getentropy_linux.c revision 291767 
1/*	$OpenBSD: getentropy_linux.c,v 1.20 2014/07/12 15:43:49 beck Exp $	*/
2
3/*
4 * Copyright (c) 2014 Theo de Raadt <deraadt@openbsd.org>
5 * Copyright (c) 2014 Bob Beck <beck@obtuse.com>
6 *
7 * Permission to use, copy, modify, and distribute this software for any
8 * purpose with or without fee is hereby granted, provided that the above
9 * copyright notice and this permission notice appear in all copies.
10 *
11 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
12 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
13 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
14 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
15 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
16 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
17 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
18 */
19#include "config.h"
20
21/*
22#define	_POSIX_C_SOURCE 199309L
23#define	_GNU_SOURCE     1
24*/
25#include <sys/types.h>
26#include <sys/param.h>
27#include <sys/ioctl.h>
28#include <sys/resource.h>
29#include <sys/syscall.h>
30#ifdef HAVE_SYS_SYSCTL_H
31#include <sys/sysctl.h>
32#endif
33#include <sys/statvfs.h>
34#include <sys/socket.h>
35#include <sys/mount.h>
36#include <sys/mman.h>
37#include <sys/stat.h>
38#include <sys/time.h>
39#include <stdlib.h>
40#include <stdint.h>
41#include <stdio.h>
42#include <termios.h>
43#include <fcntl.h>
44#include <signal.h>
45#include <string.h>
46#include <errno.h>
47#include <unistd.h>
48#include <time.h>
49#include <openssl/sha.h>
50
51#include <linux/types.h>
52#include <linux/random.h>
53#include <linux/sysctl.h>
54#ifdef HAVE_GETAUXVAL
55#include <sys/auxv.h>
56#endif
57#include <sys/vfs.h>
58
59#define REPEAT 5
60#define min(a, b) (((a) < (b)) ? (a) : (b))
61
62#define HX(a, b) \
63	do { \
64		if ((a)) \
65			HD(errno); \
66		else \
67			HD(b); \
68	} while (0)
69
70#define HR(x, l) (SHA512_Update(&ctx, (char *)(x), (l)))
71#define HD(x)	 (SHA512_Update(&ctx, (char *)&(x), sizeof (x)))
72#define HF(x)    (SHA512_Update(&ctx, (char *)&(x), sizeof (void*)))
73
74int	getentropy(void *buf, size_t len);
75
76#ifdef CAN_REFERENCE_MAIN
77extern int main(int, char *argv[]);
78#endif
79static int gotdata(char *buf, size_t len);
80#ifdef SYS_getrandom
81static int getentropy_getrandom(void *buf, size_t len);
82#endif
83static int getentropy_urandom(void *buf, size_t len);
84#ifdef SYS__sysctl
85static int getentropy_sysctl(void *buf, size_t len);
86#endif
87static int getentropy_fallback(void *buf, size_t len);
88
89int
90getentropy(void *buf, size_t len)
91{
92	int ret = -1;
93
94	if (len > 256) {
95		errno = EIO;
96		return -1;
97	}
98
99#ifdef SYS_getrandom
100	/*
101	 * Try descriptor-less getrandom()
102	 */
103	ret = getentropy_getrandom(buf, len);
104	if (ret != -1)
105		return (ret);
106	if (errno != ENOSYS)
107		return (-1);
108#endif
109
110	/*
111	 * Try to get entropy with /dev/urandom
112	 *
113	 * This can fail if the process is inside a chroot or if file
114	 * descriptors are exhausted.
115	 */
116	ret = getentropy_urandom(buf, len);
117	if (ret != -1)
118		return (ret);
119
120#ifdef SYS__sysctl
121	/*
122	 * Try to use sysctl CTL_KERN, KERN_RANDOM, RANDOM_UUID.
123	 * sysctl is a failsafe API, so it guarantees a result.  This
124	 * should work inside a chroot, or when file descriptors are
125	 * exhuasted.
126	 *
127	 * However this can fail if the Linux kernel removes support
128	 * for sysctl.  Starting in 2007, there have been efforts to
129	 * deprecate the sysctl API/ABI, and push callers towards use
130	 * of the chroot-unavailable fd-using /proc mechanism --
131	 * essentially the same problems as /dev/urandom.
132	 *
133	 * Numerous setbacks have been encountered in their deprecation
134	 * schedule, so as of June 2014 the kernel ABI still exists on
135	 * most Linux architectures. The sysctl() stub in libc is missing
136	 * on some systems.  There are also reports that some kernels
137	 * spew messages to the console.
