linux-kernel - Re: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race

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Date:   Mon, 29 Aug 2016 19:49:29 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     Rabin Vincent <rabin@....in>, Jens Axboe <axboe@...com>,
        Jan Kara <jack@...e.cz>, Al Viro <viro@...iv.linux.org.uk>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] bdev: fix NULL pointer dereference in sync()/close() race

Hello,

On Mon, Aug 29, 2016 at 11:33:41PM +0200, Vegard Nossum wrote:
> On 08/29/2016 09:55 PM, Tejun Heo wrote:
> > I think the right thing to do there is doing blkdev_get() /
> > blkdev_put() around func() invocation in iterate_bdevs() rather than
> > holding bd_mutex across the callback.  Can you please verify whether
> > that works?
> 
> Didn't work for me, I kept getting use-after-free in __blkdev_get() on
> bdev->bd_invalidated after it calls bdev->bd_disk->fops->open(). I tried
> a few related things without much luck.

I see.  It could be that it's doing blkdev_get() on a dying device.

> The only thing that worked for me without holding the mutex across the
> call was this:
...
> +		mutex_lock(&bdev->bd_mutex);
> +		bdev->bd_openers++;
> +		bdev->bd_holders++;
> +		mutex_unlock(&bdev->bd_mutex);
> +
> +		func(bdev, arg);
> +
> +		mutex_lock(&bdev->bd_mutex);
> +		bdev->bd_openers--;
> +		bdev->bd_holders--;
> +		mutex_unlock(&bdev->bd_mutex);

And this might not be too far fetched.  I think what we want is

* Bump bd_openers if there are other users already; otherwise, skip.

* blkdev_put() after the callback is finished.

Thanks.

-- 
tejun