linux-kernel - Re: Slab corruption in floppy driver module

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Message-ID: <giobtq5e46.fsf@mx10.gouders.net>
Date:	Thu, 26 Jan 2012 22:48:57 +0100
From:	Dirk Gouders <gouders@...bocholt.fh-gelsenkirchen.de>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Suresh Jayaraman <sjayaraman@...e.com>,
	LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: Slab corruption in floppy driver module

Vivek Goyal <vgoyal@...hat.com> writes:

> On Thu, Jan 26, 2012 at 10:05:32AM -0800, Tejun Heo wrote:
>> Hello,
>> 
>> On Thu, Jan 26, 2012 at 10:04:20AM -0500, Vivek Goyal wrote:
>> >  out_put_disk:
>> >  	while (dr--) {
>> >  		del_timer_sync(&motor_off_timer[dr]);
>> > -		if (disks[dr]->queue)
>> > +		if (disks[dr]->queue) {
>> >  			blk_cleanup_queue(disks[dr]->queue);
>> > +			/*
>> > +			 * The request queue reference we took at device
>> > +			 * creation time has been put by above
>> > +			 * blk_cleanup_queue(). We have not called add_disk()
>> > +			 * yet and due to failure calling put_disk(). Put disk
>> > +			 * will try to put a reference to disk->queue which is
>> > +			 * taken in add_disk(). As we have not taken that
>> > +			 * extra reference, putting extra reference down
>> > +			 * will try to access already freed queue. Clear
>> > +			 * disk->queue before calling put_disk().
>> > +			 */
>> > +			disks[dr]->queue = NULL;
>> 
>> Yeah, this looks correct to me.  It might be better to tone down the
>> comment a bit tho.  Wouldn't it be sufficient to say put_disk() isn't
>> paired with add_disk() and will put one extra time?
>
> Sure. Toned down the comment as suggested. Here is the new patch.
>
> floppy: Cleanup disk->queue before caling put_disk() if add_disk() was never called
>
> add_disk() takes gendisk reference on request queue. If driver failed during
> initialization and never called add_disk() then that extra reference is not
> taken. That reference is put in put_disk(). floppy driver allocates the
> disk, allocates queue, sets disk->queue and then relizes that floppy
> controller is not present. It tries to tear down everything and tries to
> put a reference down in put_disk() which was never taken.
>
> In such error cases cleanup disk->queue before calling put_disk() so that
> we never try to put down a reference which was never taken in first place.
>
> Reported-by: Suresh Jayaraman <sjayaraman@...e.com>
> Tested-by: Dirk Gouders <gouders@...bocholt.fh-gelsenkirchen.de>
> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> ---
>  drivers/block/floppy.c |    8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> Index: linux-2.6/drivers/block/floppy.c
> ===================================================================
> --- linux-2.6.orig/drivers/block/floppy.c	2012-01-15 09:49:14.000000000 -0500
> +++ linux-2.6/drivers/block/floppy.c	2012-01-26 14:35:14.662374464 -0500
> @@ -4368,8 +4368,14 @@ out_unreg_blkdev:
>  out_put_disk:
>  	while (dr--) {
>  		del_timer_sync(&motor_off_timer[dr]);
> -		if (disks[dr]->queue)
> +		if (disks[dr]->queue) {
>  			blk_cleanup_queue(disks[dr]->queue);
> +			/*
> +			 * put_disk() is not paired with add_disk() and
> +			 * will put queue reference one extra time. fix it.
> +			 */
> +			disks[dr]->queue = NULL;
> +		}
>  		put_disk(disks[dr]);
>  	}
>  	return err;


Probably a rare and uncommon one but it seems that the reloading case on
a machine that has a floppy controller is a different problem.  To be
sure I tested the patch on a machine that has a floppy controller and
when unloading and reloading the floppy module the log messages that I
attached to a mail earlier in this thread are still generated.

Dirk
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