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Date:   Sat, 13 Aug 2022 09:03:13 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     "Borislav Petkov" <bp@alien8.de>,
        "Sean Christopherson" <seanjc@google.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Joerg Roedel" <jroedel@suse.de>,
        "Ard Biesheuvel" <ardb@kernel.org>,
        "Andi Kleen" <ak@linux.intel.com>,
        "Sathyanarayanan Kuppuswamy" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        "David Rientjes" <rientjes@google.com>,
        "Vlastimil Babka" <vbabka@suse.cz>,
        "Tom Lendacky" <thomas.lendacky@amd.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        "Paolo Bonzini" <pbonzini@redhat.com>,
        "Ingo Molnar" <mingo@redhat.com>,
        "Varad Gautam" <varad.gautam@suse.com>,
        "Dario Faggioli" <dfaggioli@suse.com>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Mike Rapoport" <rppt@kernel.org>,
        "David Hildenbrand" <david@redhat.com>,
        "Marcelo Henrique Cerri" <marcelo.cerri@canonical.com>,
        tim.gardner@canonical.com, khalid.elmously@canonical.com,
        philip.cox@canonical.com,
        "the arch/x86 maintainers" <x86@kernel.org>, linux-mm@kvack.org,
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Subject: Re: [PATCHv7 10/14] x86/mm: Avoid load_unaligned_zeropad() stepping into
 unaccepted memory
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On Tue, Aug 9, 2022, at 4:38 AM, Kirill A. Shutemov wrote:
> On Tue, Jul 26, 2022 at 01:17:13PM -0700, Andy Lutomirski wrote:
>>=20
>>=20
>> On Tue, Jun 14, 2022, at 5:02 AM, Kirill A. Shutemov wrote:
>> > load_unaligned_zeropad() can lead to unwanted loads across page bou=
ndaries.
>> > 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 th=
ese
>> > 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 wit=
hin
>> > the guest. The guest will exit to the VMM where the only recourse i=
s to
>> > terminate the guest.
>>=20
>> Why is unaccepted memory marked present in the direct map in the firs=
t place?
>>=20
>> Having kernel code assume that every valid address is followed by
>> several bytes of memory that may be read without side effects other t=
han
>> #PF also seems like a mistake, but I probably won=E2=80=99t win that =
fight. But
>> sticking guard pages in front of definitely-not-logically present pag=
es
>> seems silly to me.  Let=E2=80=99s 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").
>

I don=E2=80=99t mean in a confidential guest =E2=80=94 I mean generally.=
 This whole model of =E2=80=9Coverrun the buffer =E2=80=94 no big deal=E2=
=80=9D is just fragile.

> --=20
>   Kiryl Shutsemau / Kirill A. Shutemov