linux-kernel - Re: [PATCH] virtio-blk: handle block_device

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Date:   Fri, 24 Apr 2020 11:42:48 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     virtualization@...ts.linux-foundation.org,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Lance Digby <ldigby@...hat.com>
Subject: Re: [PATCH] virtio-blk: handle block_device_operations callbacks
 after hot unplug

On Thu, Apr 23, 2020 at 07:13:38AM -0700, Christoph Hellwig wrote:
> 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@...hat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@...hat.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.

Perhaps a better fix is to keep a refcount in struct virtio_blk and
implement block_device_operations->release() to decrement the refcount.

That way the struct virtio_blk stays around until all block device
references are dropped.  This is similar to nbd_put() in
drivers/block/nbd.c.

Stefan

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