arch/x86/kernel/cpu/rdrand.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * This file is part of the Linux kernel.
   4  *
   5  * Copyright (c) 2011, Intel Corporation
   6  * Authors: Fenghua Yu <fenghua.yu@intel.com>,
   7  *          H. Peter Anvin <hpa@linux.intel.com>
   8  */
   9
  10 #include <asm/processor.h>
  11 #include <asm/archrandom.h>
  12 #include <asm/sections.h>
  13
  14 static int __init x86_rdrand_setup(char *s)
  15 {
  16         setup_clear_cpu_cap(X86_FEATURE_RDRAND);
  17         setup_clear_cpu_cap(X86_FEATURE_RDSEED);
  18         return 1;
  19 }
  20 __setup("nordrand", x86_rdrand_setup);
  21
  22 /*
  23  * RDRAND has Built-In-Self-Test (BIST) that runs on every invocation.
  24  * Run the instruction a few times as a sanity check.
  25  * If it fails, it is simple to disable RDRAND here.
  26  */
  27 #define SANITY_CHECK_LOOPS 8
  28
  29 #ifdef CONFIG_ARCH_RANDOM
  30 void x86_init_rdrand(struct cpuinfo_x86 *c)
  31 {
  32         unsigned int changed = 0;
  33         unsigned long tmp, prev;
  34         int i;
  35
  36         if (!cpu_has(c, X86_FEATURE_RDRAND))
  37                 return;
  38
  39         for (i = 0; i < SANITY_CHECK_LOOPS; i++) {
  40                 if (!rdrand_long(&tmp)) {
  41                         clear_cpu_cap(c, X86_FEATURE_RDRAND);
  42                         pr_warn_once("rdrand: disabled\n");
  43                         return;
  44                 }
  45         }
  46
  47         /*
  48          * Stupid sanity-check whether RDRAND does *actually* generate
  49          * some at least random-looking data.
  50          */
  51         prev = tmp;
  52         for (i = 0; i < SANITY_CHECK_LOOPS; i++) {
  53                 if (rdrand_long(&tmp)) {
  54                         if (prev != tmp)
  55                                 changed++;
  56
  57                         prev = tmp;
  58                 }
  59         }
  60
  61         if (WARN_ON_ONCE(!changed))
  62                 pr_emerg(
  63 "RDRAND gives funky smelling output, might consider not using it by booting with \"nordrand\"");
  64
  65 }
  66 #endif