linux-kernel - Re: [PATCH v8 08/40] x86/sev: Check the vmpl level

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Date:   Thu, 16 Dec 2021 15:39:26 -0800
From:   Mikolaj Lisik <lisik@...gle.com>
To:     Venu Busireddy <venu.busireddy@...cle.com>
Cc:     Brijesh Singh <brijesh.singh@....com>, 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,
        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>,
        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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 08/40] x86/sev: Check the vmpl level

On Thu, Dec 16, 2021 at 12:24 PM Venu Busireddy
<venu.busireddy@...cle.com> wrote:
>
> On 2021-12-10 09:43:00 -0600, Brijesh Singh wrote:
> > Virtual Machine Privilege Level (VMPL) feature in the SEV-SNP architecture
> > allows a guest VM to divide its address space into four levels. The level
> > can be used to provide the hardware isolated abstraction layers with a VM.
> > The VMPL0 is the highest privilege, and VMPL3 is the least privilege.
> > Certain operations must be done by the VMPL0 software, such as:
> >
> > * Validate or invalidate memory range (PVALIDATE instruction)
> > * Allocate VMSA page (RMPADJUST instruction when VMSA=1)
> >
> > The initial SEV-SNP support requires that the guest kernel is running on
> > VMPL0. Add a check to make sure that kernel is running at VMPL0 before
> > continuing the boot. There is no easy method to query the current VMPL
> > level, so use the RMPADJUST instruction to determine whether the guest is
> > running at the VMPL0.
> >
> > Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> > ---
> >  arch/x86/boot/compressed/sev.c    | 34 ++++++++++++++++++++++++++++---
> >  arch/x86/include/asm/sev-common.h |  1 +
> >  arch/x86/include/asm/sev.h        | 16 +++++++++++++++
> >  3 files changed, 48 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
> > index a0708f359a46..9be369f72299 100644
> > --- a/arch/x86/boot/compressed/sev.c
> > +++ b/arch/x86/boot/compressed/sev.c
> > @@ -212,6 +212,31 @@ static inline u64 rd_sev_status_msr(void)
> >       return ((high << 32) | low);
> >  }
> >
> > +static void enforce_vmpl0(void)
> > +{
> > +     u64 attrs;
> > +     int err;
> > +
> > +     /*
> > +      * There is no straightforward way to query the current VMPL level. The
> > +      * simplest method is to use the RMPADJUST instruction to change a page
> > +      * permission to a VMPL level-1, and if the guest kernel is launched at
> > +      * a level <= 1, then RMPADJUST instruction will return an error.
>
> Perhaps a nit. When you say "level <= 1", do you mean a level lower than or
> equal to 1 semantically, or numerically?
>

+1 to this. Additionally I found the "level-1" confusing which I
interpreted as "level minus one".

Perhaps phrasing it as "level one", or "level=1" would be more explicit?