linux-kernel - Re: [PATCHv11 6/9] efi/unaccepted: Avoid load_unaligned_zeropad() stepping into unaccepted memory

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Date:   Tue, 16 May 2023 13:03:32 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        aarcange@...hat.com, peterx@...hat.com, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv11 6/9] efi/unaccepted: Avoid load_unaligned_zeropad()
 stepping into unaccepted memory

On 5/16/23 11:35, Ard Biesheuvel wrote:
>>> Does this mean that the kernel maps memory before accepting it? As
>>> otherwise, I would assume that such an access would page fault inside
>>> the guest before triggering an exception related to the unaccepted
>>> state.
>> Yes, the kernel maps memory before accepting it (modulo things like
>> DEBUG_PAGEALLOC).
>>
> OK, and so the architecture stipulates that prefetching or other
> speculative accesses must never deliver exceptions to the host
> regarding such ranges?

I don't know of anywhere that this is explicitly written.  It's probably
implicit _somewhere_ in the reams of VMX/TDX and base SDM docs, but heck
if I know where it is. :)

If this is something anyone wants to see added to the SEPT_VE_DISABLE
documentation, please speak up.  I don't think it would be hard to get
it added and provide an explicit guarantee.