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-rw-r--r--security/landlock/fs.c1311
1 files changed, 1311 insertions, 0 deletions
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
new file mode 100644
index 0000000000..1c0c198f6f
--- /dev/null
+++ b/security/landlock/fs.c
@@ -0,0 +1,1311 @@
+// 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 | \
+ LANDLOCK_ACCESS_FS_TRUNCATE)
+/* 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;
+}
+
+/**
+ * init_layer_masks - Initialize layer masks from an access request
+ *
+ * Populates @layer_masks such that for each access right in @access_request,
+ * the bits for all the layers are set where this access right is handled.
+ *
+ * @domain: The domain that defines the current restrictions.
+ * @access_request: The requested access rights to check.
+ * @layer_masks: The layer masks to populate.
+ *
+ * Returns: An access mask where each access right bit is set which is handled
+ * in any of the active layers in @domain.
+ */
+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;
+}
+
+/**
+ * is_access_to_paths_allowed - 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:
+ * - true if the access request is granted;
+ * - false otherwise.
+ */
+static bool is_access_to_paths_allowed(
+ 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 true;
+ if (WARN_ON_ONCE(!domain || !path))
+ return true;
+ if (is_nouser_or_private(path->dentry))
+ return true;
+ if (WARN_ON_ONCE(domain->num_layers < 1 || !layer_masks_parent1))
+ return false;
+
+ if (unlikely(layer_masks_parent2)) {
+ if (WARN_ON_ONCE(!dentry_child1))
+ return false;
+ /*
+ * 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 false;
+ /* 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);
+
+ return allowed_parent1 && allowed_parent2;
+}
+
+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);
+ if (is_access_to_paths_allowed(domain, path, access_request,
+ &layer_masks, NULL, 0, NULL, NULL))
+ return 0;
+ return -EACCES;
+}
+
+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 is_access_to_paths_allowed() 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);
+ if (is_access_to_paths_allowed(
+ dom, new_dir, access_request_parent1,
+ &layer_masks_parent1, NULL, 0, NULL, NULL))
+ return 0;
+ return -EACCES;
+ }
+
+ 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.
+ */
+ if (is_access_to_paths_allowed(
+ dom, &mnt_dir, access_request_parent1, &layer_masks_parent1,
+ old_dentry, access_request_parent2, &layer_masks_parent2,
+ exchange ? new_dentry : NULL))
+ 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;
+}
+
+/* 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);
+}
+
+static int hook_path_truncate(const struct path *const path)
+{
+ return current_check_access_path(path, LANDLOCK_ACCESS_FS_TRUNCATE);
+}
+
+/* File hooks */
+
+/**
+ * get_required_file_open_access - Get access needed to open a file
+ *
+ * @file: File being opened.
+ *
+ * Returns the access rights that are required for opening the given file,
+ * depending on the file type and open mode.
+ */
+static inline access_mask_t
+get_required_file_open_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_alloc_security(struct file *const file)
+{
+ /*
+ * Grants all access rights, even if most of them are not checked later
+ * on. It is more consistent.
+ *
+ * Notably, file descriptors for regular files can also be acquired
+ * without going through the file_open hook, for example when using
+ * memfd_create(2).
+ */
+ landlock_file(file)->allowed_access = LANDLOCK_MASK_ACCESS_FS;
+ return 0;
+}
+
+static int hook_file_open(struct file *const file)
+{
+ layer_mask_t layer_masks[LANDLOCK_NUM_ACCESS_FS] = {};
+ access_mask_t open_access_request, full_access_request, allowed_access;
+ const access_mask_t optional_access = LANDLOCK_ACCESS_FS_TRUNCATE;
+ const struct landlock_ruleset *const dom =
+ landlock_get_current_domain();
+
+ if (!dom)
+ return 0;
+
+ /*
+ * Because a file may be opened with O_PATH, get_required_file_open_access()
+ * may return 0. This case will be handled with a future Landlock
+ * evolution.
+ */
+ open_access_request = get_required_file_open_access(file);
+
+ /*
+ * We look up more access than what we immediately need for open(), so
+ * that we can later authorize operations on opened files.
+ */
+ full_access_request = open_access_request | optional_access;
+
+ if (is_access_to_paths_allowed(
+ dom, &file->f_path,
+ init_layer_masks(dom, full_access_request, &layer_masks),
+ &layer_masks, NULL, 0, NULL, NULL)) {
+ allowed_access = full_access_request;
+ } else {
+ unsigned long access_bit;
+ const unsigned long access_req = full_access_request;
+
+ /*
+ * Calculate the actual allowed access rights from layer_masks.
+ * Add each access right to allowed_access which has not been
+ * vetoed by any layer.
+ */
+ allowed_access = 0;
+ for_each_set_bit(access_bit, &access_req,
+ ARRAY_SIZE(layer_masks)) {
+ if (!layer_masks[access_bit])
+ allowed_access |= BIT_ULL(access_bit);
+ }
+ }
+
+ /*
+ * For operations on already opened files (i.e. ftruncate()), it is the
+ * access rights at the time of open() which decide whether the
+ * operation is permitted. Therefore, we record the relevant subset of
+ * file access rights in the opened struct file.
+ */
+ landlock_file(file)->allowed_access = allowed_access;
+
+ if ((open_access_request & allowed_access) == open_access_request)
+ return 0;
+
+ return -EACCES;
+}
+
+static int hook_file_truncate(struct file *const file)
+{
+ /*
+ * Allows truncation if the truncate right was available at the time of
+ * opening the file, to get a consistent access check as for read, write
+ * and execute operations.
+ *
+ * Note: For checks done based on the file's Landlock allowed access, we
+ * enforce them independently of whether the current thread is in a
+ * Landlock domain, so that open files passed between independent
+ * processes retain their behaviour.
+ */
+ if (landlock_file(file)->allowed_access & LANDLOCK_ACCESS_FS_TRUNCATE)
+ return 0;
+ return -EACCES;
+}
+
+static struct security_hook_list landlock_hooks[] __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(path_truncate, hook_path_truncate),
+
+ LSM_HOOK_INIT(file_alloc_security, hook_file_alloc_security),
+ LSM_HOOK_INIT(file_open, hook_file_open),
+ LSM_HOOK_INIT(file_truncate, hook_file_truncate),
+};
+
+__init void landlock_add_fs_hooks(void)
+{
+ security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),
+ LANDLOCK_NAME);
+}