| Message ID | 058618cc56ec6611171427ad7205e37e377aa8d4.1439738240.git.len.brown@intel.com (mailing list archive) |
|---|---|
| State | Not Applicable, archived |
| Headers | show |
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 >
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 }