LXR linux/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
  14static 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
  30void 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
  67