1 //===-- safestack.cc ------------------------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the runtime support for the safe stack protection
11 // mechanism. The runtime manages allocation/deallocation of the unsafe stack
12 // for the main thread, as well as all pthreads that are created/destroyed
13 // during program execution.
15 //===----------------------------------------------------------------------===//
24 #include <sys/resource.h>
25 #include <sys/types.h>
26 #if !defined(__NetBSD__)
30 #include "interception/interception.h"
31 #include "sanitizer_common/sanitizer_common.h"
33 // TODO: The runtime library does not currently protect the safe stack beyond
34 // relying on the system-enforced ASLR. The protection of the (safe) stack can
35 // be provided by three alternative features:
37 // 1) Protection via hardware segmentation on x86-32 and some x86-64
38 // architectures: the (safe) stack segment (implicitly accessed via the %ss
39 // segment register) can be separated from the data segment (implicitly
40 // accessed via the %ds segment register). Dereferencing a pointer to the safe
41 // segment would result in a segmentation fault.
43 // 2) Protection via software fault isolation: memory writes that are not meant
44 // to access the safe stack can be prevented from doing so through runtime
45 // instrumentation. One way to do it is to allocate the safe stack(s) in the
46 // upper half of the userspace and bitmask the corresponding upper bit of the
47 // memory addresses of memory writes that are not meant to access the safe
50 // 3) Protection via information hiding on 64 bit architectures: the location
51 // of the safe stack(s) can be randomized through secure mechanisms, and the
52 // leakage of the stack pointer can be prevented. Currently, libc can leak the
53 // stack pointer in several ways (e.g. in longjmp, signal handling, user-level
54 // context switching related functions, etc.). These can be fixed in libc and
55 // in other low-level libraries, by either eliminating the escaping/dumping of
56 // the stack pointer (i.e., %rsp) when that's possible, or by using
57 // encryption/PTR_MANGLE (XOR-ing the dumped stack pointer with another secret
58 // we control and protect better, as is already done for setjmp in glibc.)
59 // Furthermore, a static machine code level verifier can be ran after code
60 // generation to make sure that the stack pointer is never written to memory,
61 // or if it is, its written on the safe stack.
63 // Finally, while the Unsafe Stack pointer is currently stored in a thread
64 // local variable, with libc support it could be stored in the TCB (thread
65 // control block) as well, eliminating another level of indirection and making
66 // such accesses faster. Alternatively, dedicating a separate register for
67 // storing it would also be possible.
69 /// Minimum stack alignment for the unsafe stack.
70 const unsigned kStackAlign = 16;
72 /// Default size of the unsafe stack. This value is only used if the stack
73 /// size rlimit is set to infinity.
74 const unsigned kDefaultUnsafeStackSize = 0x2800000;
76 /// Runtime page size obtained through sysconf
77 static unsigned pageSize;
79 // TODO: To make accessing the unsafe stack pointer faster, we plan to
80 // eventually store it directly in the thread control block data structure on
81 // platforms where this structure is pointed to by %fs or %gs. This is exactly
82 // the same mechanism as currently being used by the traditional stack
83 // protector pass to store the stack guard (see getStackCookieLocation()
84 // function above). Doing so requires changing the tcbhead_t struct in glibc
85 // on Linux and tcb struct in libc on FreeBSD.
87 // For now, store it in a thread-local variable.
89 __attribute__((visibility(
90 "default"))) __thread void *__safestack_unsafe_stack_ptr = nullptr;
93 // Per-thread unsafe stack information. It's not frequently accessed, so there
94 // it can be kept out of the tcb in normal thread-local variables.
