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Subject: Re: [PATCH BUGFIX] block, bfq: deschedule empty bfq_queues not
 referred by any process
To:     Paolo Valente <paolo.valente@linaro.org>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        ulf.hansson@linaro.org, linus.walleij@linaro.org,
        bfq-iosched@googlegroups.com, oleksandr@natalenko.name,
        Chris Evich <cevich@redhat.com>,
        Patrick Dung <patdung100@gmail.com>,
        Thorsten Schubert <tschubert@bafh.org>
References: <20191112074856.40433-1-paolo.valente@linaro.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <83a56a54-3269-ecb9-f4ae-01c3f9717279@kernel.dk>
Date:   Tue, 12 Nov 2019 07:23:23 -0800
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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