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

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Date:	Fri, 31 Jul 2015 17:06:52 +0800
From:	joeyli <jlee@...e.com>
To:	Pavel Machek <pavel@....cz>
Cc:	"Lee, Chun-Yi" <joeyli.kernel@...il.com>,
	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>
Subject: Re: [RFC PATCH 04/16] x86/efi: Generating random number in EFI stub

Hi Pavel, 

Thanks for your review.

On Tue, Jul 28, 2015 at 02:01:12PM +0200, Pavel Machek wrote:
> 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?
>

I didn't see similar definition in header.
 
> > +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?

I will try to simplify it.

> 
> > +	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.
>

I will try to avoid call cpuid many times.


Thanks a lot!
Joey Lee
 
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