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

[Zheng Hacker]

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> 于2022年9月5日周一 16:04写道：
>
> 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.
> >
> > Fix this by returning the result of ppgtt_invalidate_spt to split_2MB_gtt_entry.
> >
> > Signed-off-by: Zheng Wang
> >
> > ---
> >  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