linux-kernel - Re: [PATCH v7 00/23] arm64: Early CPU feature override, and applications to VHE, BTI and PAuth

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Date:   Mon, 8 Feb 2021 15:40:33 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Marc Zyngier <maz@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        David Brazdil <dbrazdil@...gle.com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Ajay Patil <pajay@....qualcomm.com>,
        Prasad Sodagudi <psodagud@...eaurora.org>,
        Srinivas Ramana <sramana@...eaurora.org>,
        Hector Martin <marcan@...can.st>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v7 00/23] arm64: Early CPU feature override, and
 applications to VHE, BTI and PAuth

On Mon, 8 Feb 2021 at 15:32, Will Deacon <will@...nel.org> wrote:
>
> Hi Marc,
>
> On Mon, Feb 08, 2021 at 09:57:09AM +0000, Marc Zyngier wrote:
> > It recently came to light that there is a need to be able to override
> > some CPU features very early on, before the kernel is fully up and
> > running. The reasons for this range from specific feature support
> > (such as using Protected KVM on VHE HW, which is the main motivation
> > for this work) to errata workaround (a feature is broken on a CPU and
> > needs to be turned off, or rather not enabled).
> >
> > This series tries to offer a limited framework for this kind of
> > problems, by allowing a set of options to be passed on the
> > command-line and altering the feature set that the cpufeature
> > subsystem exposes to the rest of the kernel. Note that this doesn't
> > change anything for code that directly uses the CPU ID registers.
>
> I applied this locally, but I'm seeing consistent boot failure under QEMU when
> KASAN is enabled. I tried sprinkling some __no_sanitize_address annotations
> around (see below) but it didn't help. The culprit appears to be
> early_fdt_map(), but looking a bit more closely, I'm really nervous about the
> way we call into C functions from __primary_switched. Remember -- this code
> runs _twice_ when KASLR is active: before and after the randomization. This
> also means that any memory writes the first time around can be lost due to
> the D-cache invalidation when (re-)creating the kernel page-tables.
>

Not just cache invalidation - BSS gets wiped again as well.

-- 
Ard.