53e1b68390
This follows what the kernel is doing, c.f. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5fd54ace4721fc5ce2bb5aef6318fcf17f421460.
173 lines
5.4 KiB
C
173 lines
5.4 KiB
C
/* SPDX-License-Identifier: LGPL-2.1+ */
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/***
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This file is part of systemd.
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Copyright 2010 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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***/
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#include <elf.h>
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#include <errno.h>
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#include <fcntl.h>
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#include <stdbool.h>
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#include <stdlib.h>
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#include <sys/time.h>
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#include <linux/random.h>
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#include <stdint.h>
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#if HAVE_SYS_AUXV_H
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# include <sys/auxv.h>
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#endif
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#if USE_SYS_RANDOM_H
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# include <sys/random.h>
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#else
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# include <linux/random.h>
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#endif
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#include "fd-util.h"
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#include "io-util.h"
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#include "missing.h"
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#include "random-util.h"
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#include "time-util.h"
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int acquire_random_bytes(void *p, size_t n, bool high_quality_required) {
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static int have_syscall = -1;
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_cleanup_close_ int fd = -1;
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unsigned already_done = 0;
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int r;
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/* Gathers some randomness from the kernel. This call will never block. If
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* high_quality_required, it will always return some data from the kernel,
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* regardless of whether the random pool is fully initialized or not.
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* Otherwise, it will return success if at least some random bytes were
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* successfully acquired, and an error if the kernel has no entropy whatsover
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* for us. */
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/* Use the getrandom() syscall unless we know we don't have it. */
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if (have_syscall != 0) {
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r = getrandom(p, n, GRND_NONBLOCK);
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if (r > 0) {
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have_syscall = true;
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if ((size_t) r == n)
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return 0;
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if (!high_quality_required) {
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/* Fill in the remaining bytes using pseudorandom values */
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pseudorandom_bytes((uint8_t*) p + r, n - r);
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return 0;
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}
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already_done = r;
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} else if (errno == ENOSYS)
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/* We lack the syscall, continue with reading from /dev/urandom. */
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have_syscall = false;
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else if (errno == EAGAIN) {
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/* The kernel has no entropy whatsoever. Let's remember to
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* use the syscall the next time again though.
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*
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* If high_quality_required is false, return an error so that
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* random_bytes() can produce some pseudorandom
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* bytes. Otherwise, fall back to /dev/urandom, which we know
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* is empty, but the kernel will produce some bytes for us on
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* a best-effort basis. */
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have_syscall = true;
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if (!high_quality_required)
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return -ENODATA;
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} else
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return -errno;
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}
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fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
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if (fd < 0)
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return errno == ENOENT ? -ENOSYS : -errno;
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return loop_read_exact(fd, (uint8_t*) p + already_done, n - already_done, true);
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}
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void initialize_srand(void) {
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static bool srand_called = false;
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unsigned x;
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#if HAVE_SYS_AUXV_H
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void *auxv;
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#endif
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if (srand_called)
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return;
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#if HAVE_SYS_AUXV_H
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/* The kernel provides us with 16 bytes of entropy in auxv, so let's
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* try to make use of that to seed the pseudo-random generator. It's
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* better than nothing... */
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auxv = (void*) getauxval(AT_RANDOM);
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if (auxv) {
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assert_cc(sizeof(x) <= 16);
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memcpy(&x, auxv, sizeof(x));
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} else
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#endif
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x = 0;
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x ^= (unsigned) now(CLOCK_REALTIME);
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x ^= (unsigned) gettid();
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srand(x);
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srand_called = true;
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}
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/* INT_MAX gives us only 31 bits, so use 24 out of that. */
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#if RAND_MAX >= INT_MAX
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# define RAND_STEP 3
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#else
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/* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
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# define RAND_STEP 1
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#endif
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void pseudorandom_bytes(void *p, size_t n) {
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uint8_t *q;
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initialize_srand();
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for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
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unsigned rr;
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rr = (unsigned) rand();
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#if RAND_STEP >= 3
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if ((size_t) (q - (uint8_t*) p + 2) < n)
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q[2] = rr >> 16;
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#endif
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#if RAND_STEP >= 2
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if ((size_t) (q - (uint8_t*) p + 1) < n)
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q[1] = rr >> 8;
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#endif
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q[0] = rr;
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}
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}
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void random_bytes(void *p, size_t n) {
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int r;
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r = acquire_random_bytes(p, n, false);
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if (r >= 0)
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return;
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/* If some idiot made /dev/urandom unavailable to us, or the
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* kernel has no entropy, use a PRNG instead. */
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return pseudorandom_bytes(p, n);
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}
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