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

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Message-ID: <3affa4a0-c930-45d3-927c-c81b38920c53@redhat.com>
Date: Wed, 10 Jul 2024 23:56:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Patrick Roy <roypat@...zon.co.uk>, Fuad Tabba <tabba@...gle.com>,
 pbonzini@...hat.com, akpm@...ux-foundation.org, dwmw@...zon.co.uk,
 rppt@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
 willy@...radead.org, graf@...zon.com, derekmn@...zon.com,
 kalyazin@...zon.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, dmatlack@...gle.com, chao.p.peng@...ux.intel.com,
 xmarcalx@...zon.co.uk, James Gowans <jgowans@...zon.com>
Subject: Re: [RFC PATCH 8/8] kvm: gmem: Allow restricted userspace mappings

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