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

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Date:   Thu, 16 Dec 2021 14:24:14 -0600
From:   Venu Busireddy <venu.busireddy@...cle.com>
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,
        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 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?

Venu