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From:   Matthew Auld <matthew.william.auld@gmail.com>
Date:   Tue, 27 Apr 2021 10:42:03 +0100
Message-ID: <CAM0jSHN3niCs2bQ6bsP1YmdmWcQS17wt-Yo=730q9f_zES=dBQ@mail.gmail.com>
Subject: Re: [PATCH] drm/i9i5/gt: Fix a double free in gen8_preallocate_top_level_pdp
To:     Lv Yunlong <lyl2019@mail.ustc.edu.cn>
Cc:     Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        ML dri-devel <dri-devel@lists.freedesktop.org>
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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.