linux-kernel - Re: [RFC PATCH 04/16] x86/efi: Generating random number in EFI stub

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Date:	Tue, 28 Jul 2015 14:01:12 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Lee, Chun-Yi" <joeyli.kernel@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
	linux-pm@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	Len Brown <len.brown@...el.com>,
	Josh Boyer <jwboyer@...hat.com>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Matt Fleming <matt.fleming@...el.com>,
	Jiri Kosina <jkosina@...e.cz>,
	"H. Peter Anvin" <hpa@...or.com>, "Lee, Chun-Yi" <jlee@...e.com>
Subject: Re: [RFC PATCH 04/16] x86/efi: Generating random number in EFI stub

Hi!

> This patch adds the codes for generating random number array as the
> HMAC key that will used by later EFI stub codes.
> 
> The original codes in efi_random copied from aslr and add the codes
> to accept input entropy and EFI debugging. In later patch will add
> the codes to get random number by EFI protocol. The separate codes
> can avoid impacting aslr function.
> 
> Signed-off-by: Lee, Chun-Yi <jlee@...e.com>

> +#define X86_FEATURE_EDX_TSC	(1 << 4)
> +#define X86_FEATURE_ECX_RDRAND	(1 << 30)

Can you pull it from existing includes somewhere?

> +static bool rdrand_feature(void)
> +{
> +	return (cpuid_ecx(0x1) & X86_FEATURE_ECX_RDRAND);
> +}
> +
> +static bool rdtsc_feature(void)
> +{
> +	return (cpuid_edx(0x1) & X86_FEATURE_EDX_TSC);
> +}

Are these helpers neccessary?

> +	if (rdrand_feature()) {
> +		efi_printk(sys_table, " RDRAND");
> +		if (rdrand_long(&raw)) {
> +			random ^= raw;
> +			use_i8254 = false;
> +		}
> +	}
> +
> +	if (rdtsc_feature()) {
> +		efi_printk(sys_table, " RDTSC");
> +		rdtscll(raw);
> +
> +		random ^= raw;
> +		use_i8254 = false;
> +	}

You'll do two (expensive) cpuids calls here.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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