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

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Message-ID: <658cd654-be3e-659a-20a7-f33034c231ce@amd.com>
Date:   Tue, 31 Jul 2018 08:46:10 +0200
From:   Christian König <christian.koenig@....com>
To:     Alex Deucher <alexdeucher@...il.com>,
        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>
Subject: Re: [PATCH] drm/amdgpu/pm: Fix potential Spectre v1

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.

Christian.

>
> Alex