xref: /freebsd/lib/libc/gen/getentropy.c (revision d3d381b2b194b4d24853e92eecef55f262688d1a)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
3  *
4  * Copyright (c) 2018 Conrad Meyer <cem@FreeBSD.org>
5  * All rights reserved.
6  *
7  * Redistribution and use in source and binary forms, with or without
8  * modification, are permitted provided that the following conditions
9  * are met:
10  * 1. Redistributions of source code must retain the above copyright
11  *    notice, this list of conditions and the following disclaimer.
12  * 2. Redistributions in binary form must reproduce the above copyright
13  *    notice, this list of conditions and the following disclaimer in the
14  *    documentation and/or other materials provided with the distribution.
15  *
16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26  * SUCH DAMAGE.
27  */
28 
29 #include <sys/cdefs.h>
30 __FBSDID("$FreeBSD$");
31 
32 #include <sys/param.h>
33 #include <sys/random.h>
34 #include <sys/sysctl.h>
35 
36 #include <errno.h>
37 #include <stdbool.h>
38 #include <stdlib.h>
39 
40 #include "libc_private.h"
41 
42 /* First __FreeBSD_version bump after introduction of getrandom(2) (r331279) */
43 #define GETRANDOM_FIRST 1200061
44 
45 extern int __sysctl(int *, u_int, void *, size_t *, void *, size_t);
46 
47 static size_t
48 arnd_sysctl(u_char *buf, size_t size)
49 {
50 	int mib[2];
51 	size_t len, done;
52 
53 	mib[0] = CTL_KERN;
54 	mib[1] = KERN_ARND;
55 	done = 0;
56 
57 	do {
58 		len = size;
59 		if (__sysctl(mib, 2, buf, &len, NULL, 0) == -1)
60 			return (done);
61 		done += len;
62 		buf += len;
63 		size -= len;
64 	} while (size > 0);
65 
66 	return (done);
67 }
68 
69 /*
70  * If a newer libc is accidentally installed on an older kernel, provide high
71  * quality random data anyway.  The sysctl interface is not as fast and does
72  * not block by itself, but is provided by even very old kernels.
73  */
74 static int
75 getentropy_fallback(void *buf, size_t buflen)
76 {
77 	/*
78 	 * oldp (buf) == NULL has a special meaning for sysctl that results in
79 	 * no EFAULT.  For compatibility with the kernel getrandom(2), detect
80 	 * this case and return the appropriate error.
81 	 */
82 	if (buf == NULL && buflen > 0) {
83 		errno = EFAULT;
84 		return (-1);
85 	}
86 	if (arnd_sysctl(buf, buflen) != buflen) {
87 		if (errno == EFAULT)
88 			return (-1);
89 		/*
90 		 * This cannot happen.  _arc4_sysctl() spins until the random
91 		 * device is seeded and then repeatedly reads until the full
92 		 * request is satisfied.  The only way for this to return a zero
93 		 * byte or short read is if sysctl(2) on the kern.arandom MIB
94 		 * fails.  In this case, exceping the user-provided-a-bogus-
95 		 * buffer EFAULT, give up (like for arc4random(3)'s arc4_stir).
96 		 */
97 		abort();
98 	}
99 	return (0);
100 }
101 
102 int
103 getentropy(void *buf, size_t buflen)
104 {
105 	ssize_t rd;
106 	bool have_getrandom;
107 
108 	if (buflen > 256) {
109 		errno = EIO;
110 		return (-1);
111 	}
112 
113 	have_getrandom = (__getosreldate() >= GETRANDOM_FIRST);
114 
115 	while (buflen > 0) {
116 		if (have_getrandom) {
117 			rd = getrandom(buf, buflen, 0);
118 			if (rd == -1) {
119 				switch (errno) {
120 				case ECAPMODE:
121 					/*
122 					 * Kernel >= r331280 and < r337999
123 					 * will return ECAPMODE when the
124 					 * caller is already in capability
125 					 * mode, fallback to traditional
126 					 * method in this case.
127 					 */
128 					have_getrandom = false;
129 					continue;
130 				case EINTR:
131 					continue;
132 				default:
133 					return (-1);
134 				}
135 			}
136 		} else {
137 			return (getentropy_fallback(buf, buflen));
138 		}
139 
140 		/* This cannot happen. */
141 		if (rd == 0)
142 			abort();
143 
144 		buf = (char *)buf + rd;
145 		buflen -= rd;
146 	}
147 
148 	return (0);
149 }
150