absl: fix use-after-free in Mutex/CondVar

Both Mutex and CondVar signal PerThreadSem/Waiter after satisfying the wait condition,
as the result the waiting thread may return w/o waiting on the
PerThreadSem/Waiter at all. If the waiting thread then exits, it currently
destroys Waiter object. As the result Waiter::Post can be called on
already destroyed object.
PerThreadSem/Waiter must be type-stable after creation and must not be destroyed.
The futex-based implementation is the only one that is not affected by the bug
since there is effectively nothing to destroy (maybe only UBSan/ASan
could complain about calling methods on a destroyed object).

Here is the problematic sequence of events:

1: void Mutex::Block(PerThreadSynch *s) {
2:   while (s->state.load(std::memory_order_acquire) == PerThreadSynch::kQueued) {
3:     if (!DecrementSynchSem(this, s, s->waitp->timeout)) {

4: PerThreadSynch *Mutex::Wakeup(PerThreadSynch *w) {
5:   ...
6:   w->state.store(PerThreadSynch::kAvailable, std::memory_order_release);
7:   IncrementSynchSem(this, w);
8:   ...
9: }

Consider line 6 is executed, then line 2 observes kAvailable and
line 3 is not called. The thread executing Mutex::Block returns from
the method, acquires the mutex, releases the mutex, exits and destroys
PerThreadSem/Waiter.
Now Mutex::Wakeup resumes and executes line 7 on the destroyed object. Boom!

CondVar uses a similar pattern.

Moreover the semaphore-based Waiter implementation is not even destruction-safe
(the Waiter cannot be used to signal own destruction). So even if Mutex/CondVar
would always pair Waiter::Post with Waiter::Wait before destroying PerThreadSem/Waiter,
it would still be subject to use-after-free bug on the semaphore.

PiperOrigin-RevId: 449159939
Change-Id: I497134fa8b6ce1294a422827c5f0de0e897cea31

1 files changed, 1 insertions, 6 deletions

diff --git a/absl/synchronization/internal/per_thread_sem.h b/absl/synchronization/internal/per_thread_sem.h
index 7beae8ef..90a88809 100644
--- a/absl/synchronization/internal/per_thread_sem.h
+++ b/absl/synchronization/internal/per_thread_sem.h
@@ -66,10 +66,6 @@ class PerThreadSem {
   // REQUIRES: May only be called by ThreadIdentity.
   static void Init(base_internal::ThreadIdentity* identity);
 
-  // Destroy the PerThreadSem associated with "identity".
-  // REQUIRES: May only be called by ThreadIdentity.
-  static void Destroy(base_internal::ThreadIdentity* identity);
-
   // Increments "identity"'s count.
   static inline void Post(base_internal::ThreadIdentity* identity);
 
@@ -81,8 +77,7 @@ class PerThreadSem {
   // Permitted callers.
   friend class PerThreadSemTest;
   friend class absl::Mutex;
-  friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
-  friend void ReclaimThreadIdentity(void* v);
+  friend void OneTimeInitThreadIdentity(absl::base_internal::ThreadIdentity*);
 };
 
 }  // namespace synchronization_internal