linux-kernel - Re: [PATCH v2 09/14] arm64: Enable memory encrypt for Realms

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Message-ID: <ZkW6UgrwJT6G9UN-@arm.com>
Date: Thu, 16 May 2024 08:48:34 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Suzuki K Poulose <suzuki.poulose@....com>
Cc: Steven Price <steven.price@....com>, kvm@...r.kernel.org,
	kvmarm@...ts.linux.dev, Marc Zyngier <maz@...nel.org>,
	Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Zenghui Yu <yuzenghui@...wei.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Joey Gouly <joey.gouly@....com>,
	Alexandru Elisei <alexandru.elisei@....com>,
	Christoffer Dall <christoffer.dall@....com>,
	Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
	Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Subject: Re: [PATCH v2 09/14] arm64: Enable memory encrypt for Realms

On Wed, May 15, 2024 at 11:47:02AM +0100, Suzuki K Poulose wrote:
> On 14/05/2024 19:00, Catalin Marinas wrote:
> > On Fri, Apr 12, 2024 at 09:42:08AM +0100, Steven Price wrote:
> > Can someone summarise what the point of this protection bit is? The IPA
> > memory is marked as protected/unprotected already via the RSI call and
> > presumably the RMM disables/permits sharing with a non-secure hypervisor
> > accordingly irrespective of which alias the realm guest has the linear
> > mapping mapped to. What does it do with the top bit of the IPA? Is it
> > that the RMM will prevent (via Stage 2) access if the IPA does not match
> > the requested protection? IOW, it unmaps one or the other at Stage 2?
> 
> The Realm's IPA space is split in half. The lower half is "protected"
> and all pages backing the "protected" IPA is in the Realm world and
> thus cannot be shared with the hypervisor. The upper half IPA is
> "unprotected" (backed by Non-secure PAS pages) and can be accessed
> by the Host/hyp.

What about emulated device I/O where there's no backing RAM at an IPA.
Does it need to have the top bit set?

> The RSI call (RSI_IPA_STATE_SET) doesn't make an IPA unprotected. It
> simply "invalidates" a (protected) IPA to "EMPTY" implying the Realm doesn't
> intend to use the "ipa" as RAM anymore and any access to it from
> the Realm would trigger an SEA into the Realm. The RSI call triggers an exit
> to the host with the information and is a hint to the hypervisor to reclaim
> the page backing the IPA.
> 
> Now, given we need dynamic "sharing" of pages (instead of a dedicated
> set of shared pages), "aliasing" of an IPA gives us shared pages.
> i.e., If OS wants to share a page "x" (protected IPA) with the host,
> we mark that as EMPTY via RSI call and then access the "x" with top-bit
> set (aliasing the IPA x). This fault allows the hyp to map the page backing
> IPA "x" as "unprotected" at ALIAS(x) address.

Does the RMM sanity-checks that the NS hyp mappings are protected or
unprotected depending on the IPA range?

I assume that's also the case if the NS hyp is the first one to access a
page before the realm (e.g. inbound virtio transfer; no page allocated
yet because of a realm access).

-- 
Catalin