linux-kernel - Re: blk: NULL ptr deref in blk_dequeue

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Date:	Wed, 17 Oct 2012 02:30:07 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	Jiri Kosina <jkosina@...e.cz>, Jan Kara <jack@...e.cz>,
	Tejun Heo <tj@...nel.org>, axboe@...nel.dk,
	Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: blk: NULL ptr deref in blk_dequeue_request()

On Fri, 2012-10-12 at 13:55 -0400, Sasha Levin wrote:
> Hi Ben,
> 
> On Wed, Oct 10, 2012 at 11:52 AM, Ben Hutchings <ben@...adent.org.uk> wrote:
> > On Tue, 2012-10-09 at 09:26 -0400, Sasha Levin wrote:
> >> On 10/09/2012 09:21 AM, Sasha Levin wrote:
> >> > On 10/08/2012 05:45 PM, Jiri Kosina wrote:
> >> >> On Mon, 8 Oct 2012, Jan Kara wrote:
> >> >>
> >> >>>>>> I'm still seeing this on linux-next.
> >> >>>>   I think this is floppy related (see redo_fd_request() in the stack
> >> >>>> trace). And there were quite some changes to the area recently. Adding
> >> >>>> maintainer to CC.
> >> >> Hmm ... I don't immediately see how this is happening.
> >> >>
> >> >> Sasha, could you please do git bisect on drivers/block/floppy.c between
> >> >> f6365201d and your git HEAD for starters (assuming that f6365201d works
> >> >> well for you?).
> >> >>
> >> >
> >> > A bisect on floppy.c yielded the following:
> >> >
> >> > b33d002f4b6bae912463e5a66387c498aa69b6fe is the first bad commit
> >> > commit b33d002f4b6bae912463e5a66387c498aa69b6fe
> >> > Author: Ben Hutchings <ben@...adent.org.uk>
> >> > Date:   Mon Aug 27 20:56:53 2012 -0300
> >> >
> >> >     genhd: Make put_disk() safe for disks that have not been registered
[...]
> > I see two problems:
> >
> > 1. redo_fd_request() races with tear-down of the disks, but because
> > set_next_request() checks disk->queue before doing anything this was
> > usually harmless.  Now that do_floppy_init() doesn't clear disk->queue,
> > the race condition is much easier to hit.  This may fix that problem in
> > do_floppy_init(), though there appear to be worse bugs in tear-down
> > order in floppy_module_exit():
[...]
> > 2. I made a big mistake in using the existing GENHD_FL_UP flag, as it is
> > cleared by del_gendisk().  Incremental patch below, but it should be
> > squashed into the previous patch if that branch is still rebase-able.
[...]
> I'm now seeing these instead:
[...]

Sorry, I'm not going to spend more time in the quagmire of the floppy
driver.  Whoever has this commit in their tree, please revert or drop it
as appropriate.

Ben.

-- 
Ben Hutchings
No political challenge can be met by shopping. - George Monbiot

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