1/*-
2 * SPDX-License-Identifier: BSD-2-Clause
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/param.h>
30#include <sys/errno.h>
31#include <sys/limits.h>
32#include <sys/proc.h>
33#include <sys/random.h>
34#include <sys/sysproto.h>
35#include <sys/systm.h>
36#include <sys/uio.h>
37
38#define GRND_VALIDFLAGS	(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE)
39
40/*
41 * read_random_uio(9) returns EWOULDBLOCK if a nonblocking request would block,
42 * but the Linux API name is EAGAIN.  On FreeBSD, they have the same numeric
43 * value for now.
44 */
45CTASSERT(EWOULDBLOCK == EAGAIN);
46
47static int
48kern_getrandom(struct thread *td, void *user_buf, size_t buflen,
49    unsigned int flags)
50{
51	struct uio auio;
52	struct iovec aiov;
53	int error;
54
55	if ((flags & ~GRND_VALIDFLAGS) != 0)
56		return (EINVAL);
57	if (buflen > IOSIZE_MAX)
58		return (EINVAL);
59
60	/*
61	 * Linux compatibility: We have two choices for handling Linux's
62	 * GRND_INSECURE.
63	 *
64	 * 1. We could ignore it completely (like GRND_RANDOM).  However, this
65	 * might produce the surprising result of GRND_INSECURE requests
66	 * blocking, when the Linux API does not block.
67	 *
68	 * 2. Alternatively, we could treat GRND_INSECURE requests as requests
69	 * for GRND_NONBLOCK.  Here, the surprising result for Linux programs
70	 * is that invocations with unseeded random(4) will produce EAGAIN,
71	 * rather than garbage.
72	 *
73	 * Honoring the flag in the way Linux does seems fraught.  If we
74	 * actually use the output of a random(4) implementation prior to
75	 * seeding, we leak some entropy about the initial seed to attackers.
76	 * This seems unacceptable -- it defeats the purpose of blocking on
77	 * initial seeding.
78	 *
79	 * Secondary to that concern, before seeding we may have arbitrarily
80	 * little entropy collected; producing output from zero or a handful of
81	 * entropy bits does not seem particularly useful to userspace.
82	 *
83	 * If userspace can accept garbage, insecure non-random bytes, they can
84	 * create their own insecure garbage with srandom(time(NULL)) or
85	 * similar.  Asking the kernel to produce it from the secure
86	 * getrandom(2) API seems inane.
87	 *
88	 * We elect to emulate GRND_INSECURE as an alternative spelling of
89	 * GRND_NONBLOCK (2).
90	 */
91	if ((flags & GRND_INSECURE) != 0)
92		flags |= GRND_NONBLOCK;
93
94	if (buflen == 0) {
95		td->td_retval[0] = 0;
96		return (0);
97	}
98
99	aiov.iov_base = user_buf;
100	aiov.iov_len = buflen;
101	auio.uio_iov = &aiov;
102	auio.uio_iovcnt = 1;
103	auio.uio_offset = 0;
104	auio.uio_resid = buflen;
105	auio.uio_segflg = UIO_USERSPACE;
106	auio.uio_rw = UIO_READ;
107	auio.uio_td = td;
108
109	error = read_random_uio(&auio, (flags & GRND_NONBLOCK) != 0);
110	if (error == 0)
111		td->td_retval[0] = buflen - auio.uio_resid;
112	return (error);
113}
114
115#ifndef _SYS_SYSPROTO_H_
116struct getrandom_args {
117	void		*buf;
118	size_t		buflen;
119	unsigned int	flags;
120};
121#endif
122
123int
124sys_getrandom(struct thread *td, struct getrandom_args *uap)
125{
126	return (kern_getrandom(td, uap->buf, uap->buflen, uap->flags));
127}
128