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

Mickaël Salaün
2021-03-23T15:55:47Z

On 23/03/2021 01:13, Jann Horn wrote:
> 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.)

To reach rcu_assign_pointer(landlock_inode(inode)->object, NULL) in
release_inode() or in hook_sb_delete(), the
landlock_inode(inode)->object need to be non-NULL, which implies that a
call to get_inode_object(inode) either "retry" (because release_inode is
only called by landlock_put_object, which set object->usage to 0) until
it creates a new object, or reuses the existing referenced object (and
increments object->usage). The worse case would be if
get_inode_object(inode) is called just before the
rcu_assign_pointer(landlock_inode(inode)->object, NULL) from
hook_sb_delete(), which would result in an object with a NULL underobj,
which is the expected behavior (and checked by release_inode).

The line rcu_assign_pointer(inode_sec->object, new_object) from
get_inode_object() can only be reached if the underlying inode doesn't
reference an object, in which case hook_sb_delete() will not reach the
rcu_assign_pointer(landlock_inode(inode)->object, NULL) line for this
same inode.

This works because get_inode_object(inode) is mutually exclusive to
itself with the same inode (i.e. an inode can only point to an object
that references this same inode).

I tried to explain this with the comment "Protects against concurrent
get_inode_object() calls" in get_inode_object(), and the comments just
before both rcu_assign_pointer(landlock_inode(inode)->object, NULL).

>
>
>> + 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;
>> +}