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Subject: Re: [PATCH BUGFIX V2 1/1] block, bfq: deschedule empty bfq_queues not
 referred by any process
To:     Paolo Valente <paolo.valente@linaro.org>,
        Jens Axboe <axboe@kernel.dk>
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,
        tschubert@bafh.org, patdung100@gmail.com, cevich@redhat.com
References: <20191114093311.47877-1-paolo.valente@linaro.org>
 <20191114093311.47877-2-paolo.valente@linaro.org>
From:   =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= <holger@applied-asynchrony.com>
Organization: Applied Asynchrony, Inc.
Message-ID: <89dde326-fc76-10cc-5ec9-ec5fd4dae4ac@applied-asynchrony.com>
Date:   Fri, 15 Nov 2019 19:32:50 +0100
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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@redhat.com>
> Reported-by: Patrick Dung <patdung100@gmail.com>
> Reported-by: Thorsten Schubert <tschubert@bafh.org>
> Tested-by: Thorsten Schubert <tschubert@bafh.org>
> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> Signed-off-by: Paolo Valente <paolo.valente@linaro.org>

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