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

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Date:   Sat, 13 Aug 2022 23:58:13 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>,
        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 Sat, Aug 13, 2022 at 09:04:58AM -0700, Andy Lutomirski wrote:
> 
> 
> 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 compared
> >> to greater #VE complexity or fragility.
> >> 
> >> 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() with
> > 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 the
> > 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
> preventing the host from ever completely removing a secure EPT table
> without the guest’s help, right?
> 
> Admittedly this would IMO be better behavior. Is it practical to implement?

I don't think it is better if you look from host POV. It owns resources of
the machine and it has to have a way to pull memory from uncooperative TD
at any point.

It also would require more complicated private->shared conversion as guest
has to notify TDX module about the change, not only host as we do now.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov