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

["Mickaël Salaün" <mic@digikod.net>]

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;
>> +}