blob: 97b8e421f617178489f948a71d71cdefa7079982 [file] [log] [blame]
// 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
*/
#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. */
#define ACCESS_FILE ( \
LANDLOCK_ACCESS_FS_EXECUTE | \
LANDLOCK_ACCESS_FS_WRITE_FILE | \
LANDLOCK_ACCESS_FS_READ_FILE)
/*
* @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, u32 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];
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 */
static inline u64 unmask_layers(
const struct landlock_ruleset *const domain,
const struct path *const path, const u32 access_request,
u64 layer_mask)
{
const struct landlock_rule *rule;
const struct inode *inode;
size_t i;
if (d_is_negative(path->dentry))
/* Ignore nonexistent leafs. */
return layer_mask;
inode = d_backing_inode(path->dentry);
rcu_read_lock();
rule = landlock_find_rule(domain,
rcu_dereference(landlock_inode(inode)->object));
rcu_read_unlock();
if (!rule)
return layer_mask;
/*
* An access is granted if, for each policy layer, at least one rule
* encountered on the pathwalk grants the requested accesses,
* regardless of their position in the layer stack. We must then check
* the remaining layers for each inode, from the first added layer to
* the last one.
*/
for (i = 0; i < rule->num_layers; i++) {
const struct landlock_layer *const layer = &rule->layers[i];
const u64 layer_level = BIT_ULL(layer->level - 1);
/* Checks that the layer grants access to the full request. */
if ((layer->access & access_request) == access_request) {
layer_mask &= ~layer_level;
if (layer_mask == 0)
return layer_mask;
}
}
return layer_mask;
}
static int check_access_path(const struct landlock_ruleset *const domain,
const struct path *const path, u32 access_request)
{
bool allowed = false;
struct path walker_path;
u64 layer_mask;
size_t i;
/* Make sure all layers can be checked. */
BUILD_BUG_ON(BITS_PER_TYPE(layer_mask) < LANDLOCK_MAX_NUM_LAYERS);
if (!access_request)
return 0;
if (WARN_ON_ONCE(!domain || !path))
return 0;
/*
* 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> .
*/
if ((path->dentry->d_sb->s_flags & SB_NOUSER) ||
(d_is_positive(path->dentry) &&
unlikely(IS_PRIVATE(d_backing_inode(path->dentry)))))
return 0;
if (WARN_ON_ONCE(domain->num_layers < 1))
return -EACCES;
/* Saves all layers handling a subset of requested accesses. */
layer_mask = 0;
for (i = 0; i < domain->num_layers; i++) {
if (domain->fs_access_masks[i] & access_request)
layer_mask |= BIT_ULL(i);
}
/* An access request not handled by the domain is allowed. */
if (layer_mask == 0)
return 0;
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;
layer_mask = unmask_layers(domain, &walker_path,
access_request, layer_mask);
if (layer_mask == 0) {
/* Stops when a rule from each layer grants access. */
allowed = true;
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.
*/
allowed = false;
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 = !!(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 ? 0 : -EACCES;
}
static inline int current_check_access_path(const struct path *const path,
const u32 access_request)
{
const struct landlock_ruleset *const dom =
landlock_get_current_domain();
if (!dom)
return 0;
return check_access_path(dom, path, access_request);
}
/* 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 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);
}