linux-kernel - Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into unaccepted memory

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Message-ID: <20220813211330.73bse242m4m5fz3n@box.shutemov.name>
Date:   Sun, 14 Aug 2022 00:13:30 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Sathyanarayanan Kuppuswamy 
        <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 (Intel)" <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Marcelo Henrique Cerri <marcelo.cerri@...onical.com>,
        tim.gardner@...onical.com, khalid.elmously@...onical.com,
        philip.cox@...onical.com,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping
 into unaccepted memory

On Sat, Aug 13, 2022 at 09:11:52AM -0700, Andy Lutomirski wrote:
> Now if the TD module could deliver an unrecoverable #MC instead of an
> impossible-to-handle #VE, maybe we could at least get a nice debug trace
> out?  Of course it’s not so easy to do anything with a debug trace that
> doesn’t break confidentiality.

It is not impossible-to-handle #VE, it is no #VE for the guest and exit to
the host that cannot be recovered. Yes, it is not friednly for debugging.

Our plan was to allow SEPT_VE_DISABLE=0 for debug TD. It helps with
debugging stepping on unaccepted memory as allows #VE in the guest which
leads to panic() and nice traceback.

Would it be enough?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov