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

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Message-ID: <20230516191549.tjub26jvlqymp27x@box.shutemov.name>
Date:   Tue, 16 May 2023 22:15:49 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.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 Tue, May 16, 2023 at 08:35:27PM +0200, Ard Biesheuvel wrote:
> On Tue, 16 May 2023 at 20:27, Dave Hansen <dave.hansen@...el.com> wrote:
> >
> > On 5/16/23 11:08, Ard Biesheuvel wrote:
> > >> But, this approach does not work for unaccepted memory. For TDX, a load
> > >> from unaccepted memory will not lead to a recoverable exception within
> > >> the guest. The guest will exit to the VMM where the only recourse is to
> > >> terminate the guest.
> > >>
> > > 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?
> 
> If this all works as it should, then I'm ok with leaving this here,
> but I imagine we may want to factor out some arch specific policy here
> in the future, as I don't think this would work the same on ARM.

Even if other architectures don't need this, it is harmless: we just
accept one unit ahead of time.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov