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

[Greg KH <gregkh@linuxfoundation.org>]

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