Re: [PATCH v30 07/12] landlock: Support filesystem access-control

Jann Horn
2021-03-23T00:14:54Z

On Tue, Mar 16, 2021 at 9:43 PM Mickaël Salaün <mic@digikod.net> wrote:
> Using Landlock objects and ruleset, it is possible to tag inodes
> according to a process's domain.
[...]
> +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 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 get_inode_object() calls. */
> + spin_lock(&inode->i_lock);
> + object = rcu_dereference_protected(inode_sec->object,
> + lockdep_is_held(&inode->i_lock));

rcu_dereference_protected() requires that inode_sec->object is not
concurrently changed, but I think another thread could call
get_inode_object() while we're in landlock_create_object(), and then
we could race with the NULL write in release_inode() here? (It
wouldn't actually be a UAF though because we're not actually accessing
`object` here.) Or am I missing a lock that prevents this?

In v28 this wasn't an issue because release_inode() was holding
inode->i_lock (and object->lock) during the NULL store; but in v29 and
this version the NULL store in release_inode() moved out of the locked
region. I think you could just move the NULL store in release_inode()
back up (and maybe add a comment explaining the locking rules for
landlock_inode(...)->object)?

(Or alternatively you could use rcu_dereference_raw() with a comment
explaining that the read pointer is only used to check for NULL-ness,
and that it is guaranteed that the pointer can't change if it is NULL
and we're holding the lock. But that'd be needlessly complicated, I
think.)

> + if (unlikely(object)) {
> + /* Someone else just created the object, bail out and retry. */
> + spin_unlock(&inode->i_lock);
> + kfree(new_object);
> +
> + rcu_read_lock();
> + goto retry;
> + }
> +
> + rcu_assign_pointer(inode_sec->object, new_object);
> + /*
> + * @inode will be released by hook_sb_delete() on its superblock
> + * shutdown.
> + */
> + ihold(inode);
> + spin_unlock(&inode->i_lock);
> + return new_object;
> +}