693 строки
19 KiB
C
693 строки
19 KiB
C
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// SPDX-License-Identifier: GPL-2.0-only
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/*
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* Landlock LSM - Filesystem management and hooks
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*
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* Copyright © 2016-2020 Mickaël Salaün <mic@digikod.net>
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* Copyright © 2018-2020 ANSSI
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*/
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#include <linux/atomic.h>
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#include <linux/bitops.h>
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#include <linux/bits.h>
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#include <linux/compiler_types.h>
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#include <linux/dcache.h>
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#include <linux/err.h>
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#include <linux/fs.h>
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#include <linux/init.h>
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#include <linux/kernel.h>
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#include <linux/limits.h>
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#include <linux/list.h>
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#include <linux/lsm_hooks.h>
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#include <linux/mount.h>
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#include <linux/namei.h>
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#include <linux/path.h>
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#include <linux/rcupdate.h>
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#include <linux/spinlock.h>
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#include <linux/stat.h>
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#include <linux/types.h>
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#include <linux/wait_bit.h>
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#include <linux/workqueue.h>
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#include <uapi/linux/landlock.h>
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#include "common.h"
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#include "cred.h"
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#include "fs.h"
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#include "limits.h"
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#include "object.h"
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#include "ruleset.h"
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#include "setup.h"
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/* Underlying object management */
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static void release_inode(struct landlock_object *const object)
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__releases(object->lock)
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{
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struct inode *const inode = object->underobj;
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struct super_block *sb;
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if (!inode) {
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spin_unlock(&object->lock);
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return;
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}
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/*
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* Protects against concurrent use by hook_sb_delete() of the reference
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* to the underlying inode.
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*/
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object->underobj = NULL;
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/*
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* Makes sure that if the filesystem is concurrently unmounted,
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* hook_sb_delete() will wait for us to finish iput().
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*/
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sb = inode->i_sb;
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atomic_long_inc(&landlock_superblock(sb)->inode_refs);
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spin_unlock(&object->lock);
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/*
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* Because object->underobj was not NULL, hook_sb_delete() and
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* get_inode_object() guarantee that it is safe to reset
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* landlock_inode(inode)->object while it is not NULL. It is therefore
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* not necessary to lock inode->i_lock.
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*/
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rcu_assign_pointer(landlock_inode(inode)->object, NULL);
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/*
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* Now, new rules can safely be tied to @inode with get_inode_object().
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*/
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iput(inode);
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if (atomic_long_dec_and_test(&landlock_superblock(sb)->inode_refs))
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wake_up_var(&landlock_superblock(sb)->inode_refs);
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}
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static const struct landlock_object_underops landlock_fs_underops = {
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.release = release_inode
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};
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/* Ruleset management */
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static struct landlock_object *get_inode_object(struct inode *const inode)
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{
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struct landlock_object *object, *new_object;
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struct landlock_inode_security *inode_sec = landlock_inode(inode);
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rcu_read_lock();
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retry:
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object = rcu_dereference(inode_sec->object);
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if (object) {
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if (likely(refcount_inc_not_zero(&object->usage))) {
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rcu_read_unlock();
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return object;
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}
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/*
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* We are racing with release_inode(), the object is going
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* away. Wait for release_inode(), then retry.
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*/
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spin_lock(&object->lock);
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spin_unlock(&object->lock);
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goto retry;
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}
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rcu_read_unlock();
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/*
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* If there is no object tied to @inode, then create a new one (without
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* holding any locks).
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*/
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new_object = landlock_create_object(&landlock_fs_underops, inode);
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if (IS_ERR(new_object))
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return new_object;
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/*
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* Protects against concurrent calls to get_inode_object() or
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* hook_sb_delete().
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*/
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spin_lock(&inode->i_lock);
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if (unlikely(rcu_access_pointer(inode_sec->object))) {
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/* Someone else just created the object, bail out and retry. */
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spin_unlock(&inode->i_lock);
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kfree(new_object);
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rcu_read_lock();
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goto retry;
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}
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/*
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* @inode will be released by hook_sb_delete() on its superblock
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* shutdown, or by release_inode() when no more ruleset references the
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* related object.
