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>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index aa47be71df4c..397ee05132a3 100644
--- 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.