Date:   Fri, 12 Nov 2021 21:53:28 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Sean Christopherson <seanjc@google.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Peter Gonda <pgonda@google.com>,
        Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
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On Fri, Nov 12, 2021 at 08:37:59PM +0000, Sean Christopherson wrote:
> Let userspace decide what is mapped shared and what is mapped private. 

With "userspace", you mean the *host* userspace?

> The kernel and KVM provide the APIs/infrastructure to do the actual
> conversions in a thread-safe fashion and also to enforce the current
> state, but userspace is the control plane.
>
> It would require non-trivial changes in userspace if there are multiple processes
> accessing guest memory, e.g. Peter's networking daemon example, but it _is_ fully
> solvable.  The exit to userspace means all three components (guest, kernel, 
> and userspace) have full knowledge of what is shared and what is private.  There
> is zero ambiguity:
> 
>   - if userspace accesses guest private memory, it gets SIGSEGV or whatever.  

That SIGSEGV is generated by the host kernel, I presume, after it checks
whether the memory belongs to the guest?

>   - if kernel accesses guest private memory, it does BUG/panic/oops[*]

If *it* is the host kernel, then you probably shouldn't do that -
otherwise you just killed the host kernel on which all those guests are
running.

>   - if guest accesses memory with the incorrect C/SHARED-bit, it gets killed.

Yah, that's the easy one.

> This is the direction KVM TDX support is headed, though it's obviously still a WIP.
> 
> And ideally, to avoid implicit conversions at any level, hardware vendors' ABIs
> define that:
> 
>   a) All convertible memory, i.e. RAM, starts as private.
>   b) Conversions between private and shared must be done via explicit hypercall.

I like the explicit nature of this but devil's in the detail and I'm no
virt guy...

> Without (b), userspace and thus KVM have to treat guest accesses to the incorrect
> type as implicit conversions.
> 
> [*] Sadly, fully preventing kernel access to guest private is not possible with
>     TDX, especially if the direct map is left intact.  But maybe in the future
>     TDX will signal a fault instead of poisoning memory and leaving a #MC mine.

Yah, the #MC thing sounds like someone didn't think things through. ;-\

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette