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

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Date:	Thu, 26 Jan 2012 16:56:08 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Dirk Gouders <gouders@...bocholt.fh-gelsenkirchen.de>
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

On Thu, Jan 26, 2012 at 10:48:57PM +0100, Dirk Gouders wrote:
> 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.

Ok. Thanks for the update. I had assumed that it solved both the issues.
So, module load/unload seems to be a different problem. We should still
take this patch as it solves atleast the case of floppy controller not
being present.

Jens, do you want me to post the patch in a separate mail thread?

Thanks
Vivek
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