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// Copyright 2020 The BoringSSL Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#if !defined(_GNU_SOURCE)
#define _GNU_SOURCE // needed for madvise() and MAP_ANONYMOUS on Linux.
#endif
#include "../bcm_support.h"
#include "../internal.h"
#include "internal.h"
using namespace bssl;
#if defined(OPENSSL_FORK_DETECTION_MADVISE)
#include <assert.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>
#if defined(MADV_WIPEONFORK)
static_assert(MADV_WIPEONFORK == 18);
#else
#define MADV_WIPEONFORK 18
#endif
#elif defined(OPENSSL_FORK_DETECTION_PTHREAD_ATFORK)
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#endif // OPENSSL_FORK_DETECTION_PTHREAD_ATFORK
#if defined(OPENSSL_FORK_DETECTION_MADVISE)
static int g_force_madv_wipeonfork;
static int g_force_madv_wipeonfork_enabled;
static CRYPTO_once_t g_fork_detect_once = CRYPTO_ONCE_INIT;
static StaticMutex g_fork_detect_lock;
static Atomic<uint32_t> *g_fork_detect_addr;
static uint64_t g_fork_generation;
static void init_fork_detect() {
if (g_force_madv_wipeonfork) {
return;
}
long page_size = sysconf(_SC_PAGESIZE);
if (page_size <= 0) {
return;
}
void *addr = mmap(nullptr, (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;
}
auto *const atomic = reinterpret_cast<Atomic<uint32_t> *>(addr);
atomic->store(1);
g_fork_detect_addr = atomic;
g_fork_generation = 1;
}
uint64_t bssl::CRYPTO_get_fork_generation() {
CRYPTO_once(&g_fork_detect_once, init_fork_detect);
// 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.
//
// Additionally, while the kernel will only clear WIPEONFORK at a point when a
// child process is single-threaded, the child may become multi-threaded
// before it observes this. Therefore, we must synchronize the logic below.
Atomic<uint32_t> *const flag_ptr = g_fork_detect_addr;
if (flag_ptr == nullptr) {
// Our kernel is too old to support |MADV_WIPEONFORK| or
// |g_force_madv_wipeonfork| is set.
if (g_force_madv_wipeonfork && g_force_madv_wipeonfork_enabled) {
// A constant generation number to simulate support, even if the kernel
// doesn't support it.
return 42;
}
// With Linux and clone(), we do not believe that pthread_atfork() is
// sufficient for detecting all forms of address space duplication. At this
// point we have a kernel that does not support MADV_WIPEONFORK. We could
// return the generation number from pthread_atfork() here and it would
// probably be safe in almost any situation, but to ensure safety we return
// 0 and force an entropy draw on every call.
return 0;
}
// In the common case, try to observe the flag without taking a lock. This
// avoids cacheline contention in the PRNG.
uint64_t *const generation_ptr = &g_fork_generation;
if (flag_ptr->load() != 0) {
// If we observe a non-zero flag, it is safe to read |generation_ptr|
// without a lock. The flag and generation number are fixed for this copy of
// the address space.
return *generation_ptr;
}
// The flag was zero. The generation number must be incremented, but other
// threads may have concurrently observed the zero, so take a lock before
// incrementing.
MutexWriteLock lock(&g_fork_detect_lock);
uint64_t current_generation = *generation_ptr;
if (flag_ptr->load() == 0) {
// A fork has occurred.
current_generation++;
if (current_generation == 0) {
// Zero means fork detection isn't supported, so skip that value.
current_generation = 1;
}
// We must update |generation_ptr| before |flag_ptr|. Other threads may
// observe |flag_ptr| without taking a lock.
*generation_ptr = current_generation;
flag_ptr->store(1);
}
return current_generation;
}
void bssl::CRYPTO_fork_detect_force_madv_wipeonfork_for_testing(int on) {
g_force_madv_wipeonfork = 1;
g_force_madv_wipeonfork_enabled = on;
}
#elif defined(OPENSSL_FORK_DETECTION_PTHREAD_ATFORK)
static CRYPTO_once_t g_pthread_fork_detection_once = CRYPTO_ONCE_INIT;
static uint64_t g_atfork_fork_generation;
static void we_are_forked() {
// Immediately after a fork, the process must be single-threaded.
uint64_t value = g_atfork_fork_generation + 1;
if (value == 0) {
value = 1;
}
g_atfork_fork_generation = value;
}
static void init_pthread_fork_detection() {
if (pthread_atfork(nullptr, nullptr, we_are_forked) != 0) {
abort();
}
g_atfork_fork_generation = 1;
}
uint64_t bssl::CRYPTO_get_fork_generation() {
CRYPTO_once(&g_pthread_fork_detection_once, init_pthread_fork_detection);
return g_atfork_fork_generation;
}
#elif defined(OPENSSL_DOES_NOT_FORK)
// These platforms are guaranteed not to fork, and therefore do not require
// fork detection support. Returning a constant non zero value makes BoringSSL
// assume address space duplication is not a concern and adding entropy to
// every RAND_bytes call is not needed.
uint64_t bssl::CRYPTO_get_fork_generation() { return 0xc0ffee; }
#else
// These platforms may fork, but we do not have a mitigation mechanism in
// place. Returning a constant zero value makes BoringSSL assume that address
// space duplication could have occurred on any call entropy must be added to
// every RAND_bytes call.
uint64_t bssl::CRYPTO_get_fork_generation() { return 0; }
#endif
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