linux-kernel - Re: [PATCH 2/5] blktrace: fix debugfs use after free

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Date:   Wed, 15 Apr 2020 14:48:33 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Eric Sandeen <sandeen@...deen.net>,
        Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org
Cc:     mhocko@...e.com, yukuai3@...wei.com, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>,
        syzbot+603294af2d01acfdd6da@...kaller.appspotmail.com
Subject: Re: [PATCH 2/5] blktrace: fix debugfs use after free

On 2020-04-15 10:38, Eric Sandeen wrote:
> On 4/13/20 11:18 PM, Luis Chamberlain wrote:
>> On commit 6ac93117ab00 ("blktrace: use existing disk debugfs directory")
>> merged on v4.12 Omar fixed the original blktrace code for request-based
>> drivers (multiqueue). This however left in place a possible crash, if you
>> happen to abuse blktrace in a way it was not intended.
>>
>> Namely, if you loop adding a device, setup the blktrace with BLKTRACESETUP,
>> forget to BLKTRACETEARDOWN, and then just remove the device you end up
>> with a panic:
> 
> I think this patch makes this all cleaner anyway, but - without the apparent
> loop bug mentioned by Bart which allows removal of the loop device while blktrace
> is active (if I read that right), can this still happen?

That's a great question. Even if the loop driver fix would be sufficient
to fix the blktrace debugfs use-after free I think the block layer
patches from this series are still very valuable. As explained in the
cover letter this patch series fixes more than only the blktrace debugfs
use-after-free.

Thanks,

Bart.