[PATCH v2] x86: replace RDRAND forced-reseed with simple sanity check

[Len Brown]

From: Len Brown <len.brown@xxxxxxxxx>

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@xxxxxxxxx>
---
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

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