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

 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/*
15 * RDRAND has Built-In-Self-Test (BIST) that runs on every invocation.
16 * Run the instruction a few times as a sanity check. Also make sure
17 * it's not outputting the same value over and over, which has happened
18 * as a result of past CPU bugs.
19 *
20 * If it fails, it is simple to disable RDRAND and RDSEED here.
21 */
 
22
 
23void x86_init_rdrand(struct cpuinfo_x86 *c)
24{
25	enum { SAMPLES = 8, MIN_CHANGE = 5 };
26	unsigned long sample, prev;
27	bool failure = false;
28	size_t i, changed;
29
30	if (!cpu_has(c, X86_FEATURE_RDRAND))
31		return;
32
33	for (changed = 0, i = 0; i < SAMPLES; ++i) {
34		if (!rdrand_long(&sample)) {
35			failure = true;
36			break;
 
37		}
38		changed += i && sample != prev;
39		prev = sample;
40	}
41	if (changed < MIN_CHANGE)
42		failure = true;
43
44	if (failure) {
45		clear_cpu_cap(c, X86_FEATURE_RDRAND);
46		clear_cpu_cap(c, X86_FEATURE_RDSEED);
47		pr_emerg("RDRAND is not reliable on this platform; disabling.\n");
 
 
 
 
 
 
 
 
48	}
 
 
 
 
 
49}