95 static __thread void *unsafe_stack_start = nullptr;
96 static __thread size_t unsafe_stack_size = 0;
97 static __thread size_t unsafe_stack_guard = 0;
99 using namespace __sanitizer;
101 static inline void *unsafe_stack_alloc(size_t size, size_t guard) {
102 CHECK_GE(size + guard, size);
103 void *addr = MmapOrDie(size + guard, "unsafe_stack_alloc");
104 MprotectNoAccess((uptr)addr, (uptr)guard);
105 return (char *)addr + guard;
108 static inline void unsafe_stack_setup(void *start, size_t size, size_t guard) {
109 CHECK_GE((char *)start + size, (char *)start);
110 CHECK_GE((char *)start + guard, (char *)start);
111 void *stack_ptr = (char *)start + size;
112 CHECK_EQ((((size_t)stack_ptr) & (kStackAlign - 1)), 0);
114 __safestack_unsafe_stack_ptr = stack_ptr;
115 unsafe_stack_start = start;
116 unsafe_stack_size = size;
117 unsafe_stack_guard = guard;
120 /// Thread data for the cleanup handler
121 static pthread_key_t thread_cleanup_key;
123 /// Safe stack per-thread information passed to the thread_start function
125 void *(*start_routine)(void *);
126 void *start_routine_arg;
128 void *unsafe_stack_start;
129 size_t unsafe_stack_size;
130 size_t unsafe_stack_guard;
133 /// Wrap the thread function in order to deallocate the unsafe stack when the
134 /// thread terminates by returning from its main function.
135 static void *thread_start(void *arg) {
136 struct tinfo *tinfo = (struct tinfo *)arg;
138 void *(*start_routine)(void *) = tinfo->start_routine;
139 void *start_routine_arg = tinfo->start_routine_arg;
141 // Setup the unsafe stack; this will destroy tinfo content
142 unsafe_stack_setup(tinfo->unsafe_stack_start, tinfo->unsafe_stack_size,
143 tinfo->unsafe_stack_guard);
145 // Make sure out thread-specific destructor will be called
146 pthread_setspecific(thread_cleanup_key, (void *)1);
148 return start_routine(start_routine_arg);
151 /// Linked list used to store exiting threads stack/thread information.
152 struct thread_stack_ll {
153 struct thread_stack_ll *next;
160 /// Linked list of unsafe stacks for threads that are exiting. We delay
161 /// unmapping them until the thread exits.
162 static thread_stack_ll *thread_stacks = nullptr;
163 static pthread_mutex_t thread_stacks_mutex = PTHREAD_MUTEX_INITIALIZER;
165 /// Thread-specific data destructor. We want to free the unsafe stack only after
166 /// this thread is terminated. libc can call functions in safestack-instrumented
167 /// code (like free) after thread-specific data destructors have run.
168 static void thread_cleanup_handler(void *_iter) {
169 CHECK_NE(unsafe_stack_start, nullptr);
170 pthread_setspecific(thread_cleanup_key, NULL);
172 pthread_mutex_lock(&thread_stacks_mutex);
173 // Temporary list to hold the previous threads stacks so we don't hold the
174 // thread_stacks_mutex for long.
