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 12:17:57PM +0200, David Hildenbrand wrote:
> On 17.07.19 12:04, David Hildenbrand wrote:
> > On 17.07.19 11:57, Michael S. Tsirkin wrote:
> >> 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?
> >>
> > 
> > David added
> > 
> > if (balloon->pbp
> >     && (rb != balloon->pbp->rb ) ...
> > 
> > So in case the rb changes (IOW replaced - delete old one, new one
> > added), we reset the data.
> > 
> > After a ram block was deleted, there will be no more deflation requests
> > coming in for it. This should be fine I guess.

I think it might happen that an old dangling pointer happens
to match a newly allocated one.
I think we really should just cache all data we want to take into account
and compare that.

> > 
> > 
> > However, there is another possible issue: Resets.
> > 
> > If the balloon was inflated and we reboot, the old balloon->pbp will
> > remain intact. The guest will continue using all memory until
> > virtio-balloon guest driver comes up. If the stars align, it could
> > happen that new inflation requests by the guests will result in a
> > discard of a big chunk, although the guest is re-using some parts
> > already again.
> > 
> > We would have to reset balloon->pbp during virtio_balloon_device_reset().
> > 
> 
> ... also, I think balloon->pbp is not freed when unrealizing, resulting
> in a memory leak ...
> 
> will craft some more patches.

Ught.

> -- 
> 
> Thanks,
> 
> David / dhildenb