RE: [PATCH v2 19/35] Documentation/x86: Document the new attack vector controls

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Brendan Jackman <jackmanb@xxxxxxxxxx>
> Sent: Wednesday, November 13, 2024 9:32 AM
> To: Kaplan, David <David.Kaplan@xxxxxxx>
> Cc: Manwaring, Derek <derekmn@xxxxxxxxxx>; bp@xxxxxxxxx;
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> Subject: Re: [PATCH v2 19/35] Documentation/x86: Document the new attack
> vector controls
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> On Wed, 13 Nov 2024 at 16:05, Kaplan, David <David.Kaplan@xxxxxxx> wrote:
> >
> > I don't see how ASI can ever be a user_user mitigation.  User_user attacks are
> things like influencing the indirect predictions used by another process, causing that
> process to leak data from its address space.  User_user mitigations are things like
> doing an IBPB when switching tasks.
>
> Well, in the RFC I'm currently (painfully slowly, sorry again) working on, that IBPB is
> provided by ASI. Each process has an ASI domain, ASI ensures there's an IBPB
> before we transition into any other domain that doesn't trust it (VMs trust their VMM,
> but all other transitions out of the userpace domain will flush).
>
> In practice, this is just provided by the fact that context switching currently incurs an
> asi_exit(), but that's an implementation detail, if we transitioned directly from one
> process' domain to another that would also create a flush.
>
> (But yes, maybe that being "part of ASI" is just my very ASI-centric perspective).

Ah yes, this makes sense.  As you said though, this is due to the fact that context switch incurs an asi_exit.  As a thought exercise, I wonder what would happen if there was a mitigation that was required when switching to another guest, but not to the broader host address space.  I think that would mean there would still be a need for extra flushes when switching guest->guest that may not be covered by asi_exit.  (That's theoretical though but could be a reason not to tie these together forever)

And yes, please do keep up the ASI work, I'm very much looking forward to it, it should be a big improvement.

--David Kaplan