[PATCH 60/77] virtio-balloon: Prevent guest from starting a report when we didn't request one

[Michael Roth]

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

Based on code review it appears possible for the driver to force the device
out of a stopped state when hinting by repeating the last ID it was
provided.

Prevent this by only allowing a transition to the start state when we are
in the requested state. This way the driver is only allowed to send one
descriptor that will transition the device into the start state. All others
will leave it in the stop state once it has finished.

Fixes: c13c4153f76d ("virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT")
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>
Message-Id: <20200720175115.21935.99563.stgit@localhost.localdomain>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 20a4da0f23078deeff5ea6d1e12f47d968d7c3c9)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 hw/virtio/virtio-balloon.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index e20f90dad4..a30a0c7bfa 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -466,7 +466,8 @@ static bool get_free_page_hints(VirtIOBalloon *dev)
             ret = false;
             goto out;
         }
-        if (id == dev->free_page_report_cmd_id) {
+        if (dev->free_page_report_status == FREE_PAGE_REPORT_S_REQUESTED &&
+            id == dev->free_page_report_cmd_id) {
             dev->free_page_report_status = FREE_PAGE_REPORT_S_START;
         } else {
             /*
-- 
2.17.1