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

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

[Lv Yunlong]

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@mail.ustc.edu.cn>
---
 drivers/gpu/drm/i915/gt/gen8_ppgtt.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
index 755522ced60d..3ae16945bd43 100644
--- a/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
+++ b/drivers/gpu/drm/i915/gt/gen8_ppgtt.c
@@ -630,7 +630,6 @@ static int gen8_preallocate_top_level_pdp(struct i915_ppgtt *ppgtt)
 
 		err = pin_pt_dma(vm, pde->pt.base);
 		if (err) {
-			i915_gem_object_put(pde->pt.base);
 			free_pd(vm, pde);
 			return err;
 		}
-- 
2.25.1


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

[Matthew Auld]

On Mon, 26 Apr 2021 at 13:44, Lv Yunlong <lyl2019@mail.ustc.edu.cn> 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@mail.ustc.edu.cn>

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

Pushed to drm-intel-gt-next.
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