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*/
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ihold(inode);
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rcu_assign_pointer(inode_sec->object, new_object);
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spin_unlock(&inode->i_lock);
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return new_object;
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}
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/* All access rights that can be tied to files. */
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#define ACCESS_FILE ( \
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LANDLOCK_ACCESS_FS_EXECUTE | \
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LANDLOCK_ACCESS_FS_WRITE_FILE | \
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LANDLOCK_ACCESS_FS_READ_FILE)
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/*
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* @path: Should have been checked by get_path_from_fd().
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*/
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int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
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const struct path *const path, u32 access_rights)
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{
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int err;
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struct landlock_object *object;
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/* Files only get access rights that make sense. */
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if (!d_is_dir(path->dentry) && (access_rights | ACCESS_FILE) !=
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ACCESS_FILE)
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return -EINVAL;
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if (WARN_ON_ONCE(ruleset->num_layers != 1))
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return -EINVAL;
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/* Transforms relative access rights to absolute ones. */
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access_rights |= LANDLOCK_MASK_ACCESS_FS & ~ruleset->fs_access_masks[0];
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object = get_inode_object(d_backing_inode(path->dentry));
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if (IS_ERR(object))
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return PTR_ERR(object);
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mutex_lock(&ruleset->lock);
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err = landlock_insert_rule(ruleset, object, access_rights);
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mutex_unlock(&ruleset->lock);
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/*
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* No need to check for an error because landlock_insert_rule()
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* increments the refcount for the new object if needed.
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*/
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landlock_put_object(object);
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return err;
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}
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/* Access-control management */
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static inline u64 unmask_layers(
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const struct landlock_ruleset *const domain,
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const struct path *const path, const u32 access_request,
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u64 layer_mask)
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{
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const struct landlock_rule *rule;
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const struct inode *inode;
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size_t i;
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if (d_is_negative(path->dentry))
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/* Ignore nonexistent leafs. */
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return layer_mask;
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inode = d_backing_inode(path->dentry);
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rcu_read_lock();
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rule = landlock_find_rule(domain,
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rcu_dereference(landlock_inode(inode)->object));
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rcu_read_unlock();
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if (!rule)
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return layer_mask;
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/*
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* An access is granted if, for each policy layer, at least one rule
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* encountered on the pathwalk grants the requested accesses,
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* regardless of their position in the layer stack. We must then check
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* the remaining layers for each inode, from the first added layer to
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* the last one.
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*/
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for (i = 0; i < rule->num_layers; i++) {
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const struct landlock_layer *const layer = &rule->layers[i];
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const u64 layer_level = BIT_ULL(layer->level - 1);
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/* Checks that the layer grants access to the full request. */
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if ((layer->access & access_request) == access_request) {
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layer_mask &= ~layer_level;
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if (layer_mask == 0)
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return layer_mask;
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}
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}
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return layer_mask;
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}
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static int check_access_path(const struct landlock_ruleset *const domain,
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const struct path *const path, u32 access_request)
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{
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bool allowed = false;
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struct path walker_path;
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u64 layer_mask;
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size_t i;
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/* Make sure all layers can be checked. */
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BUILD_BUG_ON(BITS_PER_TYPE(layer_mask) < LANDLOCK_MAX_NUM_LAYERS);
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if (!access_request)
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return 0;
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if (WARN_ON_ONCE(!domain || !path))
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return 0;
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/*
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* Allows access to pseudo filesystems that will never be mountable
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* (e.g. sockfs, pipefs), but can still be reachable through
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* /proc/<pid>/fd/<file-descriptor> .
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*/
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if ((path->dentry->d_sb->s_flags & SB_NOUSER) ||
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(d_is_positive(path->dentry) &&
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unlikely(IS_PRIVATE(d_backing_inode(path->dentry)))))
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return 0;
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if (WARN_ON_ONCE(domain->num_layers < 1))
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return -EACCES;
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/* Saves all layers handling a subset of requested accesses. */
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layer_mask = 0;
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for (i = 0; i < domain->num_layers; i++) {
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if (domain->fs_access_masks[i] & access_request)
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layer_mask |= BIT_ULL(i);
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}
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/* An access request not handled by the domain is allowed. */
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if (layer_mask == 0)
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return 0;
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walker_path = *path;
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path_get(&walker_path);
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/*
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* We need to walk through all the hierarchy to not miss any relevant
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* restriction.
