[PATCH v2] virtio-balloon: Disable free page reporting if page poison reporting is not enabled

[Alexander Duyck <alexander.duyck@gmail.com>]

From: Alexander Duyck <alexander.h.duyck@linux.intel.com>

We should disable free page reporting if page poisoning is enabled in the
kernel but we cannot report it via the balloon interface. This way we can
avoid the possibility of corrupting guest memory. Normally the page poison
reporting feature should always be present when free page reporting is
enabled on the hypervisor, however this allows us to correctly handle a
case of the virtio-balloon device being possibly misconfigured.

Fixes: 5d757c8d518d ("virtio-balloon: add support for providing free page reports to host")
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
---

Changes since v1:
Originally this patch also modified free page hinting, that has been removed.
Updated patch title and description.
Added a comment explaining reasoning for disabling free page reporting.

 drivers/virtio/virtio_balloon.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 51086a5afdd4..1f157d2f4952 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -1107,11 +1107,18 @@ static int virtballoon_restore(struct virtio_device *vdev)
 
 static int virtballoon_validate(struct virtio_device *vdev)
 {
-	/* Tell the host whether we care about poisoned pages. */
+	/*
+	 * Inform the hypervisor that our pages are poisoned or
+	 * initialized. If we cannot do that then we should disable
+	 * page reporting as it could potentially change the contents
+	 * of our free pages.
+	 */
 	if (!want_init_on_free() &&
 	    (IS_ENABLED(CONFIG_PAGE_POISONING_NO_SANITY) ||
 	     !page_poisoning_enabled()))
 		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_PAGE_POISON);
+	else if (!virtio_has_feature(vdev, VIRTIO_BALLOON_F_PAGE_POISON))
+		__virtio_clear_bit(vdev, VIRTIO_BALLOON_F_REPORTING);
 
 	__virtio_clear_bit(vdev, VIRTIO_F_IOMMU_PLATFORM);
 	return 0;