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 */
45 CTASSERT(EWOULDBLOCK == EAGAIN);
46
47 static int
kern_getrandom(struct thread * td,void * user_buf,size_t buflen,unsigned int flags)48 kern_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_
116 struct getrandom_args {
117 void *buf;
118 size_t buflen;
119 unsigned int flags;
120 };
121 #endif
122
123 int
sys_getrandom(struct thread * td,struct getrandom_args * uap)124 sys_getrandom(struct thread *td, struct getrandom_args *uap)
125 {
126 return (kern_getrandom(td, uap->buf, uap->buflen, uap->flags));
127 }
128