Re: [PATCH BUGFIX V2 1/1] block, bfq: deschedule empty bfq_queues not referred by any process

[Holger HoffstÃtte]

On 11/14/19 10:33 AM, Paolo Valente wrote:
> Since commit 3726112ec731 ("block, bfq: re-schedule empty queues if
> they deserve I/O plugging"), to prevent the service guarantees of a
> bfq_queue from being violated, the bfq_queue may be left busy, i.e.,
> scheduled for service, even if empty (see comments in
> __bfq_bfqq_expire() for details). But, if no process will send
> requests to the bfq_queue any longer, then there is no point in
> keeping the bfq_queue scheduled for service.
>
> In addition, keeping the bfq_queue scheduled for service, but with no
> process reference any longer, may cause the bfq_queue to be freed when
> descheduled from service. But this is assumed to never happen, and
> causes a UAF if it happens. This, in turn, caused crashes [1, 2].
>
> This commit fixes this issue by descheduling an empty bfq_queue when
> it remains with not process reference.
>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1767539
> [2] https://bugzilla.kernel.org/show_bug.cgi?id=205447
>
> Fixes: 3726112ec731 ("block, bfq: re-schedule empty queues if they deserve I/O plugging")
> Reported-by: Chris Evich <cevich@xxxxxxxxxx>
> Reported-by: Patrick Dung <patdung100@xxxxxxxxx>
> Reported-by: Thorsten Schubert <tschubert@xxxxxxxx>
> Tested-by: Thorsten Schubert <tschubert@xxxxxxxx>
> Tested-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>

Jens,

can you please also tag this for stable-5.3 before the next push?
The original problem was found on 5.3 after all, and hoping for the
stable-bot to pick it up automagically is a bit unreliable.

Thanks,
Holger