Re: [RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings

David Hildenbrand
2024-07-10T21:56:10Z

On 10.07.24 23:53, Sean Christopherson wrote:
> On Wed, Jul 10, 2024, David Hildenbrand wrote:
>> On 10.07.24 11:51, Patrick Roy wrote:
>>> On 7/9/24 22:13, David Hildenbrand wrote:
>>>> Note that just from staring at this commit, I don't understand the
>>>> motivation *why* we would want to do that.
>>>
>>> Fair - I admittedly didn't get into that as much as I probably should
>>> have. In our usecase, we do not have anything that pKVM would (I think)
>>> call "guest-private" memory. I think our memory can be better described
>>> as guest-owned, but always shared with the VMM (e.g. userspace), but
>>> ideally never shared with the host kernel. This model lets us do a lot
>>> of simplifying assumptions: Things like I/O can be handled in userspace
>>> without the guest explicitly sharing I/O buffers (which is not exactly
>>> what we would want long-term anyway, as sharing in the guest_memfd
>>> context means sharing with the host kernel), we can easily do VM
>>> snapshotting without needing things like TDX's TDH.EXPORT.MEM APIs, etc.
>>
>> Okay, so essentially you would want to use guest_memfd to only contain shard
>> memory and disallow any pinning like for secretmem.
>>
>> If so, I wonder if it wouldn't be better to simply add KVM support to
>> consume *real* secretmem memory? IIRC so far there was only demand to
>> probably remove the directmap of private memory in guest_memfd, not of
>> shared memory.
>
> It's also desirable to remove shared memory from the directmap, e.g. to prevent
> using the directmap in a cross-VM attack.
>
> I don't think we want to allow KVM to consume secretmem. That would require
> letting KVM gup() secretmem, which AIUI defeats the entire purpose of secretmem,
> and I don't think KVM should be special.

I would mean consuming secretmem via the fd, *not* via page tables / gup.

But if we also want to have the option of directmap modifications for
shared memory in guest_memfd, we could make that indeed a guest_memfd
feature.

--
Cheers,

David / dhildenb