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

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Date:   Mon, 30 Jul 2018 16:14:23 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "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 Mon, Jul 30, 2018 at 5:55 AM, Michel Dänzer <michel@...nzer.net> wrote:
> 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?

Yeah, I'm not sure.  I guess this can't hurt.

Alex