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Date: Thu, 16 Apr 2020 00:56:36 +0000 From: Luis Chamberlain <mcgrof@...nel.org> To: Eric Sandeen <sandeen@...deen.net> Cc: axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org, gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz, ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org, 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 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, 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. Luis
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