linux-kernel - Re: [PATCH Part1 RFC v4 08/36] x86/sev: check the vmpl level

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Date:   Fri, 13 Aug 2021 17:16:48 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part1 RFC v4 08/36] x86/sev: check the vmpl level

On Fri, Aug 13, 2021 at 08:13:20AM -0500, Brijesh Singh wrote:
> During the firmware initialization the PSP requires that the VMPLEn is
> set. See SNP firmware spec [1] section 8.6. To run the SNP guest you
> *must* specify a VMPL level during the vCPU creation.

Yes, that's why I said "implicit VMPL level 0"! When you don't specify
it, it should implied as 0.

Right now that "enable" bit is useless as it is *forced* to be enabled.

I sincerely hope querying the VMPL level is going to be made
straight-forwaed in future versions.

> I should not say its optional when we know from the SEV-SNP spec that
> VMPLEn must be set to launch SEV-SNP guest. I will fix the description.

It probably wasn't required when that bit was invented - why would you
call it "enable" otherwise - but some decision later made it required,
I'd guess.

> There is no easy way for a guest to query its VMPL level.

Yes, and there should be.

> The VMPL level is set during the vCPU creation. The boot cpu is
> created by the HV and thus its VMPL level is set by the HV. If HV
> chooses a lower VMPL level for the boot CPU then Linux guest will
> not be able to validate its memory because the PVALIDATE instruction
> will cause #GP when the vCPU is running at !VMPL0. The patch tries to
> detect the boot CPU VMPL level and terminate the boot.

I figured as much. All I don't like is the VMPL checking method.

> If guest is not running at VMPL0 then step #2 will cause #GP.  The check
> is prevent the #GP and terminate the boot early.

Yah, Tom helped me understand the design of the permission masks in the
RMP on IRC.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette