linux-kernel - Re: [PATCH 2/2] KVM: SEV: Configure "ALLOWED_SEV

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <c603c0c3-36cb-4429-9799-ed50bba4a59e@amd.com>
Date: Tue, 20 Aug 2024 10:26:51 -0500
From: Kim Phillips <kim.phillips@....com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	Tom Lendacky <thomas.lendacky@....com>, Michael Roth <michael.roth@....com>,
	Ashish Kalra <ashish.kalra@....com>, Nikunj A Dadhania <nikunj@....com>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Ingo Molnar <mingo@...hat.com>, "H.
 Peter Anvin" <hpa@...or.com>, Thomas Gleixner <tglx@...utronix.de>, "Kishon
 Vijay Abraham I" <kvijayab@....com>
Subject: Re: [PATCH 2/2] KVM: SEV: Configure "ALLOWED_SEV_FEATURES" VMCB Field

On 8/19/24 5:23 PM, Sean Christopherson wrote:
> On Mon, Aug 19, 2024, Kim Phillips wrote:
>> On 8/16/24 5:59 PM, Sean Christopherson wrote:
>>> On Thu, Aug 01, 2024, Kim Phillips wrote:
>>>> From: Kishon Vijay Abraham I <kvijayab@....com>
>>>>
>>>> AMD EPYC 5th generation processors have introduced a feature that allows
>>>> the hypervisor to control the SEV_FEATURES that are set for or by a
>>>> guest [1]. The ALLOWED_SEV_FEATURES feature can be used by the hypervisor
>>>> to enforce that SEV-ES and SEV-SNP guests cannot enable features that the
>>>> hypervisor does not want to be enabled.
>>>
>>> How does the host communicate to the guest which features are allowed?
>>
>> I'm not familiar with any future plans to negotiate with the guest directly,
> 
> I feel like I'm missing something.  What happens if the guest wants to enable
> PmcVirtualization and it's unexpectedly disallowed?  Does the guest simply panic?

In SNP, VMRUN will return with an exit code of VMEXIT_INVALID (0xffffffff)
if the guest tries to set it.

In SEV-ES, the hypervisor can set it, and the same thing will happen to VMRUN.

In both cases, SEV_FEATURES is saved to VMCB field GUEST_SEV_FEATURES at
offset 140h on the VMEXIT, indicating to the host which feature was
attempted but caught as not allowed.

>> but since commit ac5c48027bac ("KVM: SEV: publish supported VMSA features"),
>> userspace can retrieve sev_supported_vmsa_features via an ioctl.
>>
>>> And based on this blurb:
>>>
>>>     Some SEV features can only be used if the Allowed SEV Features Mask is enabled,
>>>     and the mask is configured to permit the corresponding feature. If the Allowed
>>>     SEV Features Mask is not enabled, these features are not available (see SEV_FEATURES
>>>     in Appendix B, Table B-4).
>>>
>>> and the appendix, this only applies to PmcVirtualization and SecureAvic.  Adding
>>> that info in the changelog would be *very* helpful.
>>
>> Ok, how about adding:
>>
>> "The PmcVirtualization and SecureAvic features explicitly require
>> ALLOWED_SEV_FEATURES to enable them before they can be used."
>>
>>> And I see that SVM_SEV_FEAT_DEBUG_SWAP, a.k.a. DebugVirtualization, is a guest
>>> controlled feature and doesn't honor ALLOWED_SEV_FEATURES.  Doesn't that mean
>>> sev_vcpu_has_debug_swap() is broken, i.e. that KVM must assume the guest can
>>> DebugVirtualization on and off at will?  Or am I missing something?
>>
>> My understanding is that users control KVM's DEBUG_SWAP setting
>> with a module parameter since commit 4dd5ecacb9a4 ("KVM: SEV: allow
>> SEV-ES DebugSwap again").  If the module parameter is not set, with
>> this patch, VMRUN will fail since the host doesn't allow DEBUG_SWAP.
> 
> But that's just KVM's view of vmsa_features.  With SNP's wonderful
> SVM_VMGEXIT_AP_CREATE, can't the guest create a VMSA with whatever sev_features
> it wants, so long as they aren't host-controllable, i.e. aren't PmcVirtualization
> or SecureAvic?

No, as above, if the guest tries any silly business the host will
get a VMEXIT_INVALID, no matter if using the feature *requires*
ALLOWED_SEV_FEATURES to be enabled and explicitly allow it (currently
PmcVirtualization or SecureAvic).

Kim