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 18:24:49 -0300
From:	Rafael Aquini <aquini@...hat.com>
To:	Luiz Capitulino <lcapitulino@...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, 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!

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

 
> 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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