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

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Date:   Tue, 31 Jul 2018 17:29:08 -0400
From:   Alex Deucher <alexdeucher@...il.com>
To:     Christian König <christian.koenig@....com>
Cc:     Michel Dänzer <michel@...nzer.net>,
        "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>
Subject: Re: [PATCH] drm/amdgpu/pm: Fix potential Spectre v1

On Tue, Jul 31, 2018 at 2:46 AM, Christian König
<christian.koenig@....com> wrote:
> Am 30.07.2018 um 22:14 schrieb Alex Deucher:
>>
>> 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.
>
>
> Well is idx actually controlable by userspace in an IOCTL? I guess the
> answer is no.
>
> On the other hand the array_index_nospec() macro makes the overhead absolute
> negligible.
>
> So I agree that we should be better safe than sorry.

Ok.  Applied.

Thanks,

Alex