Re: [PATCH] arm64: Add support for E0PD

From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: Add support for E0PD
Date: Wed, 26 Jun 2019 17:51:03 +0100	[thread overview]
Message-ID: <20190626165102.GE29672@arrakis.emea.arm.com> (raw)
In-Reply-To: <20190626160622.GA5379@sirena.org.uk>

On Wed, Jun 26, 2019 at 05:06:22PM +0100, Mark Brown wrote:
> On Wed, Jun 26, 2019 at 04:04:04PM +0100, Will Deacon wrote:
> > I think you're missing one small thing here: all v8.5 CPUs will have
> > hardware mitigations for meltdown as advertised in the ID registers.
> > However, we still force KPTI on for those CPUs if KASLR is enabled to avoid
> > it being trivially bypassed by looking at fault timings. As you point out,
> > there are two issues with that: (1) the performance impact of KPTI and (2)
> > the incompatibility with statistical profiling. It is these issues which
> > E0PD attempts to address, so whilst I'm ok with enabling it unconditionally
> > as you propose, we should go one step further and avoid enabling KPTI on
> > CPUs with E0PD even if KASLR is enabled.
> 
> I agree, I'm currently working on a patch which will disable KPTI by
> default if we've enabled E0PD - it's a bit of a faff due to how early we
> decide if we're going to use KPTI so it probably needs to be a separate
> patch anyway.

Could we not wire up this check in unmap_kernel_at_el0()? We can look at
this as a more efficient KPTI handled by the hardware.

> > We probably also need to consider the unfortunate situations where E0PD
> > is not supported by all of the CPUs in the system.
> 
> Yes, I've marked it as ARM64_CPUCAP_SYSTEM_FEATURE so it should be safe
> unless all the CPUs that don't support it are late CPUs (in which case
> it'd stop them booting) but it's not ideal as it means we won't use it
> at on mixed systems.  I did debate marking it as _WEAK so that we'd
> enable it on the CPUs that can use it but I worried that that'd be
> potentially misleading with regard to the level of hardening if the
> kernel said it was turning on E0PD.

I think this will become problematic in combination with disabling kpti.
If we decide early that it is meltdown-safe (unmap_kernel_at_el0()
returning false) because the boot CPU supports E0PD, any subsequent CPU
not having E0PD and hence requiring unmap_kernel_at_el0() will not boot.
That's fine by me as long as we have a Kconfig option to disable E0PD
and allow mixed CPU features on some custom SoCs.

-- 
Catalin

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