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: Friday, November 22, 2024 10:37 AM
> To: Manwaring, Derek <derekmn@xxxxxxxxxx>
> Cc: Kaplan, David <David.Kaplan@xxxxxxx>; bp@xxxxxxxxx;
> canellac@xxxxxxxxx; dave.hansen@xxxxxxxxxxxxxxx; hpa@xxxxxxxxx;
> jpoimboe@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; mingo@xxxxxxxxxx;
> mlipp@xxxxxxxxx; pawan.kumar.gupta@xxxxxxxxxxxxxxx;
> peterz@xxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; x86@xxxxxxxxxx
> Subject: Re: [PATCH v2 19/35] Documentation/x86: Document the new attack
> vector controls
>
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>
> On Fri, 22 Nov 2024 at 17:15, Manwaring, Derek <derekmn@xxxxxxxxxx>
> wrote:
> >
> > On 2024-11-14 at 9:32+0000, Brendan Jackman wrote:
> > > On Wed, 13 Nov 2024 at 17:19, Brendan Jackman
> <jackmanb@xxxxxxxxxx> wrote:
>
> > > A discussion off-list led me to realise that the specifics of this
> > > comment are nonsensical, I had L1TF in mind but I don't think you
> > > can exploit L1TF in a direct guest->guest attack (I'm probably still
> > > missing some nuance there). We wouldn't need to flush L1D there
> > > unless there's a new vuln.
> >
> > With Foreshadow-VMM/CVE-2018-3646 I thought you can do guest-
> >guest?
> > Since guest completely controls the physical address which ends up
> > probing L1D (as if it were a host physical address).
>
> You are almost certainly right!

Agreed, I will update my patches to fix this, so that mitigation is applied if guest->guest protection is requested.

Thanks
--David Kaplan

>
> > And agree with the flushes between different restricted address spaces
> > (e.g. context switch between guests, right?).
>
> Yeah basically, although with the RFCv2 I'm gonna be proposing this
> "tainting" model where instead of having to flush, in context switch we just
> set a flag to say "another MM* might have left data in a sidechannel". Then if
> we have that flag set on an asi_enter we flush at that point.
>
> *We could break that down further too, e.g. whether the thing that left data
> behind was a VM guest or a userspace task, if that ever influences what
> caches/buffers we wanna burn.