Re: [PATCH] bdi: fix use-after-free for bdi device

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Feb 14, 2020 at 10:50:01AM +0800, Yufen Yu wrote:
> > So, unregistering can leave ->dev along and re-registering can test
> > whether it's NULL and if not put the existing one and put a new one
> > there. Wouldn't that work?
> 
> Do you mean set bdi->dev as 'NULL' in call_rcu() callback function
> (i.e. bdi_release_device()) and test 'bdi->dev' in bdi_register_va()?
> 
> I think that may do not work.
> We cannot make sure the order of rcu callback function and re-registering.
> Then bdi_release_device() may put the new allocated device by re-registering.

No, I meant not freeing bdi->dev on deregistration and only doing so
when it actually needs to - on re-registration or release. So, sth
like the following.

* Unregister: Unregister bdi->dev but don't free it. Leave the pointer
  alone.

* Re-register: If bdi->dev is not null, initiate RCU-free and update
  bdi->dev to the new dev.

* Release: If bdi->dev is not NULL, initiate RCU-free of it.

Thanks.

-- 
tejun