Commit 0007bccc authored Aug 16, 2015 by Len Brown Committed by Thomas Gleixner Nov 25, 2015

x86: Replace RDRAND forced-reseed with simple sanity check



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>
Link: http://lkml.kernel.org/r/058618cc56ec6611171427ad7205e37e377aa8d4.1439738240.git.len.brown@intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent 679bcea8

arch/x86/kernel/cpu/rdrand.c

+12 −13

Original line number	Diff line number	Diff line
		@@ -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++;
		}

		if (count != RESEED_LOOP)
		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;
		}
		}
		#endif
		}

Admin message