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#include <linux/printk.h>
10
11#include <asm/processor.h>
12#include <asm/archrandom.h>
13#include <asm/sections.h>
14
15/*
16 * RDRAND has Built-In-Self-Test (BIST) that runs on every invocation.
17 * Run the instruction a few times as a sanity check. Also make sure
18 * it's not outputting the same value over and over, which has happened
19 * as a result of past CPU bugs.
20 *
21 * If it fails, it is simple to disable RDRAND and RDSEED here.
22 */
23
24void x86_init_rdrand(struct cpuinfo_x86 *c)
25{
26 enum { SAMPLES = 8, MIN_CHANGE = 5 };
27 unsigned long sample, prev;
28 bool failure = false;
29 size_t i, changed;
30
31 if (!cpu_has(c, X86_FEATURE_RDRAND))
32 return;
33
34 for (changed = 0, i = 0; i < SAMPLES; ++i) {
35 if (!rdrand_long(v: &sample)) {
36 failure = true;
37 break;
38 }
39 changed += i && sample != prev;
40 prev = sample;
41 }
42 if (changed < MIN_CHANGE)
43 failure = true;
44
45 if (failure) {
46 clear_cpu_cap(c, X86_FEATURE_RDRAND);
47 clear_cpu_cap(c, X86_FEATURE_RDSEED);
48 pr_emerg("RDRAND is not reliable on this platform; disabling.\n");
49 }
50}
51