linux-kernel - Re: [PATCH] x86/CPU/AMD, mm: Extend with mem

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Date:   Mon, 2 Oct 2017 08:44:21 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...e.de>, Brijesh Singh <brijesh.singh@....com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/CPU/AMD, mm: Extend with mem_encrypt=sme option

On 10/1/2017 12:16 PM, Borislav Petkov wrote:
> On Sun, Oct 01, 2017 at 12:00:31PM -0500, Brijesh Singh wrote:
>> When SEV feature is disabled, KVM will not be able to launch any SEV
>> guests.  When SEV support is available, KVM can enable it in a specific
>> VM by setting SEV bit before executing the VMRUN instruction.
> 
> So I want to be able to disable SEV and the whole code that comes with
> it in the *host*.

The ability to launch an SEV guest from the host is controlled by the SEV
feature bit.  When that is cleared (which happens in cpu/amd.c), then the
host will not be able to launch any SEV guests.

The confusion may come from the variable name.  The name sev_enabled
should probably be changed to sev_active since this variable is used as
follows:

   sme_me_mask == 0 			=> Neither SME nor SEV is active
   sme_me_mask != 0 && !sev_enabled	=> SME is active (host mode)
   sme_me_mask != 0 && sev_enabled	=> SEV is active (guest mode)

> 
>> Guest OS:
>> --------
>> Checks the MSR_AMD64_SEV to determine if SEV feature is enabled. Please
>> note that the MSR is a read-only. IOW, MSR is not intercepted by the
>> hypervisor.
>>
>> Currently, mem_encrypt=xxx and CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT
>> is don't care. We can not depend on the command line because when SEV is
>> enabled in a VM then instruction fetch will be decrypted by the
>> hardware. If we want then we can perform the comparison between the SEV
>> state obtained through MSR with user supplied command line and trigger
>> BUG() if they don't match.
> 
> And when we have supplied mem_encrypt=sme to the *host* cmdline, it
> should be impossible to start SEV guests. IOW, that feature mask test
> should not happen and I should do instead:

I think we're talking about the same thing.  You want sev_enabled to
indicate whether you can launch an SEV guest.  We would still need an
sev_active variable to distinguish between SME and SEV during kernel
execution when the sme_me_mask is non-zero.  Currently, the SEV feature
bit acts as "sev_enabled" and the sev_enabled variable acts as
"sev_active" in this scenario.

Thanks,
Tom

> 
>          } else if (!strncmp(buffer, cmd_sme, sizeof(buffer))) {
>                  sme_only = true;
> 		sev_enabled = false;
>          }
> 
> Or, respectively, not set it here as it is false already but set it at
> the end of the function like this:
> 
>          if (sme_only)
>                  return;
> 
> 	sev_enabled = true;
> }
> 
> Hmmm?
>