138	 */
139	ret = getentropy_sysctl(buf, len);
140	if (ret != -1)
141		return (ret);
142#endif /* SYS__sysctl */
143
144	/*
145	 * Entropy collection via /dev/urandom and sysctl have failed.
146	 *
147	 * No other API exists for collecting entropy.  See the large
148	 * comment block above.
149	 *
150	 * We have very few options:
151	 *     - Even syslog_r is unsafe to call at this low level, so
152	 *	 there is no way to alert the user or program.
153	 *     - Cannot call abort() because some systems have unsafe
154	 *	 corefiles.
155	 *     - Could raise(SIGKILL) resulting in silent program termination.
156	 *     - Return EIO, to hint that arc4random's stir function
157	 *       should raise(SIGKILL)
158	 *     - Do the best under the circumstances....
159	 *
160	 * This code path exists to bring light to the issue that Linux
161	 * does not provide a failsafe API for entropy collection.
162	 *
163	 * We hope this demonstrates that Linux should either retain their
164	 * sysctl ABI, or consider providing a new failsafe API which
165	 * works in a chroot or when file descriptors are exhausted.
166	 */
167#undef FAIL_INSTEAD_OF_TRYING_FALLBACK
168#ifdef FAIL_INSTEAD_OF_TRYING_FALLBACK
169	raise(SIGKILL);
170#endif
171	ret = getentropy_fallback(buf, len);
172	if (ret != -1)
173		return (ret);
174
175	errno = EIO;
176	return (ret);
177}
178
179/*
180 * Basic sanity checking; wish we could do better.
181 */
182static int
183gotdata(char *buf, size_t len)
184{
185	char	any_set = 0;
186	size_t	i;
187
188	for (i = 0; i < len; ++i)
189		any_set |= buf[i];
190	if (any_set == 0)
191		return -1;
192	return 0;
193}
194
195#ifdef SYS_getrandom
196static int
197getentropy_getrandom(void *buf, size_t len)
198{
199	int pre_errno = errno;
200	int ret;
201	if (len > 256)
202		return (-1);
203	do {
204		ret = syscall(SYS_getrandom, buf, len, 0);
205	} while (ret == -1 && errno == EINTR);
206
207	if (ret != (int)len)
208		return (-1);
209	errno = pre_errno;
210	return (0);
211}
212#endif
213
214static int
215getentropy_urandom(void *buf, size_t len)
216{
217	struct stat st;
218	size_t i;
219	int fd, cnt, flags;
220	int save_errno = errno;
221
222start:
223
224	flags = O_RDONLY;
225#ifdef O_NOFOLLOW
226	flags |= O_NOFOLLOW;
227#endif
228#ifdef O_CLOEXEC
229	flags |= O_CLOEXEC;
230#endif
231	fd = open("/dev/urandom", flags, 0);
232	if (fd == -1) {
233		if (errno == EINTR)
234			goto start;
235		goto nodevrandom;
236	}
237#ifndef O_CLOEXEC
238	fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC);
239#endif
240
241	/* Lightly verify that the device node looks sane */
242	if (fstat(fd, &st) == -1 || !S_ISCHR(st.st_mode)) {
243		close(fd);
244		goto nodevrandom;
245	}
246	if (ioctl(fd, RNDGETENTCNT, &cnt) == -1) {
247		close(fd);
248		goto nodevrandom;
249	}
250	for (i = 0; i < len; ) {
251		size_t wanted = len - i;
252		ssize_t ret = read(fd, (char*)buf + i, wanted);
253
254		if (ret == -1) {
255			if (errno == EAGAIN || errno == EINTR)
256				continue;
257			close(fd);
258			goto nodevrandom;
259		}
260		i += ret;
261	}
262	close(fd);
263	if (gotdata(buf, len) == 0) {
264		errno = save_errno;
265		return 0;		/* satisfied */
266	}
267nodevrandom:
268	errno = EIO;
