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Date:   Sat, 13 Aug 2022 09:04:58 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Dave Hansen" <dave.hansen@intel.com>,
        "Borislav Petkov" <bp@alien8.de>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     "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>,
        "Mike Rapoport" <rppt@kernel.org>,
        "David Hildenbrand" <david@redhat.com>,
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        "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 Wed, Aug 3, 2022, at 7:02 AM, Dave Hansen wrote:
> On 8/2/22 16:46, Dave Hansen wrote:
>> To sum it all up, I'm not happy with the complexity of the page
>> acceptance code either but I'm not sure that it's bad tradeoff compar=
ed
>> to greater #VE complexity or fragility.
>>=20
>> Does anyone think we should go back and really reconsider this?
>
> One other thing I remembered as I re-read my write up on this.
>
> In the "new" mode, guests never get #VE's for unaccepted memory.  They
> just exit to the host and can never be reentered.  They must be killed.
>
> In the "old" mode, I _believe_ that the guest always gets a #VE for
> non-EPT-present memory.  The #VE is basically the same no matter if the
> page is unaccepted or if the host goes out and makes a
> previously-accepted page non-present.
>
> One really nasty implication of this "old" mode is that the host can
> remove *accepted* pages that are used in the syscall gap.  That means
> that the #VE handler would need to be of the paranoid variety which
> opens up all kinds of other fun.
>
>  * "Old" - #VE's can happen in the syscall gap
>  * "New" - #VE's happen at better-defined times.  Unexpected ones are
>    fatal.
>
> There's a third option which I proposed but doesn't yet exist.  The TDX
> module _could_ separate the behavior of unaccepted memory #VE's and
> host-induced #VEs.  This way, we could use load_unaligned_zeropad() wi=
th
> impunity and handle it in the #VE handler.  At the same time, the host
> would not be allowed to remove accepted memory and cause problems in t=
he
> syscall gap.  Kinda the best of both worlds.
>
> But, I'm not sure how valuable that would be now that we have the
> (admittedly squirrelly) code to avoid load_unaligned_zeropad() #VE's.

How would that be implemented?  It would need to track which GPAs *were*=
 accepted across a host-induced unmap/remap cycle. This would involve pr=
eventing the host from ever completely removing a secure EPT table witho=
ut the guest=E2=80=99s help, right?

Admittedly this would IMO be better behavior. Is it practical to impleme=
nt?