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Date:   Tue, 9 Aug 2022 14:38:27 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Andy Lutomirski <luto@kernel.org>
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>,
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        Dave Hansen <dave.hansen@intel.com>,
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        the arch/x86 maintainers <x86@kernel.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, 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