x86, rdrand: When nordrand is specified, disable RDSEED as well

One can logically expect that when the user has specified "nordrand",
the user doesn't want any use of the CPU random number generator,
neither RDRAND nor RDSEED, so disable both.

Reported-by: Stephan Mueller <smueller@chronox.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Link: http://lkml.kernel.org/r/21542339.0lFnPSyGRS@myon.chronox.de
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/kernel-parameters.txt

@@ -2218,10 +2218,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	noreplace-smp	[X86-32,SMP] Don't replace SMP instructions
 			with UP alternatives
 
-	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.
+	nordrand	[X86] Disable kernel use of the RDRAND and
+			RDSEED instructions even if they are supported
+			by the processor.  RDRAND and RDSEED are still
+			available to user space applications.
 
 	noresume	[SWSUSP] Disables resume and restores original swap
 			space.

arch/x86/kernel/cpu/rdrand.c

@@ -27,6 +27,7 @@
 static int __init x86_rdrand_setup(char *s)
 {
 	setup_clear_cpu_cap(X86_FEATURE_RDRAND);
+	setup_clear_cpu_cap(X86_FEATURE_RDSEED);
 	return 1;
 }
 __setup("nordrand", x86_rdrand_setup);