Re: [Qemu-devel] [PATCH-for-4.1] virtio-balloon: fix QEMU crashes on pagesize > BALLOON_PAGE_SIZE

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 17, 2019 at 10:42:55AM +0200, David Hildenbrand wrote:
> 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
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Cc: David Gibson <david@gibson.dropbear.id.au>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> 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 e85d1c0d5c..669067d661 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);

I also started to wonder about this:

    if (!balloon->pbp) {
        /* Starting on a new host page */
        size_t bitlen = BITS_TO_LONGS(subpages) * sizeof(unsigned long);
        balloon->pbp = g_malloc0(sizeof(PartiallyBalloonedPage) + bitlen);
        balloon->pbp->rb = rb;
        balloon->pbp->base = host_page_base;
    }

Is keeping a pointer to a ram block like this safe? what if the ramblock
gets removed?


> -- 
> 2.21.0