[PATCH v2] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

[PATCH v2] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

[Luiz Capitulino]

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.

I didn't get this in practice, I found it by code review. On the other
hand, such an invalid virtio request will cause errors in QEMU and
fill_balloon() also performs the same check implemented by this commit.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Acked-by: Rafael Aquini <aquini@redhat.com>
---

o v2

 - Improve changelog

 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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Re: [PATCH v2] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

[Rafael Aquini]

On Wed, Jun 05, 2013 at 09:18:37PM -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.
> 
> I didn't get this in practice, I found it by code review. On the other
> hand, such an invalid virtio request will cause errors in QEMU and
> fill_balloon() also performs the same check implemented by this commit.
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> Acked-by: Rafael Aquini <aquini@redhat.com>
> ---
> 
> o v2
> 
>  - Improve changelog
> 
>  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);

Luiz, sorry for not being clearer before. I was referring to add a commentary on
code, to explain in short words why we should not get rid of this check point.

> +	if (vb->num_pfns != 0)
> +		tell_host(vb, vb->deflate_vq);
>  	mutex_unlock(&vb->balloon_lock);

If the comment is regarded as unnecessary, then just ignore my suggestion. I'm
OK with your patch. :)

Cheers!
-- Rafael

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Re: [PATCH v2] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

[Luiz Capitulino]

On Thu, 6 Jun 2013 11:13:58 -0300
Rafael Aquini <aquini@redhat.com> wrote:

> On Wed, Jun 05, 2013 at 09:18:37PM -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.
> > 
> > I didn't get this in practice, I found it by code review. On the other
> > hand, such an invalid virtio request will cause errors in QEMU and
> > fill_balloon() also performs the same check implemented by this commit.
> > 
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > Acked-by: Rafael Aquini <aquini@redhat.com>
> > ---
> > 
> > o v2
> > 
> >  - Improve changelog
> > 
> >  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);
> 
> Luiz, sorry for not being clearer before. I was referring to add a commentary on
> code, to explain in short words why we should not get rid of this check point.

Oh.

> > +	if (vb->num_pfns != 0)
> > +		tell_host(vb, vb->deflate_vq);
> >  	mutex_unlock(&vb->balloon_lock);
> 
> If the comment is regarded as unnecessary, then just ignore my suggestion. I'm
> OK with your patch. :)

IMHO, the code is clear enough.

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Re: [PATCH v2] virtio_balloon: leak_balloon(): only tell host if we got pages deflated

[Luiz Capitulino]

On Wed, 5 Jun 2013 21:18:37 -0400
Luiz Capitulino <lcapitulino@redhat.com> 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.
> 
> I didn't get this in practice, I found it by code review. On the other
> hand, such an invalid virtio request will cause errors in QEMU and
> fill_balloon() also performs the same check implemented by this commit.
> 
> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> Acked-by: Rafael Aquini <aquini@redhat.com>

Andrew, can you pick this one?

> ---
> 
> o v2
> 
>  - Improve changelog
> 
>  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);
>  }

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