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/* Copyright (c) 2020, 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. */
#if !defined(_GNU_SOURCE)
#define _GNU_SOURCE // needed for madvise() and MAP_ANONYMOUS on Linux.
#endif
#include <CNIOBoringSSL_base.h>
#include "fork_detect.h"
#if defined(OPENSSL_LINUX)
#include <sys/mman.h>
#include <unistd.h>
#include <stdlib.h>
#include <CNIOBoringSSL_type_check.h>
#include "../delocate.h"
#include "../../internal.h"
#if defined(MADV_WIPEONFORK)
OPENSSL_STATIC_ASSERT(MADV_WIPEONFORK == 18, "MADV_WIPEONFORK is not 18");
#else
#define MADV_WIPEONFORK 18
#endif
DEFINE_STATIC_ONCE(g_fork_detect_once);
DEFINE_STATIC_MUTEX(g_fork_detect_lock);
DEFINE_BSS_GET(volatile char *, g_fork_detect_addr);
DEFINE_BSS_GET(uint64_t, g_fork_generation);
DEFINE_BSS_GET(int, g_ignore_madv_wipeonfork);
static void init_fork_detect(void) {
if (*g_ignore_madv_wipeonfork_bss_get()) {
return;
}
long page_size = sysconf(_SC_PAGESIZE);
if (page_size <= 0) {
return;
}
void *addr = mmap(NULL, (size_t)page_size, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (addr == MAP_FAILED) {
return;
}
// Some versions of qemu (up to at least 5.0.0-rc4, see linux-user/syscall.c)
// ignore |madvise| calls and just return zero (i.e. success). But we need to
// know whether MADV_WIPEONFORK actually took effect. Therefore try an invalid
// call to check that the implementation of |madvise| is actually rejecting
// unknown |advice| values.
if (madvise(addr, (size_t)page_size, -1) == 0 ||
madvise(addr, (size_t)page_size, MADV_WIPEONFORK) != 0) {
munmap(addr, (size_t)page_size);
return;
}
*((volatile char *) addr) = 1;
*g_fork_detect_addr_bss_get() = addr;
*g_fork_generation_bss_get() = 1;
}
uint64_t CRYPTO_get_fork_generation(void) {
// In a single-threaded process, there are obviously no races because there's
// only a single mutator in the address space.
//
// In a multi-threaded environment, |CRYPTO_once| ensures that the flag byte
// is initialised atomically, even if multiple threads enter this function
// concurrently.
//
// In the limit, the kernel may clear WIPEONFORK pages while a multi-threaded
// process is running. (For example, because a VM was cloned.) Therefore a
// lock is used below to synchronise the potentially multiple threads that may
// concurrently observe the cleared flag.
CRYPTO_once(g_fork_detect_once_bss_get(), init_fork_detect);
// This pointer is |volatile| because the value pointed to may be changed by
// external forces (i.e. the kernel wiping the page) thus the compiler must
// not assume that it has exclusive access to it.
volatile char *const flag_ptr = *g_fork_detect_addr_bss_get();
if (flag_ptr == NULL) {
// Our kernel is too old to support |MADV_WIPEONFORK|.
return 0;
}
struct CRYPTO_STATIC_MUTEX *const lock = g_fork_detect_lock_bss_get();
uint64_t *const generation_ptr = g_fork_generation_bss_get();
CRYPTO_STATIC_MUTEX_lock_read(lock);
uint64_t current_generation = *generation_ptr;
if (*flag_ptr) {
CRYPTO_STATIC_MUTEX_unlock_read(lock);
return current_generation;
}
CRYPTO_STATIC_MUTEX_unlock_read(lock);
CRYPTO_STATIC_MUTEX_lock_write(lock);
current_generation = *generation_ptr;
if (*flag_ptr == 0) {
// A fork has occurred.
*flag_ptr = 1;
current_generation++;
if (current_generation == 0) {
current_generation = 1;
}
*generation_ptr = current_generation;
}
CRYPTO_STATIC_MUTEX_unlock_write(lock);
return current_generation;
}
void CRYPTO_fork_detect_ignore_madv_wipeonfork_for_testing(void) {
*g_ignore_madv_wipeonfork_bss_get() = 1;
}
#else // !OPENSSL_LINUX
uint64_t CRYPTO_get_fork_generation(void) { return 0; }
#endif // OPENSSL_LINUX
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