linux-kernel - Re: [PATCH RFC 3/4] mm: guest_memfd: Add option to remove guest private memory from direct map

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Message-ID: <3ea89d7f-fc29-4c80-a123-94673e526ca5@amazon.com>
Date: Thu, 15 Aug 2024 12:08:05 -0700
From: "Manwaring, Derek" <derekmn@...zon.com>
To: David Hildenbrand <david@...hat.com>, Elliot Berman
	<quic_eberman@...cinc.com>, Andrew Morton <akpm@...ux-foundation.org>, "Paolo
 Bonzini" <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>,
	"Fuad Tabba" <tabba@...gle.com>, Patrick Roy <roypat@...zon.co.uk>,
	<qperret@...gle.com>, Ackerley Tng <ackerleytng@...gle.com>
CC: <linux-coco@...ts.linux.dev>, <linux-arm-msm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, <kvm@...r.kernel.org>,
	Alexander Graf <graf@...zon.de>, Moritz Lipp <mlipp@...zon.at>, "Claudio
 Canella" <canellac@...zon.at>
Subject: Re: [PATCH RFC 3/4] mm: guest_memfd: Add option to remove guest
 private memory from direct map

On 2024-08-07 17:16-0700 Derek Manwaring wrote:
> All that said, we're also dependent on hardware not being subject to
> L1TF-style issues for the currently proposed non-CoCo method to be
> effective. We're simply clearing the Present bit while the physmap PTE
> still points to the guest physical page.

I was wrong here. The set_direct_map_invalid_noflush implementation
moves through __change_page_attr and pfn_pte, eventually arriving at
flip_protnone_guard where the PFN is inverted & thus no longer valid for
pages marked not present. So we do benefit from that prior work's extra
protection against L1TF.

Thank you for finding this, Patrick.

Derek