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authorDenys Vlasenko <vda.linux@googlemail.com>2022-05-02 14:53:14 +0200
committerDenys Vlasenko <vda.linux@googlemail.com>2022-05-02 14:53:14 +0200
commit5ba56e8b95ea84dbd7c0f7adfb9bdb1740480904 (patch)
treedb46c43aaa0a7915ea10f4dcc7a0edd6d8ec97d7 /miscutils
parent1a290f889c5103d867ba1e0715ae730b394a3a12 (diff)
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seedrng: it's not a part of util-linux, move to miscutils
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Diffstat (limited to 'miscutils')
-rw-r--r--miscutils/seedrng.c239
1 files changed, 239 insertions, 0 deletions
diff --git a/miscutils/seedrng.c b/miscutils/seedrng.c
new file mode 100644
index 000000000..8c81835f6
--- /dev/null
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1// SPDX-License-Identifier: GPL-2.0 OR MIT
2/*
3 * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
4 *
5 * SeedRNG is a simple program made for seeding the Linux kernel random number
6 * generator from seed files. It is is useful in light of the fact that the
7 * Linux kernel RNG cannot be initialized from shell scripts, and new seeds
8 * cannot be safely generated from boot time shell scripts either. It should
9 * be run once at init time and once at shutdown time. It can be run at other
10 * times on a timer as well. Whenever it is run, it writes existing seed files
11 * into the RNG pool, and then creates a new seed file. If the RNG is
12 * initialized at the time of creating a new seed file, then that new seed file
13 * is marked as "creditable", which means it can be used to initialize the RNG.
14 * Otherwise, it is marked as "non-creditable", in which case it is still used
15 * to seed the RNG's pool, but will not initialize the RNG. In order to ensure
16 * that entropy only ever stays the same or increases from one seed file to the
17 * next, old seed values are hashed together with new seed values when writing
18 * new seed files.
19 *
20 * This is based on code from <https://git.zx2c4.com/seedrng/about/>.
21 */
22//config:config SEEDRNG
23//config: bool "seedrng (1.3 kb)"
24//config: default y
25//config: help
26//config: Seed the kernel RNG from seed files, meant to be called
27//config: once during startup, once during shutdown, and optionally
28//config: at some periodic interval in between.
29
30//applet:IF_SEEDRNG(APPLET(seedrng, BB_DIR_USR_SBIN, BB_SUID_DROP))
31
32//kbuild:lib-$(CONFIG_SEEDRNG) += seedrng.o
33
34//usage:#define seedrng_trivial_usage
35//usage: "[-d DIR] [-n]"
36//usage:#define seedrng_full_usage "\n\n"
37//usage: "Seed the kernel RNG from seed files"
38//usage: "\n"
39//usage: "\n -d DIR Use seed files in DIR (default: /var/lib/seedrng)"
40//usage: "\n -n Do not credit randomness, even if creditable"
41
42#include "libbb.h"
43
44#include <linux/random.h>
45#include <sys/random.h>
46#include <sys/file.h>
47
48#ifndef GRND_INSECURE
49#define GRND_INSECURE 0x0004 /* Apparently some headers don't ship with this yet. */
50#endif
51
52#define DEFAULT_SEED_DIR "/var/lib/seedrng"
53#define CREDITABLE_SEED_NAME "seed.credit"
54#define NON_CREDITABLE_SEED_NAME "seed.no-credit"
55
56enum {
57 MIN_SEED_LEN = SHA256_OUTSIZE,
58 /* kernels < 5.18 could return short reads from getrandom()
59 * if signal is pending and length is > 256.
60 * Let's limit our reads to 256 bytes.
61 */
62 MAX_SEED_LEN = 256,
63};
64
65static size_t determine_optimal_seed_len(void)
66{
67 char poolsize_str[12];
68 unsigned poolsize;
69 int n;
70
71 n = open_read_close("/proc/sys/kernel/random/poolsize", poolsize_str, sizeof(poolsize_str) - 1);
72 if (n < 0) {
73 bb_perror_msg("can't determine pool size, assuming %u bits", MIN_SEED_LEN * 8);
74 return MIN_SEED_LEN;
75 }
76 poolsize_str[n] = '\0';
77 poolsize = (bb_strtou(poolsize_str, NULL, 10) + 7) / 8;
78 return MAX(MIN(poolsize, MAX_SEED_LEN), MIN_SEED_LEN);
79}
80
81static bool read_new_seed(uint8_t *seed, size_t len)
82{
83 bool is_creditable;
84 ssize_t ret;
85
86 ret = getrandom(seed, len, GRND_NONBLOCK);
87 if (ret == (ssize_t)len) {
88 return true;
89 }
90 if (ret < 0 && errno == ENOSYS) {
91 int fd = xopen("/dev/random", O_RDONLY);
92 struct pollfd random_fd;
93 random_fd.fd = fd;
94 random_fd.events = POLLIN;
95 is_creditable = poll(&random_fd, 1, 0) == 1;
96//This is racy. is_creditable can be set to true here, but other process
97//can consume "good" random data from /dev/urandom before we do it below.
98 close(fd);
99 } else {
100 if (getrandom(seed, len, GRND_INSECURE) == (ssize_t)len)
101 return false;
102 is_creditable = false;
103 }
104
105 /* Either getrandom() is not implemented, or
106 * getrandom(GRND_INSECURE) did not give us LEN bytes.
107 * Fallback to reading /dev/urandom.
108 */
109 errno = 0;
110 if (open_read_close("/dev/urandom", seed, len) != (ssize_t)len)
111 bb_perror_msg_and_die("can't read '%s'", "/dev/urandom");
112 return is_creditable;
113}
114
115static void seed_from_file_if_exists(const char *filename, int dfd, bool credit, sha256_ctx_t *hash)
116{
117 struct {
118 int entropy_count;
119 int buf_size;
120 uint8_t buf[MAX_SEED_LEN];
121 } req;
122 ssize_t seed_len;
123
124 seed_len = open_read_close(filename, req.buf, sizeof(req.buf));
125 if (seed_len < 0) {
126 if (errno != ENOENT)
127 bb_perror_msg_and_die("can't read '%s'", filename);
128 return;
129 }
130 xunlink(filename);
131 if (seed_len != 0) {
132 int fd;
133
134 /* We are going to use this data to seed the RNG:
135 * we believe it to genuinely containing entropy.
136 * If this just-unlinked file survives
137 * (e.g. if machine crashes _right now_)
138 * and we reuse it after reboot, this assumption
139 * would be violated. Fsync the directory to
140 * make sure file is gone:
141 */
142 fsync(dfd);
143
144//Length is not random, and taking its address spills variable to stack
145// sha256_hash(hash, &seed_len, sizeof(seed_len));
146 sha256_hash(hash, req.buf, seed_len);
147
148 req.buf_size = seed_len;
149 seed_len *= 8;
150 req.entropy_count = credit ? seed_len : 0;
151 printf("Seeding %u bits %s crediting\n",
152 (unsigned)seed_len, credit ? "and" : "without");
153 fd = xopen("/dev/urandom", O_RDONLY);
154 xioctl(fd, RNDADDENTROPY, &req);
155 if (ENABLE_FEATURE_CLEAN_UP)
156 close(fd);
157 }
158}
159
160int seedrng_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
161int seedrng_main(int argc UNUSED_PARAM, char **argv)
162{
163 const char *seed_dir;
164 int fd, dfd;
165 int i;
166 unsigned opts;
167 uint8_t new_seed[MAX_SEED_LEN];
168 size_t new_seed_len;
169 bool new_seed_creditable;
170 struct timespec timestamp;
171 sha256_ctx_t hash;
172
173 enum {
174 OPT_n = (1 << 0), /* must be 1 */
175 OPT_d = (1 << 1),
176 };
177#if ENABLE_LONG_OPTS
178 static const char longopts[] ALIGN1 =
179 "skip-credit\0" No_argument "n"
180 "seed-dir\0" Required_argument "d"
181 ;
182#endif
183
184 seed_dir = DEFAULT_SEED_DIR;
185 opts = getopt32long(argv, "nd:", longopts, &seed_dir);
186 umask(0077);
187 if (getuid() != 0)
188 bb_simple_error_msg_and_die(bb_msg_you_must_be_root);
189
190 if (mkdir(seed_dir, 0700) < 0 && errno != EEXIST)
191 bb_perror_msg_and_die("can't create directory '%s'", seed_dir);
192 dfd = xopen(seed_dir, O_DIRECTORY | O_RDONLY);
193 xfchdir(dfd);
194 /* Concurrent runs of this tool might feed the same data to RNG twice.
195 * Avoid concurrent runs by taking a blocking lock on the directory.
196 * Not checking for errors. Looking at manpage,
197 * ENOLCK "The kernel ran out of memory for allocating lock records"
198 * seems to be the only one which is likely - and if that happens,
199 * machine is OOMing (much worse problem than inability to lock...).
200 * Also, typically configured Linux machines do not fail GFP_KERNEL
201 * allocations (they trigger memory reclaim instead).
202 */
203 flock(dfd, LOCK_EX); /* would block while another copy runs */
204
205 sha256_begin(&hash);
206 sha256_hash(&hash, "SeedRNG v1 Old+New Prefix", 25);
207 clock_gettime(CLOCK_REALTIME, &timestamp);
208 sha256_hash(&hash, &timestamp, sizeof(timestamp));
209 clock_gettime(CLOCK_BOOTTIME, &timestamp);
210 sha256_hash(&hash, &timestamp, sizeof(timestamp));
211
212 for (i = 0; i <= 1; i++) {
213 seed_from_file_if_exists(i == 0 ? NON_CREDITABLE_SEED_NAME : CREDITABLE_SEED_NAME,
214 dfd,
215 /* credit? */ (opts ^ OPT_n) & i, /* 0, then 1 unless -n */
216 &hash);
217 }
218
219 new_seed_len = determine_optimal_seed_len();
220 new_seed_creditable = read_new_seed(new_seed, new_seed_len);
221//Length is not random, and taking its address spills variable to stack
222// sha256_hash(&hash, &new_seed_len, sizeof(new_seed_len));
223 sha256_hash(&hash, new_seed, new_seed_len);
224 sha256_end(&hash, new_seed + new_seed_len - SHA256_OUTSIZE);
225
226 printf("Saving %u bits of %screditable seed for next boot\n",
227 (unsigned)new_seed_len * 8, new_seed_creditable ? "" : "non-");
228 fd = xopen3(NON_CREDITABLE_SEED_NAME, O_WRONLY | O_CREAT | O_TRUNC, 0400);
229 xwrite(fd, new_seed, new_seed_len);
230 if (new_seed_creditable) {
231 /* More paranoia when we create a file which we believe contains
232 * genuine entropy: make sure disk is not full, quota isn't exceeded, etc:
233 */
234 if (fsync(fd) < 0)
235 bb_perror_msg_and_die("can't write '%s'", NON_CREDITABLE_SEED_NAME);
236 xrename(NON_CREDITABLE_SEED_NAME, CREDITABLE_SEED_NAME);
237 }
238 return EXIT_SUCCESS;
239}
generated by cgit v1.2.3-55-g6feb (git 2.39.1) at 2025-03-17 08:43:16 +0000