xref: /openbmc/linux/arch/x86/kernel/cpu/rdrand.c (revision 0d3b051a)
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 static 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
30 void 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