linux-kernel - Re: [PATCH 1/1] x86: replace RDRAND forced-reseed with simple sanity check

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Message-ID: <20150805100728.GA572@gmail.com>
Date:	Wed, 5 Aug 2015 12:07:28 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Len Brown <lenb@...nel.org>
Cc:	x86@...nel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 1/1] x86: replace RDRAND forced-reseed with simple sanity
 check


* Len Brown <lenb@...nel.org> wrote:

>  void x86_init_rdrand(struct cpuinfo_x86 *c)
>  {
>  #ifdef CONFIG_ARCH_RANDOM
>  	unsigned long tmp;
> -	int i, count, ok;
> +	int i;
>  
>  	if (!cpu_has(c, X86_FEATURE_RDRAND))
> -		return;		/* Nothing to do */
> +		return;
>  
> -	for (count = i = 0; i < RESEED_LOOP; i++) {
> -		ok = rdrand_long(&tmp);
> -		if (ok)
> -			count++;
> +	for (i = 0; i < SANITY_CHECK_LOOPS; i++) {
> +		if (!rdrand_long(&tmp)) {
> +			clear_cpu_cap(c, X86_FEATURE_RDRAND);
> +			return;

So here we should emit a printk_once() warning that something's fishy, instead of 
silently disabling a CPU feature.

Thanks,

	Ingo
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