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Subject: Re: [PATCH 2/5] blktrace: fix debugfs use after free
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     axboe@kernel.dk, viro@zeniv.linux.org.uk, bvanassche@acm.org,
        gregkh@linuxfoundation.org, rostedt@goodmis.org, mingo@redhat.com,
        jack@suse.cz, ming.lei@redhat.com, nstange@suse.de,
        akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Omar Sandoval <osandov@fb.com>,
        Hannes Reinecke <hare@suse.com>,
        Michal Hocko <mhocko@kernel.org>,
        syzbot+603294af2d01acfdd6da@syzkaller.appspotmail.com
References: <20200414041902.16769-1-mcgrof@kernel.org>
 <20200414041902.16769-3-mcgrof@kernel.org>
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From:   Eric Sandeen <sandeen@sandeen.net>
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Message-ID: <924950e6-e016-25b2-4ee1-b5ea9f752c12@sandeen.net>
Date:   Wed, 15 Apr 2020 20:02:04 -0500
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On 4/15/20 7:56 PM, Luis Chamberlain wrote:
> On Wed, Apr 15, 2020 at 12:38:26PM -0500, 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?
> 
> I have not tested that, but some modifications of the break-blktrace
> program could enable us to test that

FWIW, I modified it to modprobe & rmmod scsi_debug instead of the loop ioctls,
and the module can't be unloaded after the blktrace is started since it's busy.

Not sure that's equivalent tho.

> however I don't think the race
> would be possible after patch 3/5 "blktrace: refcount the request_queue
> during ioctl" is merged, as removal then a pending blktrace would
> refcount the request_queue and the removal would have to wait until
> the refcount is decremeneted, until after the blktrace ioctl.

I'm out of my depth in the block layer, not sure who's supposed to take
refs on what. ;)

-Eric