dentry, |
| const unsigned int flags) |
| { |
| /* old_dir refers to old_dentry->d_parent and new_dir->mnt */ |
| return current_check_refer_path(old_dentry, new_dir, new_dentry, true, |
| !!(flags & RENAME_EXCHANGE)); |
| } |
| |
| static int hook_path_mkdir(const struct path *const dir, |
| struct dentry *const dentry, const umode_t mode) |
| { |
| return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR); |
| } |
| |
| static int hook_path_mknod(const struct path *const dir, |
| struct dentry *const dentry, const umode_t mode, |
| const unsigned int dev) |
| { |
| const struct landlock_ruleset *const dom = |
| landlock_get_current_domain(); |
| |
| if (!dom) |
| return 0; |
| return check_access_path(dom, dir, get_mode_access(mode)); |
| } |
| |
| static int hook_path_symlink(const struct path *const dir, |
| struct dentry *const dentry, |
| const char *const old_name) |
| { |
| return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM); |
| } |
| |
| static int hook_path_unlink(const struct path *const dir, |
| struct dentry *const dentry) |
| { |
| return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE); |
| } |
| |
| static int hook_path_rmdir(const struct path *const dir, |
| struct dentry *const dentry) |
| { |
| return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR); |
| } |
| |
| /* File hooks */ |
| |
| static inline access_mask_t get_file_access(const struct file *const file) |
| { |
| access_mask_t access = 0; |
| |
| if (file->f_mode & FMODE_READ) { |
| /* A directory can only be opened in read mode. */ |
| if (S_ISDIR(file_inode(file)->i_mode)) |
| return LANDLOCK_ACCESS_FS_READ_DIR; |
| access = LANDLOCK_ACCESS_FS_READ_FILE; |
| } |
| if (file->f_mode & FMODE_WRITE) |
| access |= LANDLOCK_ACCESS_FS_WRITE_FILE; |
| /* __FMODE_EXEC is indeed part of f_flags, not f_mode. */ |
| if (file->f_flags & __FMODE_EXEC) |
| access |= LANDLOCK_ACCESS_FS_EXECUTE; |
| return access; |
| } |
| |
| static int hook_file_open(struct file *const file) |
| { |
| const struct landlock_ruleset *const dom = |
| landlock_get_current_domain(); |
| |
| if (!dom) |
| return 0; |
| /* |
| * Because a file may be opened with O_PATH, get_file_access() may |
| * return 0. This case will be handled with a future Landlock |
| * evolution. |
| */ |
| return check_access_path(dom, &file->f_path, get_file_access(file)); |
| } |
| |
| static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = { |
| LSM_HOOK_INIT(inode_free_security, hook_inode_free_security), |
| |
| LSM_HOOK_INIT(sb_delete, hook_sb_delete), |
| LSM_HOOK_INIT(sb_mount, hook_sb_mount), |
| LSM_HOOK_INIT(move_mount, hook_move_mount), |
| LSM_HOOK_INIT(sb_umount, hook_sb_umount), |
| LSM_HOOK_INIT(sb_remount, hook_sb_remount), |
| LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot), |
| |
| LSM_HOOK_INIT(path_link, hook_path_link), |
| LSM_HOOK_INIT(path_rename, hook_path_rename), |
| LSM_HOOK_INIT(path_mkdir, hook_path_mkdir), |
| LSM_HOOK_INIT(path_mknod, hook_path_mknod), |
| LSM_HOOK_INIT(path_symlink, hook_path_symlink), |
| LSM_HOOK_INIT(path_unlink, hook_path_unlink), |
| LSM_HOOK_INIT(path_rmdir, hook_path_rmdir), |
| |
| LSM_HOOK_INIT(file_open, hook_file_open), |
| }; |
| |
| __init void landlock_add_fs_hooks(void) |
| { |
| security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks), |
| LANDLOCK_NAME); |
| } |