linux-kernel - Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when changing C-bit

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Date:   Thu, 10 Feb 2022 17:48:55 +0100
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,
        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>,
        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>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when
 changing C-bit

On Wed, Feb 09, 2022 at 12:10:15PM -0600, Brijesh Singh wrote:
> The set_memory_{encrypted,decrypted}() are used for changing the pages
> from decrypted (shared) to encrypted (private) and vice versa.
> When SEV-SNP is active, the page state transition needs to go through
> additional steps done by the guest.
> 
> If the page is transitioned from shared to private, then perform the
> following after the encryption attribute is set in the page table:
> 
> 1. Issue the page state change VMGEXIT to add the memory region in
>    the RMP table.
> 2. Validate the memory region after the RMP entry is added.
> 
> To maintain the security guarantees, if the page is transitioned from
> private to shared, then perform the following before encryption attribute
> is removed from the page table:
> 
> 1. Invalidate the page.
> 2. Issue the page state change VMGEXIT to remove the page from RMP table.
> 
> To change the page state in the RMP table, use the Page State Change
> VMGEXIT defined in the GHCB specification.
> 
> The GHCB specification provides the flexibility to use either 4K or 2MB
> page size in during the page state change (PSC) request. For now use the
> 4K page size for all the PSC until RMP page size tracking is supported
> in the kernel.

This commit message sounds familiar because I've read it before - patch
18 - and it looks copied. So I've turned into a simple one which says it
all:

    x86/mm: Validate memory when changing the C-bit

    Add the needed functionality to change pages state from shared
    to private and vice-versa using the Page State Change VMGEXIT as
    documented in the GHCB spec.

Thx.

-- 
Regards/Gruss,
    Boris.

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