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

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Message-ID: <20220809113827.fchtnyzy44z5fuis@box.shutemov.name>
Date:   Tue, 9 Aug 2022 14:38:27 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        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>,
        Dave Hansen <dave.hansen@...el.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 Tue, Jul 26, 2022 at 01:17:13PM -0700, Andy Lutomirski wrote:
> 
> 
> On Tue, Jun 14, 2022, at 5:02 AM, Kirill A. Shutemov wrote:
> > load_unaligned_zeropad() can lead to unwanted loads across page boundaries.
> > The unwanted loads are typically harmless. But, they might be made to
> > totally unrelated or even unmapped memory. load_unaligned_zeropad()
> > relies on exception fixup (#PF, #GP and now #VE) to recover from these
> > unwanted loads.
> >
> > 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.
> 
> Why is unaccepted memory marked present in the direct map in the first place?
> 
> Having kernel code assume that every valid address is followed by
> several bytes of memory that may be read without side effects other than
> #PF also seems like a mistake, but I probably won’t win that fight. But
> sticking guard pages in front of definitely-not-logically present pages
> seems silly to me.  Let’s just not map it.

It would mean no 1G pages in direct mapping for TDX as we accept 2M a
time.

> (What if MMIO memory is mapped next to regular memory?  Doing random
> unaligned reads that cross into MMIO seems unwise.)

MMIO is shared, not unaccpted private. We already handle the situation.
See 1e7769653b06 ("x86/tdx: Handle load_unaligned_zeropad() page-cross to
a shared page").

-- 
  Kiryl Shutsemau / Kirill A. Shutemov