linux-kernel - Re: [PATCH v6 19/42] x86/mm: Add support to validate memory when changing C-bit

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Date:   Wed, 10 Nov 2021 19:43:59 +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>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v6 19/42] x86/mm: Add support to validate memory when
 changing C-bit

On Wed, Nov 10, 2021 at 08:21:21AM -0600, Brijesh Singh wrote:
> I am assuming you mean add some compile time check to ensure that desc will
> fit in the shared buffer ?

No:

struct ghcb {

	...

        u8 shared_buffer[2032];

so that memcpy needs to do:

	memcpy(ghcb->shared_buffer, desc, min_t(int, 2032, sizeof(*desc)));

with that 2032 behind a proper define, ofc.

> I can drop the overlap comment to avoid the confusion, as you pointed it
> more of the future thing. Basically overlap is the below condition
> 
> set_memory_private(gfn=0, page_size=2m)
> set_memory_private(gfn=10, page_size=4k)
> 
> The RMPUPDATE instruction will detect overlap on the second call and return
> an error to the guest. After we add the support to track the page validation
> state (either in bitmap or page flag), the second call will not be issued
> and thus avoid an overlap errors. For now, we use the page_size=4k for all
> the page state changes from the kernel.

Yah, sounds like the comment is not needed now. You could put this in
the commit message, though.

Thx.

-- 
Regards/Gruss,
    Boris.

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