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Date:   Mon, 11 Oct 2021 09:49:15 -0700
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Subject: Re: [PATCH v8 06/11] x86/traps: Add #VE support for TDX guest
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To:     Sean Christopherson <seanjc@google.com>,
        Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>, Deep Shah <sdeep@vmware.com>,
        VMware Inc <pv-drivers@vmware.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, Peter H Anvin <hpa@zytor.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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 <YWRS41aZH9A6fekt@google.com>
From:   Andi Kleen <ak@linux.intel.com>
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> 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.

This has been addressed. The TDX module will support a mode that forbids 
unmapping pages permanently, and Linux is going to check/enforce that 
this mode is enabled. The patch for the check is not included in the 
posted patches yet though.


>
> 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,

It's a TDPARAMS setting


-Andi