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

[Ard Biesheuvel]

On Mon, 8 Feb 2021 at 15:32, Will Deacon <will@xxxxxxxxxx> 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.