Re: [PATCH v6 09/18] arm64: enable ptrauth earlier

From: Vincenzo Frascino <vincenzo.frascino@arm.com>
To: Amit Daniel Kachhap <amit.kachhap@arm.com>,
	linux-arm-kernel@lists.infradead.org
Cc: Mark Rutland <mark.rutland@arm.com>,
	Kees Cook <keescook@chromium.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Mark Brown <broonie@kernel.org>,
	James Morse <james.morse@arm.com>,
	Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v6 09/18] arm64: enable ptrauth earlier
Date: Tue, 10 Mar 2020 15:45:13 +0000	[thread overview]
Message-ID: <4000de90-6196-364d-dcad-5476d85e18e0@arm.com> (raw)
In-Reply-To: <1583476525-13505-10-git-send-email-amit.kachhap@arm.com>

Hi Amit,

On 3/6/20 6:35 AM, Amit Daniel Kachhap wrote:
> From: Kristina Martsenko <kristina.martsenko@arm.com>
> 
> When the kernel is compiled with pointer auth instructions, the boot CPU
> needs to start using address auth very early, so change the cpucap to
> account for this.
> 
> Pointer auth must be enabled before we call C functions, because it is
> not possible to enter a function with pointer auth disabled and exit it
> with pointer auth enabled. Note, mismatches between architected and
> IMPDEF algorithms will still be caught by the cpufeature framework (the
> separate *_ARCH and *_IMP_DEF cpucaps).
> 
> Note the change in behavior: if the boot CPU has address auth and a
> late CPU does not, then the late CPU is parked by the cpufeature
> framework. Also, if the boot CPU does not have address auth and the late
> CPU has then the late cpu will still boot but with ptrauth feature
> disabled.
> 
> Leave generic authentication as a "system scope" cpucap for now, since
> initially the kernel will only use address authentication.
> 

I can't find in this patch were CPU_STUCK_REASON_NO_PTRAUTH is set. Maybe I am
missing something. Please feel free to correct me if I am wrong.

My expectation is that you should call early_park_cpu to do that if the
secondary does not support PTRAUTH similar to what you did in v2 of this series:

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)

and then check it during the secondary_startup, similar to what happens for
52BIT_VA for example.

In this way "update_early_cpu_boot_status" would update the
CPU_STUCK_REASON_NO_PTRAUTH flag.

[...]

-- 
Regards,
Vincenzo

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