[Qemu-devel] [PATCH-for-4.1 v3 2/6] virtio-balloon: Fix QEMU crashes on pagesize > BALLOON_PAGE_SIZE

[David Hildenbrand <david@redhat.com>]

We are using the wrong functions to set/clear bits, effectively touching
multiple bits, writing out of range of the bitmap, resulting in memory
corruptions. We have to use set_bit()/clear_bit() instead.

Can easily be reproduced by starting a qemu guest on hugetlbfs memory,
inflating the balloon. QEMU crashes. This never could have worked
properly - especially, also pages would have been discarded when the
first sub-page would be inflated (the whole bitmap would be set).

While testing I realized, that on hugetlbfs it is pretty much impossible
to discard a page - the guest just frees the 4k sub-pages in random order
most of the time. I was only able to discard a hugepage a handful of
times - so I hope that now works correctly.

Fixes: ed48c59875b6 ("virtio-balloon: Safely handle BALLOON_PAGE_SIZE <
                     host page size")
Fixes: b27b32391404 ("virtio-balloon: Fix possible guest memory corruption
                     with inflates & deflates")
Cc: qemu-stable@nongnu.org #v4.0.0
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/virtio/virtio-balloon.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
index 515abf6553..a78d2d2184 100644
--- a/hw/virtio/virtio-balloon.c
+++ b/hw/virtio/virtio-balloon.c
@@ -94,9 +94,8 @@ static void balloon_inflate_page(VirtIOBalloon *balloon,
         balloon->pbp->base = host_page_base;
     }
 
-    bitmap_set(balloon->pbp->bitmap,
-               (ram_offset - balloon->pbp->base) / BALLOON_PAGE_SIZE,
-               subpages);
+    set_bit((ram_offset - balloon->pbp->base) / BALLOON_PAGE_SIZE,
+            balloon->pbp->bitmap);
 
     if (bitmap_full(balloon->pbp->bitmap, subpages)) {
         /* We've accumulated a full host page, we can actually discard
@@ -140,9 +139,8 @@ static void balloon_deflate_page(VirtIOBalloon *balloon,
          * for a guest to do this in practice, but handle it anyway,
          * since getting it wrong could mean discarding memory the
          * guest is still using. */
-        bitmap_clear(balloon->pbp->bitmap,
-                     (ram_offset - balloon->pbp->base) / BALLOON_PAGE_SIZE,
-                     subpages);
+        clear_bit((ram_offset - balloon->pbp->base) / BALLOON_PAGE_SIZE,
+                  balloon->pbp->bitmap);
 
         if (bitmap_empty(balloon->pbp->bitmap, subpages)) {
             g_free(balloon->pbp);
-- 
2.21.0