Re: [PATCH v1 2/9] KVM: x86/mmu: Add support for prewrite page tracking

["Mickaël Salaün" <mic@digikod.net>]

On 05/05/2023 18:28, Sean Christopherson wrote:
> On Fri, May 05, 2023, Micka�l Sala�n wrote:
>> diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
>> index eb186bc57f6a..a7fb4ff888e6 100644
>> --- a/arch/x86/include/asm/kvm_page_track.h
>> +++ b/arch/x86/include/asm/kvm_page_track.h
>> @@ -3,6 +3,7 @@
>>   #define _ASM_X86_KVM_PAGE_TRACK_H
>>   
>>   enum kvm_page_track_mode {
>> +	KVM_PAGE_TRACK_PREWRITE,
> 
> Heh, just when I decide to finally kill off support for multiple modes[1] :-)
> 
> My assessment from that changelog still holds true for this case:
> 
>    Drop "support" for multiple page-track modes, as there is no evidence
>    that array-based and refcounted metadata is the optimal solution for
>    other modes, nor is there any evidence that other use cases, e.g. for
>    access-tracking, will be a good fit for the page-track machinery in
>    general.
>    
>    E.g. one potential use case of access-tracking would be to prevent guest
>    access to poisoned memory (from the guest's perspective).  In that case,
>    the number of poisoned pages is likely to be a very small percentage of
>    the guest memory, and there is no need to reference count the number of
>    access-tracking users, i.e. expanding gfn_track[] for a new mode would be
>    grossly inefficient.  And for poisoned memory, host userspace would also
>    likely want to trap accesses, e.g. to inject #MC into the guest, and that
>    isn't currently supported by the page-track framework.
>    
>    A better alternative for that poisoned page use case is likely a
>    variation of the proposed per-gfn attributes overlay (linked), which
>    would allow efficiently tracking the sparse set of poisoned pages, and by
>    default would exit to userspace on access.
> 
> Of particular relevance:
> 
>    - Using the page-track machinery is inefficient because the guest is likely
>      going to write-protect a minority of its memory.  And this
> 
>        select KVM_EXTERNAL_WRITE_TRACKING if KVM
> 
>      is particularly nasty because simply enabling HEKI in the Kconfig will cause
>      KVM to allocate rmaps and gfn tracking.
> 
>    - There's no need to reference count the protection, i.e. 15 of the 16 bits of
>      gfn_track are dead weight.
> 
>    - As proposed, adding a second "mode" would double the cost of gfn tracking.
> 
>    - Tying the protections to the memslots will create an impossible-to-maintain
>      ABI because the protections will be lost if the owning memslot is deleted and
>      recreated.
> 
>    - The page-track framework provides incomplete protection and will lead to an
>      ongoing game of whack-a-mole, e.g. this patch catches the obvious cases by
>      adding calls to kvm_page_track_prewrite(), but misses things like kvm_vcpu_map().
> 
>    - The scaling and maintenance issues will only get worse if/when someone tries
>      to support dropping read and/or execute permissions, e.g. for execute-only.
> 
>    - The code is x86-only, and is likely to stay that way for the foreseeable
>      future.
> 
> The proposed alternative is to piggyback the memory attributes implementation[2]
> that is being added (if all goes according to plan) for confidential VMs.  This
> use case (dropping permissions) came up not too long ago[3], which is why I have
> a ready-made answer).
> 
> I have no doubt that we'll need to solve performance and scaling issues with the
> memory attributes implementation, e.g. to utilize xarray multi-range support
> instead of storing information on a per-4KiB-page basis, but AFAICT, the core
> idea is sound.  And a very big positive from a maintenance perspective is that
> any optimizations, fixes, etc. for one use case (CoCo vs. hardening) should also
> benefit the other use case.
> 
> [1] https://lore.kernel.org/all/20230311002258.852397-22-seanjc@google.com
> [2] https://lore.kernel.org/all/Y2WB48kD0J4VGynX@google.com
> [3] https://lore.kernel.org/all/Y1a1i9vbJ%2FpVmV9r@google.com

I agree, I used this mechanism because it was easier at first to rely on 
a previous work, but while I was working on the MBEC support, I realized 
that it's not the optimal way to do it.

I was thinking about using a new special EPT bit similar to 
EPT_SPTE_HOST_WRITABLE, but it may not be portable though. What do you 
think?