zram: use __bio_add_page for adding single page to bio

[linux-block.git] / security / landlock / fs.c

// 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);
}