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*/
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while (true) {
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struct dentry *parent_dentry;
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layer_mask = unmask_layers(domain, &walker_path,
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access_request, layer_mask);
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if (layer_mask == 0) {
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/* Stops when a rule from each layer grants access. */
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allowed = true;
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break;
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}
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jump_up:
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if (walker_path.dentry == walker_path.mnt->mnt_root) {
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if (follow_up(&walker_path)) {
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/* Ignores hidden mount points. */
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goto jump_up;
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} else {
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/*
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* Stops at the real root. Denies access
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* because not all layers have granted access.
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*/
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allowed = false;
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break;
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}
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}
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if (unlikely(IS_ROOT(walker_path.dentry))) {
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/*
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* Stops at disconnected root directories. Only allows
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* access to internal filesystems (e.g. nsfs, which is
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* reachable through /proc/<pid>/ns/<namespace>).
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*/
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allowed = !!(walker_path.mnt->mnt_flags & MNT_INTERNAL);
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break;
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}
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parent_dentry = dget_parent(walker_path.dentry);
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dput(walker_path.dentry);
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walker_path.dentry = parent_dentry;
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}
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path_put(&walker_path);
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return allowed ? 0 : -EACCES;
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}
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static inline int current_check_access_path(const struct path *const path,
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const u32 access_request)
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{
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const struct landlock_ruleset *const dom =
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landlock_get_current_domain();
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if (!dom)
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return 0;
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return check_access_path(dom, path, access_request);
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}
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/* Inode hooks */
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static void hook_inode_free_security(struct inode *const inode)
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{
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/*
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* All inodes must already have been untied from their object by
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* release_inode() or hook_sb_delete().
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*/
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WARN_ON_ONCE(landlock_inode(inode)->object);
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}
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/* Super-block hooks */
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/*
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* Release the inodes used in a security policy.
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*
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* Cf. fsnotify_unmount_inodes() and invalidate_inodes()
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*/
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static void hook_sb_delete(struct super_block *const sb)
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{
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struct inode *inode, *prev_inode = NULL;
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if (!landlock_initialized)
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return;
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spin_lock(&sb->s_inode_list_lock);
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list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
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struct landlock_object *object;
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/* Only handles referenced inodes. */
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if (!atomic_read(&inode->i_count))
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continue;
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/*
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* Protects against concurrent modification of inode (e.g.
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* from get_inode_object()).
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*/
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spin_lock(&inode->i_lock);
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/*
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* Checks I_FREEING and I_WILL_FREE to protect against a race
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* condition when release_inode() just called iput(), which
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* could lead to a NULL dereference of inode->security or a
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* second call to iput() for the same Landlock object. Also
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* checks I_NEW because such inode cannot be tied to an object.
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*/
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if (inode->i_state & (I_FREEING | I_WILL_FREE | I_NEW)) {
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spin_unlock(&inode->i_lock);
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continue;
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}
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rcu_read_lock();
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object = rcu_dereference(landlock_inode(inode)->object);
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if (!object) {
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rcu_read_unlock();
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spin_unlock(&inode->i_lock);
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continue;
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}
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/* Keeps a reference to this inode until the next loop walk. */
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__iget(inode);
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spin_unlock(&inode->i_lock);
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/*
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* If there is no concurrent release_inode() ongoing, then we
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* are in charge of calling iput() on this inode, otherwise we
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* will just wait for it to finish.
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*/
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spin_lock(&object->lock);
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if (object->underobj == inode) {
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object->underobj = NULL;
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spin_unlock(&object->lock);
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rcu_read_unlock();
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/*
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* Because object->underobj was not NULL,
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* release_inode() and get_inode_object() guarantee
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* that it is safe to reset
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* landlock_inode(inode)->object while it is not NULL.
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* It is therefore not necessary to lock inode->i_lock.
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*/
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rcu_assign_pointer(landlock_inode(inode)->object, NULL);
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/*
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* At this point, we own the ihold() reference that was
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* originally set up by get_inode_object() and the
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* __iget() reference that we just set in this loop
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* walk. Therefore the following call to iput() will
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* not sleep nor drop the inode because there is now at
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* least two references to it.
