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SubjectRe: [PATCH v2] x86: replace RDRAND forced-reseed with simple sanity check
From
Ingo, PeterA,

ping.

Please let me know if you require a revision to this patch,
or if you would like me to re-send it.

thanks,
-Len


On Sun, Aug 16, 2015 at 11:20 AM, Len Brown <lenb@kernel.org> wrote:
> From: Len Brown <len.brown@intel.com>
>
> x86_init_rdrand() was added with 2 goals:
>
> 1. Sanity check that the built-in-self-test circuit on the Digital
> Random Number Generator (DRNG) is not complaining. As RDRAND
> HW self-checks on every invocation, this goal is achieved
> by simply invoking RDRAND and checking its return code.
>
> 2. Force a full re-seed of the random number generator.
> This was done out of paranoia to benefit the most un-sophisticated
> DRNG implementation conceivable in the architecture,
> an implementation that does not exist, and unlikely ever will.
> This worst-case full-re-seed is achieved by invoking
> a 64-bit RDRAND 8192 times.
>
> Unfortunately, this worst-case re-seed costs O(1,000us).
> Magnifying this cost, it is done from identify_cpu(), which is the
> synchronous critical path to bring a processor on-line -- repeated
> for every logical processor in the system at boot and resume from S3.
>
> As it is very expensive, and of highly dubious value,
> we delete the worst-case re-seed from the kernel.
>
> We keep the 1st goal -- sanity check the hardware,
> and mark it absent if it complains.
>
> This change reduces the cost of x86_init_rdrand() by a factor of 1,000x,
> to O(1us) from O(1,000us).
>
> Signed-off-by: Len Brown <len.brown@intel.com>
> ---
> v2: add printk_once() on disabling rdrand, per feedback from Ingo Molnar
> ---
> arch/x86/kernel/cpu/rdrand.c | 25 ++++++++++++-------------
> 1 file changed, 12 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c
> index 136ac74..819d949 100644
> --- a/arch/x86/kernel/cpu/rdrand.c
> +++ b/arch/x86/kernel/cpu/rdrand.c
> @@ -33,28 +33,27 @@ static int __init x86_rdrand_setup(char *s)
> __setup("nordrand", x86_rdrand_setup);
>
> /*
> - * Force a reseed cycle; we are architecturally guaranteed a reseed
> - * after no more than 512 128-bit chunks of random data. This also
> - * acts as a test of the CPU capability.
> + * RDRAND has Built-In-Self-Test (BIST) that runs on every invocation.
> + * Run the instruction a few times as a sanity check.
> + * If it fails, it is simple to disable RDRAND here.
> */
> -#define RESEED_LOOP ((512*128)/sizeof(unsigned long))
> +#define SANITY_CHECK_LOOPS 8
>
> 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);
> + printk_once(KERN_WARNING "rdrand: disabled\n");
> + return;
> + }
> }
> -
> - if (count != RESEED_LOOP)
> - clear_cpu_cap(c, X86_FEATURE_RDRAND);
> #endif
> }
> --
> 2.5.0.330.g130be8e
>



--
Len Brown, Intel Open Source Technology Center


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