linux-kernel - Re: [PATCH v8 06/11] x86/traps: Add #VE support for TDX guest

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Date:   Mon, 11 Oct 2021 15:06:11 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>, Deep Shah <sdeep@...are.com>,
        VMware Inc <pv-drivers@...are.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Peter H Anvin <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 06/11] x86/traps: Add #VE support for TDX guest

On Sat, Oct 09, 2021, Lai Jiangshan wrote:
> On Tue, Oct 5, 2021 at 10:54 AM Kuppuswamy Sathyanarayanan
> <sathyanarayanan.kuppuswamy@...ux.intel.com> wrote:
> 
> >
> > The entry paths do not access TD-shared memory, MMIO regions or use
> > those specific MSRs, instructions, CPUID leaves that might generate #VE.
> > In addition, all interrupts including NMIs are blocked by the hardware
> > starting with #VE delivery until TDGETVEINFO is called.  This eliminates
> > the chance of a #VE during the syscall gap or paranoid entry paths and
> > simplifies #VE handling.

Minor clarification: it eliminates the chance of a #VE during the syscall gap
_if the VMM is benign_.  If the VMM is malicious, it can unmap and remap the
syscall page to induce an EPT Violation #VE due to the page not being accepted.

> Hello
> 
> If the reason is applied to #VE, I think it can be applied to SVM-ES's
> #VC too.  (I wish the entry code for #VC to be simplified since I'm
> moving some the asm entry code to C code)
>
> And I'm sorry I haven't read all the emails.
> Has the question asked by Andy Lutomirski been answered in any emails?
> 
> https://lore.kernel.org/lkml/CALCETrU9XypKbj-TrXLB3CPW6=MZ__5ifLz0ckbB=c=Myegn9Q@mail.gmail.com/

This question?

  Can the hypervisor cause an already-accepted secure-EPT page to transition to
  the unaccepted state?

Yep.  I wrote the above before following the link, I should have guessed which
question it was :-)

IIRC, the proposed middle ground was to add a TDCALL and/or TDPARAMS setting that
would allow the guest to opt-out of EPT Violation #VE due to page not accepted,
and instead terminate the VM on such a condition.  The caveat is that that would
require the kernel to never take an "page not accepted #VE" when doing lazy page
acceptance, but that was deemed doable.

That also raises the question of whether Andy's NAK applies to SEV-SNP without
support for "Enhanced SYSCALL Behavior"[*], otherwise SEV-SNP has the same "#VC
in syscall gap" attack.

[*] https://www.amd.com/system/files/TechDocs/57115.pdf