linux-kernel - Re: [Intel-gfx] [PATCH v3] drm/i915/gvt: fix double free bug in split_2MB_gtt

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Message-Id: <20221219075700.220058-1-zyytlz.wz@163.com>
Date:   Mon, 19 Dec 2022 15:57:00 +0800
From:   Zheng Wang <zyytlz.wz@....com>
To:     zhi.a.wang@...el.com
Cc:     1002992920@...com, airlied@...il.com, airlied@...ux.ie,
        alex000young@...il.com, dri-devel@...ts.freedesktop.org,
        gregkh@...uxfoundation.org, hackerzheng666@...il.com,
        intel-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org,
        joonas.lahtinen@...ux.intel.com, linux-kernel@...r.kernel.org,
        security@...nel.org, tvrtko.ursulin@...ux.intel.com,
        zhenyuw@...ux.intel.com, zyytlz.wz@....com
Subject: Re: [Intel-gfx] [PATCH v3] drm/i915/gvt: fix double free bug in split_2MB_gtt_entry

Hi Zhi,

Thanks again for your reply and clear explaination about the function.
I still have some doubt about the fix. Here is a invoke chain :
ppgtt_populate_spt
  ->ppgtt_populate_shadow_entry
    ->split_2MB_gtt_entry
As far as I'm concerned, when something error happens in DMA mapping,
which will make intel_gvt_dma_map_guest_page return none-zero code,
It will invoke ppgtt_invalidate_spt and call ppgtt_free_spt,which will
finally free spt by kfree. But the caller doesn't notice that and frees
spt by calling ppgtt_free_spt again. This is a typical UAF/Double Free
vulnerability. So I think the key point is about how to handle spt properly.
The handle newly allocated spt (aka sub_spt) is not the root cause of this
issue. Could you please give me more advice about how to fix this security
bug? Besides, I'm not sure if there are more similar problems in othe location.

Best regards,
Zheng Wang

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2.25.1