Re: [PATCH] drm/i9i5/gt: Fix a double free in gen8_preallocate_top_level_pdp

From: Matthew Auld
Date: Tue Apr 27 2021 - 05:42:33 EST

On Mon, 26 Apr 2021 at 13:44, Lv Yunlong <lyl2019@xxxxxxxxxxxxxxxx> wrote:
> Our code analyzer reported a double free bug.
> In gen8_preallocate_top_level_pdp, pde and pde->pt.base are allocated
> via alloc_pd(vm) with one reference. If pin_pt_dma() failed, pde->pt.base
> is freed by i915_gem_object_put() with a reference dropped. Then free_pd
> calls free_px() defined in intel_ppgtt.c, which calls i915_gem_object_put()
> to put pde->pt.base again.
> As pde->pt.base is protected by refcount, so the second put will not free
> pde->pt.base actually. But, maybe it is better to remove the first put?
> Fixes: 82adf901138cc ("drm/i915/gt: Shrink i915_page_directory's slab bucket")
> Signed-off-by: Lv Yunlong <lyl2019@xxxxxxxxxxxxxxxx>

Yes, it looks like this fixes a potential use-after-free. Thanks for the patch,
Reviewed-by: Matthew Auld <matthew.auld@xxxxxxxxx>

Pushed to drm-intel-gt-next.