x86, random: Verify RDRAND functionality and allow it to be disabled

If the CPU declares that RDRAND is available, go through a guranteed
reseed sequence, and make sure that it is actually working (producing
data.)   If it does not, disable the CPU feature flag.

Allow RDRAND to be disabled on the command line (as opposed to at
compile time) for a user who has special requirements with regards to
random numbers.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index aa47be71df4c12ddeb15f7abead764279b20f113..397ee05132a36d1aec542ba9d9e2b67967df8f0a 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1764,6 +1764,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
        noresidual      [PPC] Don't use residual data on PReP machines.
 
+       nordrand        [X86] Disable the direct use of the RDRAND
+                       instruction even if it is supported by the
+                       processor.  RDRAND is still available to user
+                       space applications.
+
        noresume        [SWSUSP] Disables resume and restores original swap
                        space.
 

diff --git a/arch/x86/include/asm/archrandom.h b/arch/x86/include/asm/archrandom.h
index b7b5bc02e361c0d3904df77b29f1ef4a9a6d3061..0d9ec770f2f8770f8e5a320300ea47f3bf39b9bb 100644 (file)
--- a/arch/x86/include/asm/archrandom.h
+++ b/arch/x86/include/asm/archrandom.h
@@ -70,4 +70,6 @@ GET_RANDOM(arch_get_random_int, unsigned int, RDRAND_INT, ASM_NOP3);
 
 #endif  /* CONFIG_ARCH_RANDOM */
 
+extern void x86_init_rdrand(struct cpuinfo_x86 *c);
+
 #endif /* ASM_X86_ARCHRANDOM_H */

diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
index 6042981d0309c492bdfb5032312a8e4f7cb0a5d1..0e3a82a41a6673d3aac217d6320c29242f049a62 100644 (file)
--- a/arch/x86/kernel/cpu/Makefile
+++ b/arch/x86/kernel/cpu/Makefile
@@ -15,6 +15,7 @@ CFLAGS_common.o               := $(nostackp)
 obj-y                  := intel_cacheinfo.o scattered.o topology.o
 obj-y                  += proc.o capflags.o powerflags.o common.o
 obj-y                  += vmware.o hypervisor.o sched.o mshyperv.o
+obj-y                  += rdrand.o
 
 obj-$(CONFIG_X86_32)   += bugs.o
 obj-$(CONFIG_X86_64)   += bugs_64.o

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 22a073d7fbff25df173d42ef710d67da1a70c077..8dbd9294f4f20738bd9094d7d79b8b42a3cb409e 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -15,6 +15,7 @@
 #include <asm/stackprotector.h>
 #include <asm/perf_event.h>
 #include <asm/mmu_context.h>
+#include <asm/archrandom.h>
 #include <asm/hypervisor.h>
 #include <asm/processor.h>
 #include <asm/sections.h>
@@ -857,6 +858,7 @@ static void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
 #endif
 
        init_hypervisor(c);
+       x86_init_rdrand(c);
 
        /*
         * Clear/Set all flags overriden by options, need do it

diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c
new file mode 100644 (file)
index 0000000..feca286
--- /dev/null
+++ b/arch/x86/kernel/cpu/rdrand.c
@@ -0,0 +1,73 @@
+/*
+ * This file is part of the Linux kernel.
+ *
+ * Copyright (c) 2011, Intel Corporation
+ * Authors: Fenghua Yu <fenghua.yu@intel.com>,
+ *          H. Peter Anvin <hpa@linux.intel.com>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin St - Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ */
+
+#include <asm/processor.h>
+#include <asm/archrandom.h>
+#include <asm/sections.h>
+
+static int __init x86_rdrand_setup(char *s)
+{
+       setup_clear_cpu_cap(X86_FEATURE_RDRAND);
+       return 1;
+}
+__setup("nordrand", x86_rdrand_setup);
+
+/* We can't use arch_get_random_long() here since alternatives haven't run */
+static inline int rdrand_long(unsigned long *v)
+{
+       int ok;
+       asm volatile("1: " RDRAND_LONG "\n\t"
+                    "jc 2f\n\t"
+                    "decl %0\n\t"
+                    "jnz 1b\n\t"
+                    "2:"
+                    : "=r" (ok), "=a" (*v)
+                    : "0" (RDRAND_RETRY_LOOPS));
+       return ok;
+}
+
+/*
+ * 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.
+ */
+#define RESEED_LOOP ((512*128)/sizeof(unsigned long))
+
+void __cpuinit x86_init_rdrand(struct cpuinfo_x86 *c)
+{
+#ifdef CONFIG_ARCH_RANDOM
+       unsigned long tmp;
+       int i, count, ok;
+
+       if (!cpu_has(c, X86_FEATURE_RDRAND))
+               return;         /* Nothing to do */
+
+       for (count = i = 0; i < RESEED_LOOP; i++) {
+               ok = rdrand_long(&tmp);
+               if (ok)
+                       count++;
+       }
+
+       if (count != RESEED_LOOP)
+               clear_cpu_cap(c, X86_FEATURE_RDRAND);
+#endif
+}