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

[Jann Horn <jannh@google.com>]

On Tue, Mar 23, 2021 at 4:54 PM Mickaël Salaün <mic@digikod.net> wrote:
> 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,

Yes.

> 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).

But it can be that landlock_inode(inode)->object only becomes non-NULL
after get_inode_object() has checked
rcu_dereference(inode_sec->object).

> 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 scenario I'm talking about doesn't involve hook_sb_delete().

> 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,

Yes.

> 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).

To clarify: You can concurrently call get_inode_object() multiple
times on the same inode, right? There are no locks held on entry to
that function.

> 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).

The scenario I'm talking about is:

Initially the inode does not have an associated landlock_object. There
are two threads A and B. Thread A is going to execute
get_inode_object(). Thread B is going to execute get_inode_object()
followed immediately by landlock_put_object().

thread A: enters get_inode_object()
thread A: rcu_dereference(inode_sec->object) returns NULL
thread A: enters landlock_create_object()
thread B: enters get_inode_object()
thread B: rcu_dereference(inode_sec->object) returns NULL
thread B: calls landlock_create_object()
thread B: sets inode_sec->object while holding inode->i_lock
thread B: leaves get_inode_object()
thread B: enters landlock_put_object()
thread B: object->usage drops to 0, object->lock is taken
thread B: calls release_inode()
thread B: drops object->lock
thread A: returns from landlock_create_object()
thread A: takes inode->i_lock

At this point, thread B will run:

    rcu_assign_pointer(landlock_inode(inode)->object, NULL);

while thread A runs:

    rcu_dereference_protected(inode_sec->object,
        lockdep_is_held(&inode->i_lock));

meaning there is a (theoretical) data race, since
rcu_dereference_protected() doesn't use READ_ONCE().

> >> +       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;
> >> +}