269	return -1;
270}
271
272#ifdef SYS__sysctl
273static int
274getentropy_sysctl(void *buf, size_t len)
275{
276	static int mib[] = { CTL_KERN, KERN_RANDOM, RANDOM_UUID };
277	size_t i;
278	int save_errno = errno;
279
280	for (i = 0; i < len; ) {
281		size_t chunk = min(len - i, 16);
282
283		/* SYS__sysctl because some systems already removed sysctl() */
284		struct __sysctl_args args = {
285			.name = mib,
286			.nlen = 3,
287			.oldval = (char *)buf + i,
288			.oldlenp = &chunk,
289		};
290		if (syscall(SYS__sysctl, &args) != 0)
291			goto sysctlfailed;
292		i += chunk;
293	}
294	if (gotdata(buf, len) == 0) {
295		errno = save_errno;
296		return (0);			/* satisfied */
297	}
298sysctlfailed:
299	errno = EIO;
300	return -1;
301}
302#endif /* SYS__sysctl */
303
304static int cl[] = {
305	CLOCK_REALTIME,
306#ifdef CLOCK_MONOTONIC
307	CLOCK_MONOTONIC,
308#endif
309#ifdef CLOCK_MONOTONIC_RAW
310	CLOCK_MONOTONIC_RAW,
311#endif
312#ifdef CLOCK_TAI
313	CLOCK_TAI,
314#endif
315#ifdef CLOCK_VIRTUAL
316	CLOCK_VIRTUAL,
317#endif
318#ifdef CLOCK_UPTIME
319	CLOCK_UPTIME,
320#endif
321#ifdef CLOCK_PROCESS_CPUTIME_ID
322	CLOCK_PROCESS_CPUTIME_ID,
323#endif
324#ifdef CLOCK_THREAD_CPUTIME_ID
325	CLOCK_THREAD_CPUTIME_ID,
326#endif
327};
328
329static int
330getentropy_fallback(void *buf, size_t len)
331{
332	uint8_t results[SHA512_DIGEST_LENGTH];
333	int save_errno = errno, e, pgs = getpagesize(), faster = 0, repeat;
334	static int cnt;
335	struct timespec ts;
336	struct timeval tv;
337	struct rusage ru;
338	sigset_t sigset;
339	struct stat st;
340	SHA512_CTX ctx;
341	static pid_t lastpid;
342	pid_t pid;
343	size_t i, ii, m;
344	char *p;
345
346	pid = getpid();
347	if (lastpid == pid) {
348		faster = 1;
349		repeat = 2;
350	} else {
351		faster = 0;
352		lastpid = pid;
353		repeat = REPEAT;
354	}
355	for (i = 0; i < len; ) {
356		int j;
357		SHA512_Init(&ctx);
358		for (j = 0; j < repeat; j++) {
359			HX((e = gettimeofday(&tv, NULL)) == -1, tv);
360			if (e != -1) {
361				cnt += (int)tv.tv_sec;
362				cnt += (int)tv.tv_usec;
363			}
364
365			for (ii = 0; ii < sizeof(cl)/sizeof(cl[0]); ii++)
366				HX(clock_gettime(cl[ii], &ts) == -1, ts);
367
368			HX((pid = getpid()) == -1, pid);
369			HX((pid = getsid(pid)) == -1, pid);
370			HX((pid = getppid()) == -1, pid);
371			HX((pid = getpgid(0)) == -1, pid);
372			HX((e = getpriority(0, 0)) == -1, e);
373
374			if (!faster) {
375				ts.tv_sec = 0;
376				ts.tv_nsec = 1;
377				(void) nanosleep(&ts, NULL);
378			}
379
380			HX(sigpending(&sigset) == -1, sigset);
381			HX(sigprocmask(SIG_BLOCK, NULL, &sigset) == -1,
382			    sigset);
383
384#ifdef CAN_REFERENCE_MAIN
385			HF(main);		/* an addr in program */
386#endif
387			HF(getentropy);	/* an addr in this library */
388			HF(printf);		/* an addr in libc */
389			p = (char *)&p;
390			HD(p);		/* an addr on stack */
391			p = (char *)&errno;
392			HD(p);		/* the addr of errno */
393
394			if (i == 0) {
395				struct sockaddr_storage ss;
396				struct statvfs stvfs;
397				struct termios tios;
398				struct statfs stfs;
399				socklen_t ssl;
400				off_t off;
401
402				/*
403				 * Prime-sized mappings encourage fragmentation;
404				 * thus exposing some address entropy.
405				 */
406				struct mm {
407					size_t	npg;
408					void	*p;
409				} mm[] =	 {
410					{ 17, MAP_FAILED }, { 3, MAP_FAILED },
411					{ 11, MAP_FAILED }, { 2, MAP_FAILED },
412					{ 5, MAP_FAILED }, { 3, MAP_FAILED },
413					{ 7, MAP_FAILED }, { 1, MAP_FAILED },
414					{ 57, MAP_FAILED }, { 3, MAP_FAILED },
415					{ 131, MAP_FAILED }, { 1, MAP_FAILED },
416				};
417
418				for (m = 0; m < sizeof mm/sizeof(mm[0]); m++) {
419					HX(mm[m].p = mmap(NULL,
420					    mm[m].npg * pgs,
421					    PROT_READ|PROT_WRITE,
422					    MAP_PRIVATE|MAP_ANON, -1,
423					    (off_t)0), mm[m].p);
424					if (mm[m].p != MAP_FAILED) {
425						size_t mo;
426
427						/* Touch some memory... */
428						p = mm[m].p;
429						mo = cnt %
430						    (mm[m].npg * pgs - 1);
431						p[mo] = 1;
432						cnt += (int)((long)(mm[m].p)
433						    / pgs);
434					}
435
436					/* Check cnts and times... */
437					for (ii = 0; ii < sizeof(cl)/sizeof(cl[0]);
438					    ii++) {
439						HX((e = clock_gettime(cl[ii],
440						    &ts)) == -1, ts);
441						if (e != -1)
442							cnt += (int)ts.tv_nsec;
443					}
444
445					HX((e = getrusage(RUSAGE_SELF,
446					    &ru)) == -1, ru);
447					if (e != -1) {
448						cnt += (int)ru.ru_utime.tv_sec;
449						cnt += (int)ru.ru_utime.tv_usec;
450					}
451				}
452
453				for (m = 0; m < sizeof mm/sizeof(mm[0]); m++) {
454					if (mm[m].p != MAP_FAILED)
455						munmap(mm[m].p, mm[m].npg * pgs);
456					mm[m].p = MAP_FAILED;
457				}
458
459				HX(stat(".", &st) == -1, st);
460				HX(statvfs(".", &stvfs) == -1, stvfs);
461				HX(statfs(".", &stfs) == -1, stfs);
462
463				HX(stat("/", &st) == -1, st);
464				HX(statvfs("/", &stvfs) == -1, stvfs);
465				HX(statfs("/", &stfs) == -1, stfs);
466
467				HX((e = fstat(0, &st)) == -1, st);
468				if (e == -1) {
469					if (S_ISREG(st.st_mode) ||
470					    S_ISFIFO(st.st_mode) ||
471					    S_ISSOCK(st.st_mode)) {
472						HX(fstatvfs(0, &stvfs) == -1,
473						    stvfs);
474						HX(fstatfs(0, &stfs) == -1,
475						    stfs);
476						HX((off = lseek(0, (off_t)0,
477						    SEEK_CUR)) < 0, off);
478					}
479					if (S_ISCHR(st.st_mode)) {
480						HX(tcgetattr(0, &tios) == -1,
481						    tios);
482					} else if (S_ISSOCK(st.st_mode)) {
483						memset(&ss, 0, sizeof ss);
484						ssl = sizeof(ss);
485						HX(getpeername(0,
486						    (void *)&ss, &ssl) == -1,
487						    ss);
488					}
489				}
490
491				HX((e = getrusage(RUSAGE_CHILDREN,
492				    &ru)) == -1, ru);
493				if (e != -1) {
494					cnt += (int)ru.ru_utime.tv_sec;
495					cnt += (int)ru.ru_utime.tv_usec;
496				}
497			} else {
498				/* Subsequent hashes absorb previous result */
499				HD(results);
500			}
501
502			HX((e = gettimeofday(&tv, NULL)) == -1, tv);
503			if (e != -1) {
504				cnt += (int)tv.tv_sec;
505				cnt += (int)tv.tv_usec;
506			}
507
508			HD(cnt);
509		}
510#ifdef HAVE_GETAUXVAL
511#  ifdef AT_RANDOM
512		/* Not as random as you think but we take what we are given */
513		p = (char *) getauxval(AT_RANDOM);
514		if (p)
515			HR(p, 16);
516#  endif
517#  ifdef AT_SYSINFO_EHDR
518		p = (char *) getauxval(AT_SYSINFO_EHDR);
519		if (p)
520			HR(p, pgs);
521#  endif
522#  ifdef AT_BASE
523		p = (char *) getauxval(AT_BASE);
524		if (p)
525			HD(p);
526#  endif
527#endif /* HAVE_GETAUXVAL */
528
529		SHA512_Final(results, &ctx);
530		memcpy((char*)buf + i, results, min(sizeof(results), len - i));
531		i += min(sizeof(results), len - i);
532	}
533	memset(results, 0, sizeof results);
534	if (gotdata(buf, len) == 0) {
535		errno = save_errno;
536		return 0;		/* satisfied */
537	}
538	errno = EIO;
539	return -1;
540}
541