175 thread_stack_ll *temp_stacks = thread_stacks;
176 thread_stacks = nullptr;
177 pthread_mutex_unlock(&thread_stacks_mutex);
179 pid_t pid = getpid();
180 tid_t tid = GetTid();
182 // Free stacks for dead threads
183 thread_stack_ll **stackp = &temp_stacks;
185 thread_stack_ll *stack = *stackp;
187 if (stack->pid != pid ||
188 (internal_iserror(TgKill(stack->pid, stack->tid, 0), &error) &&
190 UnmapOrDie(stack->stack_base, stack->size);
191 *stackp = stack->next;
194 stackp = &stack->next;
197 thread_stack_ll *cur_stack =
198 (thread_stack_ll *)malloc(sizeof(thread_stack_ll));
199 cur_stack->stack_base = (char *)unsafe_stack_start - unsafe_stack_guard;
200 cur_stack->size = unsafe_stack_size + unsafe_stack_guard;
201 cur_stack->pid = pid;
202 cur_stack->tid = tid;
204 pthread_mutex_lock(&thread_stacks_mutex);
205 // Merge thread_stacks with the current thread's stack and any remaining
207 *stackp = thread_stacks;
208 cur_stack->next = temp_stacks;
209 thread_stacks = cur_stack;
210 pthread_mutex_unlock(&thread_stacks_mutex);
212 unsafe_stack_start = nullptr;
215 static void EnsureInterceptorsInitialized();
217 /// Intercept thread creation operation to allocate and setup the unsafe stack
218 INTERCEPTOR(int, pthread_create, pthread_t *thread,
219 const pthread_attr_t *attr,
220 void *(*start_routine)(void*), void *arg) {
221 EnsureInterceptorsInitialized();
226 pthread_attr_getstacksize(attr, &size);
227 pthread_attr_getguardsize(attr, &guard);
229 // get pthread default stack size
230 pthread_attr_t tmpattr;
231 pthread_attr_init(&tmpattr);
232 pthread_attr_getstacksize(&tmpattr, &size);
233 pthread_attr_getguardsize(&tmpattr, &guard);
234 pthread_attr_destroy(&tmpattr);
238 CHECK_EQ((size & (kStackAlign - 1)), 0);
239 CHECK_EQ((guard & (pageSize - 1)), 0);
241 void *addr = unsafe_stack_alloc(size, guard);
242 struct tinfo *tinfo =
243 (struct tinfo *)(((char *)addr) + size - sizeof(struct tinfo));
244 tinfo->start_routine = start_routine;
245 tinfo->start_routine_arg = arg;
246 tinfo->unsafe_stack_start = addr;
247 tinfo->unsafe_stack_size = size;
248 tinfo->unsafe_stack_guard = guard;
250 return REAL(pthread_create)(thread, attr, thread_start, tinfo);
253 static BlockingMutex interceptor_init_lock(LINKER_INITIALIZED);
254 static bool interceptors_inited = false;
256 static void EnsureInterceptorsInitialized() {
257 BlockingMutexLock lock(&interceptor_init_lock);
258 if (interceptors_inited) return;
260 // Initialize pthread interceptors for thread allocation
261 INTERCEPT_FUNCTION(pthread_create);
263 interceptors_inited = true;
266 extern "C" __attribute__((visibility("default")))
267 #if !SANITIZER_CAN_USE_PREINIT_ARRAY
268 // On ELF platforms, the constructor is invoked using .preinit_array (see below)
269 __attribute__((constructor(0)))
271 void __safestack_init() {
272 // Determine the stack size for the main thread.
273 size_t size = kDefaultUnsafeStackSize;
277 if (getrlimit(RLIMIT_STACK, &limit) == 0 && limit.rlim_cur != RLIM_INFINITY)
278 size = limit.rlim_cur;
280 // Allocate unsafe stack for main thread
281 void *addr = unsafe_stack_alloc(size, guard);
283 unsafe_stack_setup(addr, size, guard);
284 pageSize = sysconf(_SC_PAGESIZE);
286 // Setup the cleanup handler
287 pthread_key_create(&thread_cleanup_key, thread_cleanup_handler);
290 #if SANITIZER_CAN_USE_PREINIT_ARRAY
291 // On ELF platforms, run safestack initialization before any other constructors.
292 // On other platforms we use the constructor attribute to arrange to run our
293 // initialization early.
295 __attribute__((section(".preinit_array"),
296 used)) void (*__safestack_preinit)(void) = __safestack_init;
301 __attribute__((visibility("default"))) void *__get_unsafe_stack_bottom() {
302 return unsafe_stack_start;
306 __attribute__((visibility("default"))) void *__get_unsafe_stack_top() {
307 return (char*)unsafe_stack_start + unsafe_stack_size;
311 __attribute__((visibility("default"))) void *__get_unsafe_stack_start() {
312 return unsafe_stack_start;
316 __attribute__((visibility("default"))) void *__get_unsafe_stack_ptr() {
317 return __safestack_unsafe_stack_ptr;