Re: [PATCH -next] blk-throttle: Set BIO_THROTTLED when bio has been throttled

[QiuLaibin]

On 2021/12/14 4:12, Tejun Heo wrote:
> On Mon, Dec 13, 2021 at 05:24:00PM +0800, QiuLaibin wrote:
> > > * This function is called synchronously on the issue path. The bio isn't
> > >     seen by the queue and device driver yet and nothing can race to issue it
> > >     before this function returns.
> >
> > The bio is under throttle here, this submit_bio return directly. And
> > current process will queue a dispatch work by
> > throtl_schedule_pending_timer() to submit this bio before BIO_THROTTLED flag
> > set. If the bio is completed quickly after the dispatch work is queued, UAF
> > of bio will happen.
>
> You are right, the timer can get to it. Can't it be solved by just
> reordering spin_unlock and setting BIO_THROTTLED?

I think it can be solved by setting BIO_THROTTLED before queue dispatch 
work.

> > > * Now we're not setting BIO_THROTTLED when we're taking a different return
> > >     path through the out_unlock label and risks calling back into blk_throtl
> > >     again on the same bio.
> >
> > In my opinion, This flag can prevent the request from being throttled
> > multiple times. If the request itself does not need to be throttled, the
> > result of repeated entry will be the same.
> > If necessary, I think we can use other methods to achieve this effect for
> > request does not need to be throttled.
>
> So that we don't change anything regarding this?

I am thinking of adding a new bio tag (like BIO_THROTTLE_BYPASS) to 
avoid those requests which do not need to be throttled to enter the 
throttle multiple times.
> Thanks.