linux-kernel - Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

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Date:   Mon, 15 Nov 2021 10:18:16 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     brijesh.singh@....com, Dave Hansen <dave.hansen@...el.com>,
        Peter Gonda <pgonda@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support


On 11/12/21 2:37 PM, Sean Christopherson wrote:
> On Fri, Nov 12, 2021, Borislav Petkov wrote:
>> On Fri, Nov 12, 2021 at 07:48:17PM +0000, Sean Christopherson wrote:
>>> Yes, but IMO inducing a fault in the guest because of _host_ bug is wrong.
>>

In the automatic change proposal, both the the host and a guest bug will 
cause a guest to get the #VC and then the guest can decide whether it 
wants to proceed or terminate. If it chooses to move, it can poison the 
page and log it for future examination.

>> What do you suggest instead?
> 
> Let userspace decide what is mapped shared and what is mapped private.  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.
>    - if kernel accesses guest private memory, it does BUG/panic/oops[*]
>    - if guest accesses memory with the incorrect C/SHARED-bit, it gets killed.
> 
> This is the direction KVM TDX support is headed, though it's obviously still a WIP.
> 

Just curious, in this approach, how do you propose handling the host 
kexec/kdump? If a kexec/kdump occurs while the VM is still active, the 
new kernel will encounter the #PF (RMP violation) because some pages are 
still marked 'private' in the RMP table.



> 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.
> 
> 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.
> 

thanks