RE: [PATCH v3 20/35] x86/bugs: Define attack vectors

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Sent: Thursday, February 20, 2025 4:05 PM
> To: Borislav Petkov <bp@xxxxxxxxx>
> Cc: Kaplan, David <David.Kaplan@xxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Pawan Gupta
> <pawan.kumar.gupta@xxxxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Dave
> Hansen <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; H . Peter Anvin
> <hpa@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3 20/35] x86/bugs: Define attack vectors
>
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> On Tue, Feb 18, 2025 at 09:52:03AM +0100, Borislav Petkov wrote:
> > On Mon, Feb 17, 2025 at 11:05:01PM -0800, Josh Poimboeuf wrote:
> > > IMO, make them generic from the start, then there's less churn and
> > > it's easy to port the other arches.
> > >
> > > If we went with putting everything in "mitigations=", making them
> > > generic would be the obvious way to go anyway.
> >
> > Just to make sure we're all on the same page: we obviously cannot
> > enable and test and support a mitigaion on another arch like, say, arm64, or so.
> >
> > This needs to come from the respective arch maintainers themselves and
> > they'll have to say, yes, pls, enable it and we'll support it. We
> > should not go "oh, this would be a good idea to do on all arches"
> > without hearing from them first, even if it is a good idea on its face.
> >
> > That's why those are x86-only as they should be initially.
>
> I wasn't suggesting that this patch set should *enable* it on all arches.  Of course
> that would need to be reviewed by the respective arch maintainers.
>
> But looking ahead, this *will* be needed for the other arches, for the same reason
> we have a generic mitigations=off.  It's a user problem, not an arch-specific one.
> Users need a simple interface that works everywhere.  That's why I suggested
> integrating it into "mitigations=".
>

Talked with Boris on the side, he is ok with supporting this in mitigations=, with a warning message if you try to use these controls on yet-unsupported architectures.

Going back to the command line definition, I think that to help make the selection clearer we could consider the following format:

mitigations=[on/off],[attack vectors]

For example:

"mitigations=on,no_user_kernel" to enable all attack vectors except user->kernel
"mitigations=off,guest_host" to disable all vectors except guest->host

By requiring either 'on' or 'off' first, I think that makes it more obvious what the default would be.  My concern is something like 'mitigations=no_user_kernel' doesn't immediately make it clear that other mitigations are going to be enabled.  If the correct format is not followed, the kernel can print a warning and just fall back to the defaults.

This format would only be required if you're going to use attack vector controls, of course.

Thoughts?

--David Kaplan