On Wed, Jul 17, 2019 at 01:28:19PM +0200, David Hildenbrand wrote:
> On 17.07.19 13:22, Michael S. Tsirkin wrote:
> > On Wed, Jul 17, 2019 at 01:10:21PM +0200, David Hildenbrand wrote:
> >> On 17.07.19 13:06, Michael S. Tsirkin wrote:
> >>> 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-sta...@nongnu.org #v4.0.0
> >>>>>>> Cc: Stefan Hajnoczi <stefa...@redhat.com>
> >>>>>>> Cc: David Gibson <da...@gibson.dropbear.id.au>
> >>>>>>> Cc: Michael S. Tsirkin <m...@redhat.com>
> >>>>>>> Cc: Igor Mammedov <imamm...@redhat.com>
> >>>>>>> Signed-off-by: David Hildenbrand <da...@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.
> >>
> >> That's true. I think just remembering and comparing the GPA base address
> >> would be sufficient.
> >
> > Well we need to know the bitmap size allocated, too.
> > And I guess when we are ready to free we should
> > re-check it just in case.
>
> Right, either that or the page size, which is orthogonal.
>
> >
> >> However, I don't consider this here to trigger easily. We would need
> >> some crazy memory unplug+replug going on while using the balloon. So I
> >> assume we can just rework this part after 4.1
> >
> > Dangling pointers are just a recipe for CVEs. I'd rather rework it now.
> >
>
> If they are not dereferences, I don't consider it an ultimate problem.
The following pattern is highly unsafe if p has been freed and
reused:
if (d->p == p)
use p->foo
and this is because we can now have copies of d->p->foo != p->foo
resulting in inconsistencies.
> But yeah, I'll look into that tomorrow. Can you pick up these patches in
> the meantime?
>
> Thanks!
Sure, thanks!
> --
>
> Thanks,
>
> David / dhildenb
