linux-kernel - Re: [RFC PATCH v3 0/6] Direct Map Removal for guest

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Message-ID: <4aa0ccf4-ebbe-4244-bc85-8bc8dcd14e74@redhat.com>
Date: Thu, 31 Oct 2024 10:50:10 +0100
From: David Hildenbrand <david@...hat.com>
To: Patrick Roy <roypat@...zon.co.uk>, tabba@...gle.com,
 quic_eberman@...cinc.com, seanjc@...gle.com, pbonzini@...hat.com,
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Subject: Re: [RFC PATCH v3 0/6] Direct Map Removal for guest_memfd

On 30.10.24 14:49, Patrick Roy wrote:
> Unmapping virtual machine guest memory from the host kernel's direct map
> is a successful mitigation against Spectre-style transient execution
> issues: If the kernel page tables do not contain entries pointing to
> guest memory, then any attempted speculative read through the direct map
> will necessarily be blocked by the MMU before any observable
> microarchitectural side-effects happen. This means that Spectre-gadgets
> and similar cannot be used to target virtual machine memory. Roughly 60%
> of speculative execution issues fall into this category [1, Table 1].
> 
> This patch series extends guest_memfd with the ability to remove its
> memory from the host kernel's direct map, to be able to attain the above
> protection for KVM guests running inside guest_memfd.
> 
> === Changes to v2 ===
> 
> - Handle direct map removal for physically contiguous pages in arch code
>    (Mike R.)
> - Track the direct map state in guest_memfd itself instead of at the
>    folio level, to prepare for huge pages support (Sean C.)
> - Allow configuring direct map state of not-yet faulted in memory
>    (Vishal A.)
> - Pay attention to alignment in ftrace structs (Steven R.)
> 
> Most significantly, I've reduced the patch series to focus only on
> direct map removal for guest_memfd for now, leaving the whole "how to do
> non-CoCo VMs in guest_memfd" for later. If this separation is
> acceptable, then I think I can drop the RFC tag in the next revision
> (I've mainly kept it here because I'm not entirely sure what to do with
> patches 3 and 4).

Hi,

keeping upcoming "shared and private memory in guest_memfd" in mind, I 
assume the focus would be to only remove the direct map for private memory?

So in the current upstream state, you would only be removing the direct 
map for private memory, currently translating to "encrypted"/"protected" 
memory that is inaccessible either way already.

Correct?

-- 
Cheers,

David / dhildenb