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

[Alex Young]

Thanks for your reply.

We think that when intel_gvt_dma_map_guest_page() fails,
ppgtt_invalidate_spt is called to handle this error.

If the ppgtt_invalidate_spt is successful to kfree the spt object,
then in the ppgtt_populate_spt function there  is no need to kfree the
spt again.

And if the ppgtt_invalidate_spt failed, then in the ppgtt_populate_spt
function there is need to kfree the spt for error handling.

This is our fix, if it's not right, we are glad to discuss with you.

Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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