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

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

[Len Brown]

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>
---
 arch/x86/kernel/cpu/rdrand.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c
index 136ac74..b86817e 100644
--- a/arch/x86/kernel/cpu/rdrand.c
+++ b/arch/x86/kernel/cpu/rdrand.c
@@ -33,28 +33,26 @@ 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);
+			return;
+		}
 	}
-
-	if (count != RESEED_LOOP)
-		clear_cpu_cap(c, X86_FEATURE_RDRAND);
 #endif
 }
-- 
2.5.0

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

[Pavel Machek]

On Fri 2015-07-31 11:27:39, Len Brown 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.

If we trust built-in-self-test... why do we need to do this at all? We
should check the return value at every call, anyway...

									Pavel

> 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>
> ---
>  arch/x86/kernel/cpu/rdrand.c | 24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c
> index 136ac74..b86817e 100644
> --- a/arch/x86/kernel/cpu/rdrand.c
> +++ b/arch/x86/kernel/cpu/rdrand.c
> @@ -33,28 +33,26 @@ 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);
> +			return;
> +		}
>  	}
> -
> -	if (count != RESEED_LOOP)
> -		clear_cpu_cap(c, X86_FEATURE_RDRAND);
>  #endif
>  }

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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

[Jeff Epler]

On Fri, Jul 31, 2015 at 11:27:39AM -0400, Len Brown wrote:
>  	if (!cpu_has(c, X86_FEATURE_RDRAND))
> -		return;		/* Nothing to do */
> +		return;

Why remove this comment?

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

[Thomas Gleixner]

On Sun, 2 Aug 2015, Jeff Epler wrote:

> On Fri, Jul 31, 2015 at 11:27:39AM -0400, Len Brown wrote:
> >  	if (!cpu_has(c, X86_FEATURE_RDRAND))
> > -		return;		/* Nothing to do */
> > +		return;
> 
> Why remove this comment?

Because it's pointless.
 

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

[Len Brown]

> If we trust built-in-self-test...
> why do we need to do this at all? We
> should check the return value at every call, anyway...

Yes, we do trust built-in-self-test.
Yes, we do check for errors on on every call, not just here in boot.

The sanity check at boot from the kernel allows Linux to disable
the feature, preventing user-space from thrashing trying to use it.

There is also a line of reasoning that if the circuit is going to fail,
chances are that it will fail immediately.
I have no reason to believe that the circuit will fail in the field
either at run-time or at boot-time.  But that line of reasoning
suggests that O(1 usec) to check at boot is a prudent investment --
it is certainly a better investment per time than may of the other
things Linux does.

thanks,
Len Brown, Intel Open Source Technology Center

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

[Ingo Molnar]

* Len Brown <lenb@kernel.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