linux-kernel - RE: [PATCH v3 00/14] arm64: Support for running as a guest in Arm CCA

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Date: Fri, 7 Jun 2024 01:38:15 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Steven Price <steven.price@....com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>
CC: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
	Will Deacon <will@...nel.org>, James Morse <james.morse@....com>, Oliver
 Upton <oliver.upton@...ux.dev>, Suzuki K Poulose <suzuki.poulose@....com>,
	Zenghui Yu <yuzenghui@...wei.com>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.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" <linux-coco@...ts.linux.dev>, Ganapatrao
 Kulkarni <gankulkarni@...amperecomputing.com>
Subject: RE: [PATCH v3 00/14] arm64: Support for running as a guest in Arm CCA

From: Steven Price <steven.price@....com> Sent: Wednesday, June 5, 2024 2:30 AM
> 
> This series adds support for running Linux in a protected VM under the
> Arm Confidential Compute Architecture (CCA). This has been updated
> following the feedback from the v2 posting[1]. Thanks for the feedback!
> Individual patches have a change log for v3.
> 
> The biggest change from v2 is fixing set_memory_{en,de}crypted() to
> perform a break-before-make sequence. Note that only the virtual address
> supplied is flipped between shared and protected, so if e.g. a vmalloc()
> address is passed the linear map will still point to the (now invalid)
> previous IPA. Attempts to access the wrong address may trigger a
> Synchronous External Abort. However any code which attempts to access
> the 'encrypted' alias after set_memory_decrypted() is already likely to
> be broken on platforms that implement memory encryption, so I don't
> expect problems.

In the case of a vmalloc() address, load_unaligned_zeropad() could still
make an access to the underlying pages through the linear address. In
CoCo guests on x86, both the vmalloc PTE and the linear map PTE are
flipped, so the load_unaligned_zeropad() problem can occur only during
the transition between decrypted and encrypted. But even then, the
exception handlers have code to fixup this case and allow everything to
proceed normally.

I haven't looked at the code in your patches, but do you handle that case,
or somehow prevent it?

Thanks,

Michael