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Date:   Thu, 23 Apr 2020 07:13:38 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Stefan Hajnoczi <stefanha@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Lance Digby <ldigby@redhat.com>
Subject: Re: [PATCH] virtio-blk: handle block_device_operations callbacks
 after hot unplug
Message-ID: <20200423141338.GA29646@infradead.org>
References: <20200423123717.139141-1-stefanha@redhat.com>
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In-Reply-To: <20200423123717.139141-1-stefanha@redhat.com>
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On Thu, Apr 23, 2020 at 01:37:17PM +0100, Stefan Hajnoczi wrote:
> A virtio_blk block device can still be referenced after hot unplug by
> userspace processes that hold the file descriptor.  In this case
> virtblk_getgeo() can be invoked after virtblk_remove() was called.  For
> example, a program that has /dev/vdb open can call ioctl(HDIO_GETGEO)
> after hot unplug.
> 
> Fix this by clearing vblk->disk->private_data and checking that the
> virtio_blk driver instance is still around in virtblk_getgeo().
> 
> Note that the virtblk_getgeo() function itself is guaranteed to remain
> in memory after hot unplug because the virtio_blk module refcount is
> still held while a block device reference exists.
> 
> Originally-by: Lance Digby <ldigby@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  drivers/block/virtio_blk.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 93468b7c6701..b50cdf37a6f7 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -300,6 +300,10 @@ static int virtblk_getgeo(struct block_device *bd, struct hd_geometry *geo)
>  {
>  	struct virtio_blk *vblk = bd->bd_disk->private_data;
>  
> +	/* Driver instance has been removed */
> +	if (!vblk)
> +		return -ENOTTY;

If this ever hits you have a nasty race condition and this is not
going to fix it, as it could be removed just after this check as well.