linux-kernel - Re: [PATCH] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

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Date:	Wed, 5 Jun 2013 19:08:44 -0400
From:	Luiz Capitulino <lcapitulino@...hat.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH] virtio_balloon: leak_balloon(): only tell host if we
 got pages deflated

On Wed, 5 Jun 2013 18:24:49 -0300
Rafael Aquini <aquini@...hat.com> wrote:

> On Wed, Jun 05, 2013 at 05:10:31PM -0400, Luiz Capitulino wrote:
> > The balloon_page_dequeue() function can return NULL. If it does for
> > the first page being freed, then leak_balloon() will create a
> > scatter list with len=0. Which in turn seems to generate an invalid
> > virtio request.
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@...hat.com>
> > ---
> > 
> > PS: I didn't get this in practice. I found it by code review. On the other
> >     hand, automatic-ballooning was able to put such invalid requests in
> >     the virtqueue and QEMU would explode...
> >
> 
> Nice catch! The patch looks sane and replicates the check done at
> fill_balloon(). I think we also could use this P.S. as a commentary 
> to let others aware of this scenario. Thanks Luiz!

Want me to respin?

> Acked-by: Rafael Aquini <aquini@...hat.com>

Thanks for your review!

> 
>  
> > PPS: Very lightly tested
> >
> >  drivers/virtio/virtio_balloon.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index bd3ae32..71af7b5 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -191,7 +191,8 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
> >  	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> >  	 * is true, we *have* to do it in this order
> >  	 */
> > -	tell_host(vb, vb->deflate_vq);
> > +	if (vb->num_pfns != 0)
> > +		tell_host(vb, vb->deflate_vq);
> >  	mutex_unlock(&vb->balloon_lock);
> >  	release_pages_by_pfn(vb->pfns, vb->num_pfns);
> >  }
> > -- 
> > 1.8.1.4
> > 
> 

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