linux-kernel - Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

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Date: Thu, 20 Jun 2024 20:11:33 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: David Hildenbrand <david@...hat.com>, Fuad Tabba <tabba@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	John Hubbard <jhubbard@...dia.com>,
	Elliot Berman <quic_eberman@...cinc.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <shuah@...nel.org>, Matthew Wilcox <willy@...radead.org>,
	maz@...nel.org, kvm@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, pbonzini@...hat.com
Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

On Thu, Jun 20, 2024 at 01:30:29PM -0700, Sean Christopherson wrote:
> I.e. except for blatant bugs, e.g. use-after-free, we need to be able to guarantee
> with 100% accuracy that there are no outstanding mappings when converting a page
> from shared=>private.  Crossing our fingers and hoping that short-term GUP will
> have gone away isn't enough.

To be clear it is not crossing fingers. If the page refcount is 0 then
there are no references to that memory anywhere at all. It is 100%
certain.

It may take time to reach zero, but when it does it is safe.

Many things rely on this property, including FSDAX.

> For non-CoCo VMs, I expect we'll want to be much more permissive, but I think
> they'll be a complete non-issue because there is no shared vs. private to worry
> about.  We can simply allow any and all userspace mappings for guest_memfd that is
> attached to a "regular" VM, because a misbehaving userspace only loses whatever
> hardening (or other benefits) was being provided by using guest_memfd.  I.e. the
> kernel and system at-large isn't at risk.

It does seem to me like guest_memfd should really focus on the private
aspect.

If we need normal memfd enhancements of some kind to work better with
KVM then that may be a better option than turning guest_memfd into
memfd.

Jason