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+//===-- safestack.cpp -----------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt 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" {
+__attribute__((visibility(
+ "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,
+ MAP_PRIVATE | MAP_ANON, -1, 0);
+ SFS_CHECK(MAP_FAILED != addr);
+ 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,
+ tinfo->unsafe_stack_guard);
+
+ // 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);
+
+ pthread_mutex_lock(&thread_stacks_mutex);
+ // 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;
+ pthread_mutex_unlock(&thread_stacks_mutex);
+
+ 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;
+ free(stack);
+ } 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;
+
+ pthread_mutex_lock(&thread_stacks_mutex);
+ // 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;
+ pthread_mutex_unlock(&thread_stacks_mutex);
+
+ 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) {
+ EnsureInterceptorsInitialized();
+ 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_init(&tmpattr);
+ pthread_attr_getstacksize(&tmpattr, &size);
+ pthread_attr_getguardsize(&tmpattr, &guard);
+ pthread_attr_destroy(&tmpattr);
+ }
+
+ SFS_CHECK(size);
+ 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)
+ return;
+
+ // Initialize pthread interceptors for thread allocation
+ INTERCEPT_FUNCTION(pthread_create);
+
+ interceptors_inited = true;
+}
+
+} // namespace
+
+extern "C" __attribute__((visibility("default")))
+#if !SANITIZER_CAN_USE_PREINIT_ARRAY
+// On ELF platforms, the constructor is invoked using .preinit_array (see below)
+__attribute__((constructor(0)))
+#endif
+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);
+}
+
+#if SANITIZER_CAN_USE_PREINIT_ARRAY
+// 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" {
+__attribute__((section(".preinit_array"),
+ used)) void (*__safestack_preinit)(void) = __safestack_init;
+}
+#endif
+
+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;
+}