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*/
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iput(inode);
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} else {
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spin_unlock(&object->lock);
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rcu_read_unlock();
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}
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if (prev_inode) {
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/*
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* At this point, we still own the __iget() reference
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* that we just set in this loop walk. Therefore we
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* can drop the list lock and know that the inode won't
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* disappear from under us until the next loop walk.
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*/
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spin_unlock(&sb->s_inode_list_lock);
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/*
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* We can now actually put the inode reference from the
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* previous loop walk, which is not needed anymore.
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*/
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iput(prev_inode);
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cond_resched();
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spin_lock(&sb->s_inode_list_lock);
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}
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prev_inode = inode;
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}
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spin_unlock(&sb->s_inode_list_lock);
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/* Puts the inode reference from the last loop walk, if any. */
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if (prev_inode)
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iput(prev_inode);
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/* Waits for pending iput() in release_inode(). */
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wait_var_event(&landlock_superblock(sb)->inode_refs, !atomic_long_read(
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&landlock_superblock(sb)->inode_refs));
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}
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|
|
||
|
/*
|
||
|
* 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 inline u32 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;
|
||
|
}
|
||
|
}
|
||
|
|
||
|
/*
|
||
|
* Creating multiple links or renaming may lead to privilege escalations if not
|
||
|
* handled properly. Indeed, we must be sure that the source doesn't gain more
|
||
|
* privileges by being accessible from the destination. This is getting more
|
||
|
* complex when dealing with multiple layers. The whole picture can be seen as
|
||
|
* a multilayer partial ordering problem. A future version of Landlock will
|
||
|
* deal with that.
|
||
|
*/
|
||
|
static int hook_path_link(struct dentry *const old_dentry,
|
||
|
const struct path *const new_dir,
|
||
|
struct dentry *const new_dentry)
|
||
|
{
|
||
|
const struct landlock_ruleset *const dom =
|
||
|
landlock_get_current_domain();
|
||
|
|
||
|
if (!dom)
|
||
|
return 0;
|
||
|
/* The mount points are the same for old and new paths, cf. EXDEV. */
|
||
|
if (old_dentry->d_parent != new_dir->dentry)
|
||
|
/* Gracefully forbids reparenting. */
|
||
|
return -EXDEV;
|
||
|
if (unlikely(d_is_negative(old_dentry)))
|
||
|
return -ENOENT;
|
||
|
return check_access_path(dom, new_dir,
|
||
|
get_mode_access(d_backing_inode(old_dentry)->i_mode));
|
||
|
}
|
||
|
|
||
|
static inline u32 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;
|
||
|
}
|
||
|
|
||
|
