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

[Christian KÃnig]

Am 30.07.2018 um 22:14 schrieb Alex Deucher:
> On Mon, Jul 30, 2018 at 5:55 AM, Michel DÃnzer <michel@xxxxxxxxxxx> 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@xxxxxxxxxxxxxx> 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