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
x86_init_rdrand(struct cpuinfo_x86 * c)23 void 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 }
50