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//===-- safestack.cpp -----------------------------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements the runtime support for the safe stack protection
// mechanism. The runtime manages allocation/deallocation of the unsafe stack
// for the main thread, as well as all pthreads that are created/destroyed
// during program execution.
#include "safestack_platform.h"
#include "safestack_util.h"
#include <errno.h>
#include <sys/resource.h>
#include "interception/interception.h"
using namespace safestack;
// TODO: To make accessing the unsafe stack pointer faster, we plan to
// eventually store it directly in the thread control block data structure on
// platforms where this structure is pointed to by %fs or %gs. This is exactly
// the same mechanism as currently being used by the traditional stack
// protector pass to store the stack guard (see getStackCookieLocation()
// function above). Doing so requires changing the tcbhead_t struct in glibc
// on Linux and tcb struct in libc on FreeBSD.
// For now, store it in a thread-local variable.
extern "C" {
"default"))) __thread void *__safestack_unsafe_stack_ptr = nullptr;
namespace {
// TODO: The runtime library does not currently protect the safe stack beyond
// relying on the system-enforced ASLR. The protection of the (safe) stack can
// be provided by three alternative features:
// 1) Protection via hardware segmentation on x86-32 and some x86-64
// architectures: the (safe) stack segment (implicitly accessed via the %ss
// segment register) can be separated from the data segment (implicitly
// accessed via the %ds segment register). Dereferencing a pointer to the safe
// segment would result in a segmentation fault.
// 2) Protection via software fault isolation: memory writes that are not meant
// to access the safe stack can be prevented from doing so through runtime
// instrumentation. One way to do it is to allocate the safe stack(s) in the
// upper half of the userspace and bitmask the corresponding upper bit of the
// memory addresses of memory writes that are not meant to access the safe
// stack.
// 3) Protection via information hiding on 64 bit architectures: the location
// of the safe stack(s) can be randomized through secure mechanisms, and the
// leakage of the stack pointer can be prevented. Currently, libc can leak the
// stack pointer in several ways (e.g. in longjmp, signal handling, user-level
// context switching related functions, etc.). These can be fixed in libc and
// in other low-level libraries, by either eliminating the escaping/dumping of
// the stack pointer (i.e., %rsp) when that's possible, or by using
// encryption/PTR_MANGLE (XOR-ing the dumped stack pointer with another secret
// we control and protect better, as is already done for setjmp in glibc.)
// Furthermore, a static machine code level verifier can be ran after code
// generation to make sure that the stack pointer is never written to memory,
// or if it is, its written on the safe stack.
// Finally, while the Unsafe Stack pointer is currently stored in a thread
// local variable, with libc support it could be stored in the TCB (thread
// control block) as well, eliminating another level of indirection and making
// such accesses faster. Alternatively, dedicating a separate register for
// storing it would also be possible.
/// Minimum stack alignment for the unsafe stack.
const unsigned kStackAlign = 16;
/// Default size of the unsafe stack. This value is only used if the stack
/// size rlimit is set to infinity.
const unsigned kDefaultUnsafeStackSize = 0x2800000;
// Per-thread unsafe stack information. It's not frequently accessed, so there
// it can be kept out of the tcb in normal thread-local variables.
__thread void *unsafe_stack_start = nullptr;
__thread size_t unsafe_stack_size = 0;
__thread size_t unsafe_stack_guard = 0;
inline void *unsafe_stack_alloc(size_t size, size_t guard) {
SFS_CHECK(size + guard >= size);
void *addr = Mmap(nullptr, size + guard, PROT_READ | PROT_WRITE,
Mprotect(addr, guard, PROT_NONE);
return (char *)addr + guard;
inline void unsafe_stack_setup(void *start, size_t size, size_t guard) {
SFS_CHECK((char *)start + size >= (char *)start);
SFS_CHECK((char *)start + guard >= (char *)start);
void *stack_ptr = (char *)start + size;
SFS_CHECK((((size_t)stack_ptr) & (kStackAlign - 1)) == 0);
__safestack_unsafe_stack_ptr = stack_ptr;
unsafe_stack_start = start;
unsafe_stack_size = size;
unsafe_stack_guard = guard;
/// Thread data for the cleanup handler
pthread_key_t thread_cleanup_key;
/// Safe stack per-thread information passed to the thread_start function
struct tinfo {
void *(*start_routine)(void *);
void *start_routine_arg;
void *unsafe_stack_start;
size_t unsafe_stack_size;
size_t unsafe_stack_guard;
/// Wrap the thread function in order to deallocate the unsafe stack when the
/// thread terminates by returning from its main function.
void *thread_start(void *arg) {
struct tinfo *tinfo = (struct tinfo *)arg;
void *(*start_routine)(void *) = tinfo->start_routine;
void *start_routine_arg = tinfo->start_routine_arg;
// Setup the unsafe stack; this will destroy tinfo content
unsafe_stack_setup(tinfo->unsafe_stack_start, tinfo->unsafe_stack_size,
// Make sure out thread-specific destructor will be called
pthread_setspecific(thread_cleanup_key, (void *)1);
return start_routine(start_routine_arg);
/// Linked list used to store exiting threads stack/thread information.
