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/* Copyright (c) 2014, Google Inc.
*
* Permission to use, copy, modify, and/or distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
* SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
* OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
#include <openssl/rand.h>
#if !defined(OPENSSL_WINDOWS)
#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <openssl/thread.h>
#include <openssl/mem.h>
/* This file implements a PRNG by reading from /dev/urandom, optionally with a
* fork-safe buffer.
*
* If buffering is enabled then it maintains a global, linked list of buffers.
* Threads which need random bytes grab a buffer from the list under a lock and
* copy out the bytes that they need. In the rare case that the buffer is
* empty, it's refilled from /dev/urandom outside of the lock.
*
* Large requests are always serviced from /dev/urandom directly.
*
* Each buffer contains the PID of the process that created it and it's tested
* against the current PID each time. Thus processes that fork will discard all
* the buffers filled by the parent process. There are two problems with this:
*
* 1) glibc maintains a cache of the current PID+PPID and, if this cache isn't
* correctly invalidated, the getpid() will continue to believe that
* it's the old process. Glibc depends on the glibc wrappers for fork,
* vfork and clone being used in order to invalidate the getpid() cache.
*
* 2) If a process forks, dies and then its child forks, it's possible that
* the third process will end up with the same PID as the original process.
* If the second process never used any random values then this will mean
* that the third process has stale, cached values and won't notice.
*/
/* BUF_SIZE is intended to be a 4K allocation with malloc overhead. struct
* rand_buffer also fits in this space and the remainder is entropy. */
#define BUF_SIZE (4096 - 16)
/* rand_buffer contains unused, random bytes. These structures form a linked
* list via the |next| pointer, which is NULL in the final element. */
struct rand_buffer {
size_t used; /* used contains the number of bytes of |rand| that have
been consumed. */
struct rand_buffer *next;
pid_t pid; /* pid contains the pid at the time that the buffer was
created so that data is not duplicated after a fork. */
pid_t ppid; /* ppid contains the parent pid in order to try and reduce
the possibility of duplicated PID confusing the
detection of a fork. */
uint8_t rand[];
};
/* rand_bytes_per_buf is the number of actual entropy bytes in a buffer. */
static const size_t rand_bytes_per_buf = BUF_SIZE - sizeof(struct rand_buffer);
/* list_head is the start of a global, linked-list of rand_buffer objects. It's
* protected by CRYPTO_LOCK_RAND. */
static struct rand_buffer *list_head;
/* urandom_fd is a file descriptor to /dev/urandom. It's protected by
* CRYPTO_LOCK_RAND. */
static int urandom_fd = -2;
/* urandom_buffering controls whether buffering is enabled (1) or not (0). This
* is protected by CRYPTO_LOCK_RAND. */
static int urandom_buffering = 0;
/* urandom_get_fd_locked returns a file descriptor to /dev/urandom. The caller
* of this function must hold CRYPTO_LOCK_RAND. */
static int urandom_get_fd_locked() {
if (urandom_fd != -2)
return urandom_fd;
urandom_fd = open("/dev/urandom", O_RDONLY | O_NOCTTY);
return urandom_fd;
}
/* RAND_cleanup frees all buffers, closes any cached file descriptor
* and resets the global state. */
void RAND_cleanup(void) {
struct rand_buffer *cur;
CRYPTO_w_lock(CRYPTO_LOCK_RAND);
while ((cur = list_head)) {
list_head = cur->next;
OPENSSL_free(cur);
}
if (urandom_fd >= 0) {
close(urandom_fd);
}
urandom_fd = -2;
list_head = NULL;
CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
}
/* read_full reads exactly |len| bytes from |fd| into |out| and returns 1. In
* the case of an error it returns 0. */
static char read_full(int fd, uint8_t *out, size_t len) {
ssize_t r;
while (len > 0) {
do {
r = read(fd, out, len);
} while (r == -1 && errno == EINTR);
if (r <= 0) {
return 0;
}
out += r;
len -= r;
}
return 1;
}
/* urandom_rand_pseudo_bytes puts |num| random bytes into |out|. It returns
* one on success and zero otherwise. */
int RAND_bytes(uint8_t *out, size_t requested) {
int fd;
struct rand_buffer *buf;
size_t todo;
pid_t pid, ppid;
if (requested == 0) {
return 1;
}
CRYPTO_w_lock(CRYPTO_LOCK_RAND);
fd = urandom_get_fd_locked();
if (fd < 0) {
CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
abort();
return 0;
}
/* If buffering is not enabled, or if the request is large, then the
* result comes directly from urandom. */
if (!urandom_buffering || requested > BUF_SIZE / 2) {
CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
if (!read_full(fd, out, requested)) {
abort();
return 0;
}
return 1;
}
pid = getpid();
ppid = getppid();
for (;;) {
buf = list_head;
if (buf && buf->pid == pid && buf->ppid == ppid &&
rand_bytes_per_buf - buf->used >= requested) {
memcpy(out, &buf->rand[buf->used], requested);
buf->used += requested;
CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
return 1;
}
/* If we don't immediately have enough entropy with the correct
* PID, remove the buffer from the list in order to gain
* exclusive access and unlock. */
if (buf) {
list_head = buf->next;
}
CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
if (!buf) {
buf = (struct rand_buffer *)OPENSSL_malloc(BUF_SIZE);
/* The buffer doesn't contain any random bytes yet
* so we mark it as fully used so that it will be
* filled below. */
buf->used = rand_bytes_per_buf;
buf->next = NULL;
buf->pid = pid;
buf->ppid = ppid;
}
if (buf->pid == pid && buf->ppid == ppid) {
break;
}
/* We have forked and so cannot use these bytes as they
* may have been used in another process. */
OPENSSL_free(buf);
CRYPTO_w_lock(CRYPTO_LOCK_RAND);
}
while (requested > 0) {
todo = rand_bytes_per_buf - buf->used;
if (todo > requested) {
todo = requested;
}
memcpy(out, &buf->rand[buf->used], todo);
requested -= todo;
out += todo;
buf->used += todo;
if (buf->used < rand_bytes_per_buf) {
break;
}
if (!read_full(fd, buf->rand, rand_bytes_per_buf)) {
OPENSSL_free(buf);
abort();
return 0;
}
buf->used = 0;
}
CRYPTO_w_lock(CRYPTO_LOCK_RAND);
assert(list_head != buf);
buf->next = list_head;
list_head = buf;
CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
return 1;
}
#endif /* !OPENSSL_WINDOWS */