linux-kernel - Re: [PATCH] drm/amdgpu/pm: Fix potential Spectre v1

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Date:   Mon, 30 Jul 2018 11:55:08 +0200
From:   Michel Dänzer <michel@...nzer.net>
To:     Alex Deucher <alexdeucher@...il.com>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     "David (ChunMing) Zhou" <David1.Zhou@....com>,
        David Airlie <airlied@...ux.ie>,
        LKML <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH] drm/amdgpu/pm: Fix potential Spectre v1

On 2018-07-24 10:53 PM, Alex Deucher wrote:
> On Mon, Jul 23, 2018 at 12:32 PM, Gustavo A. R. Silva
> <gustavo@...eddedor.com> wrote:
>> idx can be indirectly controlled by user-space, hence leading to a
>> potential exploitation of the Spectre variant 1 vulnerability.
>>
>> This issue was detected with the help of Smatch:
>>
>> drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c:408 amdgpu_set_pp_force_state()
>> warn: potential spectre issue 'data.states'
>>
>> Fix this by sanitizing idx before using it to index data.states
> 
> Is this actually necessary?  We already check that idx is valid a few
> lines before:
>         if (ret || idx >= ARRAY_SIZE(data.states)) {
>                         count = -EINVAL;
>                         goto fail;
>                 }

A Spectre attack would be based on idx ending up too large, but the CPU
speculatively executing the following code assuming idx <
ARRAY_SIZE(data.states), and extracting information from the incorrectly
speculated code via side channels.

I'm not sure if that's actually possible in this case, but better safe
than sorry?


-- 
Earthling Michel Dänzer               |               http://www.amd.com
Libre software enthusiast             |             Mesa and X developer