struct thread_stack_ll {
struct thread_stack_ll *next;
void *stack_base;
size_t size;
pid_t pid;
ThreadId tid;
/// Linked list of unsafe stacks for threads that are exiting. We delay
/// unmapping them until the thread exits.
thread_stack_ll *thread_stacks = nullptr;
pthread_mutex_t thread_stacks_mutex = PTHREAD_MUTEX_INITIALIZER;
/// Thread-specific data destructor. We want to free the unsafe stack only after
/// this thread is terminated. libc can call functions in safestack-instrumented
/// code (like free) after thread-specific data destructors have run.
void thread_cleanup_handler(void *_iter) {
SFS_CHECK(unsafe_stack_start != nullptr);
pthread_setspecific(thread_cleanup_key, NULL);
// Temporary list to hold the previous threads stacks so we don't hold the
// thread_stacks_mutex for long.
thread_stack_ll *temp_stacks = thread_stacks;
thread_stacks = nullptr;
pid_t pid = getpid();
ThreadId tid = GetTid();
// Free stacks for dead threads
thread_stack_ll **stackp = &temp_stacks;
while (*stackp) {
thread_stack_ll *stack = *stackp;
if (stack->pid != pid ||
(-1 == TgKill(stack->pid, stack->tid, 0) && errno == ESRCH)) {
Munmap(stack->stack_base, stack->size);
*stackp = stack->next;
} else
stackp = &stack->next;
thread_stack_ll *cur_stack =
(thread_stack_ll *)malloc(sizeof(thread_stack_ll));
cur_stack->stack_base = (char *)unsafe_stack_start - unsafe_stack_guard;
cur_stack->size = unsafe_stack_size + unsafe_stack_guard;
cur_stack->pid = pid;
cur_stack->tid = tid;
// Merge thread_stacks with the current thread's stack and any remaining
// temp_stacks
*stackp = thread_stacks;
cur_stack->next = temp_stacks;
thread_stacks = cur_stack;
unsafe_stack_start = nullptr;
void EnsureInterceptorsInitialized();
/// Intercept thread creation operation to allocate and setup the unsafe stack
INTERCEPTOR(int, pthread_create, pthread_t *thread,
const pthread_attr_t *attr,
void *(*start_routine)(void*), void *arg) {
size_t size = 0;
size_t guard = 0;
if (attr) {
pthread_attr_getstacksize(attr, &size);
pthread_attr_getguardsize(attr, &guard);
} else {
// get pthread default stack size
pthread_attr_t tmpattr;
pthread_attr_getstacksize(&tmpattr, &size);
pthread_attr_getguardsize(&tmpattr, &guard);
size = RoundUpTo(size, kStackAlign);
void *addr = unsafe_stack_alloc(size, guard);
// Put tinfo at the end of the buffer. guard may be not page aligned.
// If that is so then some bytes after addr can be mprotected.
struct tinfo *tinfo =
(struct tinfo *)(((char *)addr) + size - sizeof(struct tinfo));
tinfo->start_routine = start_routine;
tinfo->start_routine_arg = arg;
tinfo->unsafe_stack_start = addr;
tinfo->unsafe_stack_size = size;
tinfo->unsafe_stack_guard = guard;
return REAL(pthread_create)(thread, attr, thread_start, tinfo);
pthread_mutex_t interceptor_init_mutex = PTHREAD_MUTEX_INITIALIZER;
bool interceptors_inited = false;
void EnsureInterceptorsInitialized() {
MutexLock lock(interceptor_init_mutex);
if (interceptors_inited)
// Initialize pthread interceptors for thread allocation
interceptors_inited = true;
} // namespace
extern "C" __attribute__((visibility("default")))
// On ELF platforms, the constructor is invoked using .preinit_array (see below)
void __safestack_init() {
// Determine the stack size for the main thread.
size_t size = kDefaultUnsafeStackSize;
size_t guard = 4096;
struct rlimit limit;
if (getrlimit(RLIMIT_STACK, &limit) == 0 && limit.rlim_cur != RLIM_INFINITY)
size = limit.rlim_cur;
// Allocate unsafe stack for main thread
void *addr = unsafe_stack_alloc(size, guard);
unsafe_stack_setup(addr, size, guard);
// Setup the cleanup handler
pthread_key_create(&thread_cleanup_key, thread_cleanup_handler);
// On ELF platforms, run safestack initialization before any other constructors.
// On other platforms we use the constructor attribute to arrange to run our
// initialization early.
extern "C" {
used)) void (*__safestack_preinit)(void) = __safestack_init;
extern "C"
__attribute__((visibility("default"))) void *__get_unsafe_stack_bottom() {
return unsafe_stack_start;
extern "C"
__attribute__((visibility("default"))) void *__get_unsafe_stack_top() {
return (char*)unsafe_stack_start + unsafe_stack_size;
extern "C"
__attribute__((visibility("default"))) void *__get_unsafe_stack_start() {
return unsafe_stack_start;
extern "C"
__attribute__((visibility("default"))) void *__get_unsafe_stack_ptr() {
return __safestack_unsafe_stack_ptr;