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}