[Intel-gfx] [PATCH] drm/i915/gvt: fix double-free bug in split_2MB_gtt_entry.

[Zheng Hacker]

Hi Zhenyu,

Very glad for your reply. I agree that the bug is hard to trigger in
userspace. But it is possible to happen in some specific scene. For
example, if calling pfn_valid failed, the bug will be triggered. And
it did happened as the [1] commit description illustrates.

As for the patch, I think your plan is the best. We need to free the
spt only in bad case.

[1] https://github.com/torvalds/linux/commit/39b4cbadb9a95bf3f13ea102d6ec841940916ee2

Regards,
Zheng Wang

Zhenyu Wang <zhenyuw at linux.intel.com> 于2022年9月7日周三 11:33写道：

>
> On 2022.09.06 19:36:56 +0800, Zheng Hacker wrote:
> > Hi Greg,
> >
> > Alex has explained how we figured out the patch. We did analyze the
> > code and found it possible to reach the vulnerability code. But we
> > have no physical device in hand to test the driver. So we'd like to
> > discuss with developers to see if the issue exists or not.
> >
> > Best regards,
> > Zheng Wang.
> >
> > Greg KH <gregkh at linuxfoundation.org> ???2022???9???5????????? 16:04?????????
> > >
> > > On Mon, Sep 05, 2022 at 03:46:09PM +0800, Zheng Hacker wrote:
> > > > I rewrote the letter. Hope it works.
> > > >
> > > > There is a double-free security bug in split_2MB_gtt_entry.
> > > >
> > > > Here is a calling chain :
> > > > ppgtt_populate_spt->ppgtt_populate_shadow_entry->split_2MB_gtt_entry.
> > > > If intel_gvt_dma_map_guest_page failed, it will call
> > > > ppgtt_invalidate_spt, which will finally call ppgtt_free_spt and
> > > > kfree(spt). But the caller does not notice that, and it will call
> > > > ppgtt_free_spt again in error path.
> > > >
>
> It's a little mess in code so in theory it might be possible but
> intel_gvt_dma_map_guest_page won't fail in practise...
>
> > > > Fix this by returning the result of ppgtt_invalidate_spt to split_2MB_gtt_entry.
> > > >
>
> I don't see why changing ret value can fix this issue, as it doesn't change
> any behavior e.g caller of ppgtt_populate_spt to handle possible different error return.
>
> As current code looks assuming that ppgtt_invalidate_spt would free spt in good case,
> I think the real cleanup should split that assumption and handle free in error case properly.
>
> > > > Signed-off-by: Zheng Wang
>
> This misses proper email address.
>
> thanks
>
> > > >
> > > > ---
> > > >  drivers/gpu/drm/i915/gvt/gtt.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/i915/gvt/gtt.c b/drivers/gpu/drm/i915/gvt/gtt.c
> > > > index ce0eb03709c3..9f14fded8c0c 100644
> > > > --- a/drivers/gpu/drm/i915/gvt/gtt.c
> > > > +++ b/drivers/gpu/drm/i915/gvt/gtt.c
> > > > @@ -1215,7 +1215,7 @@ static int split_2MB_gtt_entry(struct intel_vgpu *vgpu,
> > > >                 ret = intel_gvt_dma_map_guest_page(vgpu, start_gfn + sub_index,
> > > >                                                    PAGE_SIZE, &dma_addr);
> > > >                 if (ret) {
> > > > -                       ppgtt_invalidate_spt(spt);
> > > > +                       ret = ppgtt_invalidate_spt(spt);
> > > >                         return ret;
> > >
> > > But now you just lost the original error, shouldn't this succeed even if
> > > intel_gvt_dma_map_guest_page() failed?
> > >
> > > And how are you causing intel_gvt_dma_map_guest_page() to fail in a real
> > > system?
> > >
> > > thanks,
> > >
> > > greg k-h