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 16:04:04 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     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 11:33, Kirill A. Shutemov wrote:
> For context: there's a way configure TDX environment to trigger #VE on
> such accesses and it is default. But Linux requires such #VEs to be
> disabled as it opens attack vector from the host to the guest: host can
> pull any private page from under kernel at any point and trigger such #VE.
> If it happens in just a right time in syscall gap or NMI entry code it can
> be exploitable.

I'm kinda uncomfortable with saying it's exploitable.

It really boils down to not wanting to deal with managing a new IST
exception.  While the NMI IST implementation is about as good as we can
get it, I believe there are still holes in it (even if we consider only
how it interacts with #MC).  The more IST users we add, the more holes
there are.

You add the fact that an actual adversary can induce the exceptions
instead of (rare and mostly random) radiation that causes #MC, and it
makes me want to either curl up in a little ball or pursue a new career.

So, exploitable?  Dunno.  Do I want to touch an #VE/IST implementation?
No way, not with a 10 foot pole.