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

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Message-ID: <11728bc1-7b59-1623-b517-d1a0d57eb275@intel.com>
Date:   Mon, 19 Dec 2022 08:22:01 +0000
From:   "Wang, Zhi A" <zhi.a.wang@...el.com>
To:     Zheng Wang <zyytlz.wz@....com>
CC:     "1002992920@...com" <1002992920@...com>,
        "airlied@...il.com" <airlied@...il.com>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "alex000young@...il.com" <alex000young@...il.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "hackerzheng666@...il.com" <hackerzheng666@...il.com>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "joonas.lahtinen@...ux.intel.com" <joonas.lahtinen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "security@...nel.org" <security@...nel.org>,
        "tvrtko.ursulin@...ux.intel.com" <tvrtko.ursulin@...ux.intel.com>,
        "zhenyuw@...ux.intel.com" <zhenyuw@...ux.intel.com>
Subject: Re: [Intel-gfx] [PATCH v3] drm/i915/gvt: fix double free bug in
 split_2MB_gtt_entry

On 12/19/2022 9:57 AM, Zheng Wang wrote:
> 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
> 

I think it is a case-by-case thing. For example:

The current scenario in this function looks like below:

caller pass spt a
function
	alloc spt b
	something error
	free spt a
	return error

The problem is: the function wrongly frees the spt a instead free what 
it allocates.

A proper fix should be:

caller pass spt a
function
	alloc spt b
	something error
	*free spt b*
	return error

Thanks,
Zhi.