Re: [RFC 0/3] block: address blktrace use-after-free

[Luis Chamberlain]

On Tue, Apr 7, 2020 at 1:00 PM Luis Chamberlain <mcgrof@xxxxxxxxxx> wrote:
>
> On Tue, Apr 07, 2020 at 10:47:01AM +0800, yukuai (C) wrote:
> > On 2020/4/3 16:19, Ming Lei wrote:
> >
> > > BTW, Yu Kuai posted one patch for this issue, looks that approach
> > > is simpler:
> > >
> > > https://lore.kernel.org/linux-block/20200324132315.22133-1-yukuai3@xxxxxxxxxx/
> > >
> > >
> >
> > I think the issue might not be fixed with the patch seires.
> >
> > At first, I think there are two key points for the issure:
> > 1. The final release of queue is delayed in a workqueue
> > 2. The creation of 'q->debugfs_dir' might failed(only if 1 exist)
> > And if we can fix any of the above problem, the UAF issue will be fixed.
> > (BTW, I did not come up with a good idea for problem 1, and my approach
> > is for problem 2.)
> >
> > The third patch "block: avoid deferral of blk_release_queue() work" is
> > not enough to fix problem 1:
> > a. if CONFIG_DEBUG_KOBJECT_RELEASE is enable:
> > static void kobject_release(struct kref *kref)
> > {
> >         struct kobject *kobj = container_of(kref, struct kobject, kref);
> > #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
> >         unsigned long delay = HZ + HZ * (get_random_int() & 0x3);
> >         pr_info("kobject: '%s' (%p): %s, parent %p (delayed %ld)\n",
> >                 âkobject_name(kobj), kobj, __func__, kobj->parent, delay);
> >         INIT_DELAYED_WORK(&kobj->release, kobject_delayed_cleanup);
> >
> >         schedule_delayed_work(&kobj->release, delay);
> > #else
> >         kobject_cleanup(kobj);
> > #endif
> > }
> > b. when 'kobject_put' is called from blk_cleanup_queue, can we make sure
> > it is the last reference?
>
> You are right, I think I know the fix for this now. Will run some more
> tests.

Yeap, we were just not refcounting during blktrace. I'll send a fix as
part of the series.

  Luis