linux-kernel - Re: [PATCH BUGFIX] block, bfq: deschedule empty bfq

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Date:   Tue, 12 Nov 2019 07:23:23 -0800
From:   Jens Axboe <axboe@...nel.dk>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name,
        Chris Evich <cevich@...hat.com>,
        Patrick Dung <patdung100@...il.com>,
        Thorsten Schubert <tschubert@...h.org>
Subject: Re: [PATCH BUGFIX] block, bfq: deschedule empty bfq_queues not
 referred by any process

On 11/11/19 11:48 PM, 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

Applied, thanks.

-- 
Jens Axboe