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
45extern int __sysctl(int *, u_int, void *, size_t *, void *, size_t);
46
47static size_t
48arnd_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 */
74static int
75getentropy_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
102int
103getentropy(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