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

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Message-ID: <Zo8C1Rz1eR96gQ1E@google.com>
Date: Wed, 10 Jul 2024 14:53:25 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: David Hildenbrand <david@...hat.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 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.