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Date: Wed, 15 Apr 2020 20:02:04 -0500 From: Eric Sandeen <sandeen@...deen.net> To: Luis Chamberlain <mcgrof@...nel.org> 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 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
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