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 struct landlock_ruleset *const dom =
|
||
|
landlock_get_current_domain();
|
||
|
|
||
|
if (!dom)
|
||
|
return 0;
|
||
|
/* The mount points are the same for old and new paths, cf. EXDEV. */
|
||
|
if (old_dir->dentry != new_dir->dentry)
|
||
|
/* Gracefully forbids reparenting. */
|
||
|
return -EXDEV;
|
||
|
if (unlikely(d_is_negative(old_dentry)))
|
||
|
return -ENOENT;
|
||
|
/* RENAME_EXCHANGE is handled because directories are the same. */
|
||
|
return check_access_path(dom, old_dir, maybe_remove(old_dentry) |
|
||
|
maybe_remove(new_dentry) |
|
||
|
get_mode_access(d_backing_inode(old_dentry)->i_mode));
|
||
|
}
|
||
|
|
||
|
static int hook_path_mkdir(const struct path *const dir,
|
||
|
struct dentry *const dentry, const umode_t mode)
|
||
|
{
|
||
|
return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_DIR);
|
||
|
}
|
||
|
|
||
|
static int hook_path_mknod(const struct path *const dir,
|
||
|
struct dentry *const dentry, const umode_t mode,
|
||
|
const unsigned int dev)
|
||
|
{
|
||
|
const struct landlock_ruleset *const dom =
|
||
|
landlock_get_current_domain();
|
||
|
|
||
|
if (!dom)
|
||
|
return 0;
|
||
|
return check_access_path(dom, dir, get_mode_access(mode));
|
||
|
}
|
||
|
|
||
|
static int hook_path_symlink(const struct path *const dir,
|
||
|
struct dentry *const dentry, const char *const old_name)
|
||
|
{
|
||
|
return current_check_access_path(dir, LANDLOCK_ACCESS_FS_MAKE_SYM);
|
||
|
}
|
||
|
|
||
|
static int hook_path_unlink(const struct path *const dir,
|
||
|
struct dentry *const dentry)
|
||
|
{
|
||
|
return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_FILE);
|
||
|
}
|
||
|
|
||
|
static int hook_path_rmdir(const struct path *const dir,
|
||
|
struct dentry *const dentry)
|
||
|
{
|
||
|
return current_check_access_path(dir, LANDLOCK_ACCESS_FS_REMOVE_DIR);
|
||
|
}
|
||
|
|
||
|
/* File hooks */
|
||
|
|
||
|
static inline u32 get_file_access(const struct file *const file)
|
||
|
{
|
||
|
u32 access = 0;
|
||
|
|
||
|
if (file->f_mode & FMODE_READ) {
|
||
|
/* A directory can only be opened in read mode. */
|
||
|
if (S_ISDIR(file_inode(file)->i_mode))
|
||
|
return LANDLOCK_ACCESS_FS_READ_DIR;
|
||
|
access = LANDLOCK_ACCESS_FS_READ_FILE;
|
||
|
}
|
||
|
if (file->f_mode & FMODE_WRITE)
|
||
|
access |= LANDLOCK_ACCESS_FS_WRITE_FILE;
|
||
|
/* __FMODE_EXEC is indeed part of f_flags, not f_mode. */
|
||
|
if (file->f_flags & __FMODE_EXEC)
|
||
|
access |= LANDLOCK_ACCESS_FS_EXECUTE;
|
||
|
return access;
|
||
|
}
|
||
|
|
||
|
static int hook_file_open(struct file *const file)
|
||
|
{
|
||
|
const struct landlock_ruleset *const dom =
|
||
|
landlock_get_current_domain();
|
||
|
|
||
|
if (!dom)
|
||
|
return 0;
|
||
|
/*
|
||
|
* Because a file may be opened with O_PATH, get_file_access() may
|
||
|
* return 0. This case will be handled with a future Landlock
|
||
|
* evolution.
|
||
|
*/
|
||
|
return check_access_path(dom, &file->f_path, get_file_access(file));
|
||
|
}
|
||
|
|
||
|
static struct security_hook_list landlock_hooks[] __lsm_ro_after_init = {
|
||
|
LSM_HOOK_INIT(inode_free_security, hook_inode_free_security),
|
||
|
|
||
|
LSM_HOOK_INIT(sb_delete, hook_sb_delete),
|
||
|
LSM_HOOK_INIT(sb_mount, hook_sb_mount),
|
||
|
LSM_HOOK_INIT(move_mount, hook_move_mount),
|
||
|
LSM_HOOK_INIT(sb_umount, hook_sb_umount),
|
||
|
LSM_HOOK_INIT(sb_remount, hook_sb_remount),
|
||
|
LSM_HOOK_INIT(sb_pivotroot, hook_sb_pivotroot),
|
||
|
|
||
|
LSM_HOOK_INIT(path_link, hook_path_link),
|
||
|
LSM_HOOK_INIT(path_rename, hook_path_rename),
|
||
|
LSM_HOOK_INIT(path_mkdir, hook_path_mkdir),
|
||
|
LSM_HOOK_INIT(path_mknod, hook_path_mknod),
|
||
|
LSM_HOOK_INIT(path_symlink, hook_path_symlink),
|
||
|
LSM_HOOK_INIT(path_unlink, hook_path_unlink),
|
||
|
LSM_HOOK_INIT(path_rmdir, hook_path_rmdir),
|
||
|
|
||
|
LSM_HOOK_INIT(file_open, hook_file_open),
|
||
|
};
|
||
|
|
||
|
__init void landlock_add_fs_hooks(void)
|
||
|
{
|
||
|
security_add_hooks(landlock_hooks, ARRAY_SIZE(landlock_hooks),
|
||
|
LANDLOCK_NAME);
|
||
|
}
|