Re: [PATCH v3 06/16] arm64: enable ptrauth earlier

From: Catalin Marinas <catalin.marinas@arm.com>
To: Amit Kachhap <amit.kachhap@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Kees Cook <keescook@chromium.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Richard Henderson <richard.henderson@linaro.org>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	James Morse <james.morse@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
	Mark Brown <Mark.Brown@arm.com>,
	Vincenzo Frascino <Vincenzo.Frascino@arm.com>,
	Will Deacon <will@kernel.org>, Dave Martin <Dave.Martin@arm.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 06/16] arm64: enable ptrauth earlier
Date: Fri, 17 Jan 2020 12:00:50 +0000	[thread overview]
Message-ID: <20200117120050.GI26090@arrakis.emea.arm.com> (raw)
In-Reply-To: <2d14a778-5fdf-7735-6fb1-57de3aa3e275@arm.com>

On Fri, Jan 17, 2020 at 04:13:06PM +0530, Amit Kachhap wrote:
> On 1/16/20 9:54 PM, Catalin Marinas wrote:
> > On Mon, Dec 16, 2019 at 02:17:08PM +0530, Amit Daniel Kachhap wrote:
> > > diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> > > index 5aaf1bb..c59c28f 100644
> > > --- a/arch/arm64/kernel/head.S
> > > +++ b/arch/arm64/kernel/head.S
[...]
> > > +ENTRY(__cpu_secondary_checkptrauth)
> > > +#ifdef CONFIG_ARM64_PTR_AUTH
> > > +	/* Check if the CPU supports ptrauth */
> > > +	mrs	x2, id_aa64isar1_el1
> > > +	ubfx	x2, x2, #ID_AA64ISAR1_APA_SHIFT, #8
> > > +	cbnz	x2, 1f
> > > +alternative_if ARM64_HAS_ADDRESS_AUTH
> > > +	mov	x3, 1
> > > +alternative_else
> > > +	mov	x3, 0
> > > +alternative_endif
> > > +	cbz	x3, 1f
> > > +	/* Park the mismatched secondary CPU */
> > > +	early_park_cpu CPU_STUCK_REASON_NO_PTRAUTH
> > > +#endif
> > > +1:	ret
> > > +ENDPROC(__cpu_secondary_checkptrauth)
> > 
> > Do we actually need to park secondary CPUs early? Let's say a secondary
> > CPU doesn't have PAC, __cpu_setup won't set the corresponding SCTLR_EL1
> > bits and the instructions are NOPs. Wouldn't the cpufeature framework
> > park it later anyway?
> 
> In the current cpufeature framework, such missing cpufeature in
> secondary cpu will lead to kernel panic (inside check_early_cpufeatures)
> and not cpu offline. However Kristina in her RFC V2 [1] added such
> feature to park it.

I remember discussing how to avoid the kernel panic with her at the
time.

> Later for moving the enabling ptrauth to assembly this work got dropped.
> Suzuki provided the template code for doing that [2].
> 
> Later James suggested to do this like existing
> __cpu_secondary_check52bitva which parks the secondary cpu very early
> and also to save wasted cpu cycles [3].

I don't really care about a few cycles lost during boot.

> So your question is still valid that it can be done in cpufeature. Let
> me know your opinion that which one is better.

My preference is for Kristina's approach. The 52-bit VA is slightly
different (as is VHE) as we cannot guarantee the secondary CPU to even
reach the CPU framework. With PAC, I don't see why it would fail
reaching the C code, so I'd prefer a more readable C implementation than
the assembler one.

Anyway, I'm open to counterarguments here.

-- 
Catalin

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