Re: [PATCH v8 2/3] arm64: random: Add data to pool from setup_arch()

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Jan 07, 2020 at 06:35:39PM +0000, Mark Brown wrote:
> Since the arm64 ARCH_RANDOM implementation is not available until
> cpufeature has determined the system capabilities it can't be used by
> the generic random code to initialize the entropy pool for early use.
> Instead explicitly add some data to the pool from setup_arch() if the
> boot CPU supports v8.5-RNG, this is the point recommended by the generic
> code.

This structure looks good to me.

> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/include/asm/archrandom.h | 24 ++++++++++++++++++++++++
>  arch/arm64/kernel/setup.c           |  2 ++
>  2 files changed, 26 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
> index 364177954bef..1d9b94fa9c45 100644
> --- a/arch/arm64/include/asm/archrandom.h
> +++ b/arch/arm64/include/asm/archrandom.h
> @@ -59,9 +59,33 @@ static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
>  	return ok;
>  }
>  
> +/*
> + * Our ARCH_RANDOM implementation does not function until relatively
> + * late in the boot when cpufeature has detertmined system
> + * capabilities so the core code can't use arch_get_random*() to
> + * initialize, instead we call this function to inject data from
> + * setup_arch() if the boot CPU supports v8.5-RNG.
> + */
> +static inline void arm64_add_early_rndr_entropy(void)

Can we mark this as __init so that people aren't tempted to use it at
runtime? I realsie it's inline, but it's at least something sparse can
check.

> +{
> +	unsigned long val;
> +	int i;
> +
> +	/* Open code as we run prior to the first call to cpufeature. */
> +	val = read_sysreg_s(SYS_ID_AA64ISAR0_EL1);
> +	if (!((val >> ID_AA64ISAR0_RNDR_SHIFT) & 0xf))
> +		return;
> +
> +	/* Add multiple values to mirror the generic code. */
> +	for (i = 0; i < 16; i++)
> +		if (__arm64_rndr(&val))
> +			add_device_randomness(&val, sizeof(val));
> +}

Given the next patch also needs to check the I reckon it is worth
factoring the ID register check into a helper:

/* Used on the boot CPU before the CPU feature framework is up */
static inline bool __init __early_cpu_has_rndr(void)
{
	unsigned long ftr = read_sysreg_s(SYS_ID_AA64ISAR0_EL1);
	return (ftr >> ID_AA64ISAR0_RNDR_SHIFT) & 0xf;
}

> +
>  #else
>  
>  static inline bool __arm64_rndr(unsigned long *v) { return false; }
> +static inline void arm64_add_early_rndr_entropy(void) { }

... with a stub here:

static inline bool __init __early_cpu_has_rndr(void) { return false; }

... so the KASLR code can just check __early_cpu_has_rndr().

Thanks,
Mark.

>  
>  #endif /* CONFIG_ARCH_RANDOM */
>  #endif /* _ASM_ARCHRANDOM_H */
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 56f664561754..170842965a32 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -344,6 +344,8 @@ void __init setup_arch(char **cmdline_p)
>  	/* Init percpu seeds for random tags after cpus are set up. */
>  	kasan_init_tags();
>  
> +	arm64_add_early_rndr_entropy();
> +
>  #ifdef CONFIG_ARM64_SW_TTBR0_PAN
>  	/*
>  	 * Make sure init_thread_info.ttbr0 always generates translation
> -- 
> 2.20.1
> 

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