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Subject: Re: [PATCH RFC] KVM: x86: tell guests if the exposed SMT topology is
 trustworthy
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <943488A8-2DD7-4471-B3C7-9F21A0B0BCF9@dinechin.org>
Date:   Thu, 7 Nov 2019 17:02:26 +0200
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>, kvm@vger.kernel.org,
        x86@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Jim Mattson <jmattson@google.com>,
        linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
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Message-Id: <713ECF67-6A6C-4956-8AC6-7F4C05961328@oracle.com>
References: <20191105161737.21395-1-vkuznets@redhat.com>
 <20191105193749.GA20225@linux.intel.com>
 <20191105232500.GA25887@linux.intel.com>
 <943488A8-2DD7-4471-B3C7-9F21A0B0BCF9@dinechin.org>
To:     Christophe de Dinechin <christophe.de.dinechin@gmail.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On 7 Nov 2019, at 16:00, Christophe de Dinechin =
<christophe.de.dinechin@gmail.com> wrote:
>=20
>=20
>=20
>> On 6 Nov 2019, at 00:25, Sean Christopherson =
<sean.j.christopherson@intel.com> wrote:
>>=20
>> On Tue, Nov 05, 2019 at 11:37:50AM -0800, Sean Christopherson wrote:
>>> On Tue, Nov 05, 2019 at 05:17:37PM +0100, Vitaly Kuznetsov wrote:
>>>> Virtualized guests may pick a different strategy to mitigate =
hardware
>>>> vulnerabilities when it comes to hyper-threading: disable SMT =
completely,
>>>> use core scheduling, or, for example, opt in for STIBP. Making the
>>>> decision, however, requires an extra bit of information which is =
currently
>>>> missing: does the topology the guest see match hardware or if it is =
'fake'
>>>> and two vCPUs which look like different cores from guest's =
perspective can
>>>> actually be scheduled on the same physical core. Disabling SMT or =
doing
>>>> core scheduling only makes sense when the topology is trustworthy.
>>>>=20
>>>> Add two feature bits to KVM: KVM_FEATURE_TRUSTWORTHY_SMT with the =
meaning
>>>> that KVM_HINTS_TRUSTWORTHY_SMT bit answers the question if the =
exposed SMT
>>>> topology is actually trustworthy. It would, of course, be possible =
to get
>>>> away with a single bit (e.g. 'KVM_FEATURE_FAKE_SMT') and not lose =
backwards
>>>> compatibility but the current approach looks more straightforward.
>>>=20
>>> I'd stay away from "trustworthy", especially if this is controlled =
by
>>> userspace.  Whether or not the hint is trustworthy is purely up to =
the
>>> guest.  Right now it doesn't really matter, but that will change as =
we
>>> start moving pieces of the host out of the guest's TCB.
>>>=20
>>> It may make sense to split the two (or even three?) cases, e.g.
>>> KVM_FEATURE_NO_SMT and KVM_FEATURE_ACCURATE_TOPOLOGY.  KVM can =
easily
>>> enforce NO_SMT _today_, i.e. allow it to be set if and only if SMT =
is
>>> truly disabled.  Verifying that the topology exposed to the guest is =
legit
>>> is a completely different beast.
>>=20
>> Scratch the ACCURATE_TOPOLOGY idea, I doubt there's a real use case =
for
>> setting ACCURATE_TOPOLOGY and not KVM_HINTS_REALTIME.  A feature flag =
to
>> state that SMT is disabled seems simple and useful.

A bit such as NoNonArchitecturalCoreSharing can be set even when host =
SMT is enabled.
For example, when host use core-scheduling to group together vCPUs that =
run as sibling hyperthreads.
Therefore, I wouldn=E2=80=99t want to tie the feature-flag semantics to =
host SMT being enabled/disabled.
It=E2=80=99s just true that this bit can be set when host SMT is =
disabled.

>=20
> I share that concern about the naming, although I do see some
> value in exposing the cpu_smt_possible() result. I think it=E2=80=99s =
easier
> to state that something does not work than to state something does
> work.
>=20
> Also, with respect to mitigation, we may want to split the two cases
> that Paolo outlined, i.e. have KVM_HINTS_REALTIME,
> KVM_HINTS_CORES_CROSSTALK and
> KVM_HINTS_CORES_LEAKING,
> where CORES_CROSSTALKS indicates there may be some
> cross-talk between what the guest thinks are isolated cores,
> and CORES_LEAKING indicates that cores may leak data
> to some other guest.
>=20
> The problem with my approach is that it is shouting =E2=80=9Cdon=E2=80=99=
t trust me=E2=80=9D
> a bit too loudly.

I don=E2=80=99t see a value in exposing CORES_LEAKING to guest. As guest =
have nothing to do with it.

-Liran