windows: Avoid potential races with canceling threads

The previous code did not synchronize some of the flags, so it was
possible that `cancel()` did not work properly (e.g. no APC queued).

This change uses atomics to set/read the flags, it also avoids using
the global thread lock for synchronization as getting the condvar before
queuing the APC avoids having to sync with `docancel()`.  However, to
make sure we don't use a stale and potentially already freed condvar
when the thread is canceled a new lock is added and held until the
condvar has been signaled (using atomics for that member doesn't work as
the ordering rules enforced by atomics can not avoid that the thread in
`cancel()` potentially reads a stale value, or the race between reading
and signaling it).

diff --git a/src/libstrongswan/threading/windows/thread.c b/src/libstrongswan/threading/windows/thread.c
index 03925c9f417290f38640aca7638312e45e5d6b28..a2c8dfe1946c01a4d99a816dd7dc3b5a7a42892a 100644 (file)
--- a/src/libstrongswan/threading/windows/thread.c
+++ b/src/libstrongswan/threading/windows/thread.c
@@ -109,6 +109,11 @@ static hashtable_t *threads;
  */
 static spinlock_t *threads_lock;
 
+/**
+ * Lock for setting condvars on threads
+ */
+static spinlock_t *condvar_lock;
+
 /**
  * Counter to assign printable thread IDs
  */
@@ -325,9 +330,9 @@ void thread_set_active_condvar(CONDITION_VARIABLE *condvar)
 
        thread = get_current_thread();
 
-       threads_lock->lock(threads_lock);
+       condvar_lock->lock(condvar_lock);
        thread->condvar = condvar;
-       threads_lock->unlock(threads_lock);
+       condvar_lock->unlock(condvar_lock);
 
        /* this is a cancellation point, as condvar wait is one */
        SleepEx(0, TRUE);
@@ -340,10 +345,6 @@ static void WINAPI docancel(ULONG_PTR dwParam)
 {
        private_thread_t *this = (private_thread_t*)dwParam;
 
-       /* make sure cancel() does not access this anymore */
-       threads_lock->lock(threads_lock);
-       threads_lock->unlock(threads_lock);
-
        end_thread(this);
        ExitThread(0);
 }
@@ -351,20 +352,20 @@ static void WINAPI docancel(ULONG_PTR dwParam)
 METHOD(thread_t, cancel, void,
        private_thread_t *this)
 {
-       this->canceled = TRUE;
-       if (this->cancelability)
+       atomic_set_bool(&this->canceled, TRUE);
+       if (atomic_get_bool(&this->cancelability) &&
+               cas_bool(&this->cancel_pending, FALSE, TRUE))
        {
-               threads_lock->lock(threads_lock);
-               if (!this->cancel_pending)
+               CONDITION_VARIABLE *cv;
+
+               condvar_lock->lock(condvar_lock);
+               cv = this->condvar;
+               QueueUserAPC(docancel, this->handle, (uintptr_t)this);
+               if (cv)
                {
-                       this->cancel_pending = TRUE;
-                       QueueUserAPC(docancel, this->handle, (uintptr_t)this);
-                       if (this->condvar)
-                       {
-                               WakeAllConditionVariable(this->condvar);
-                       }
+                       WakeAllConditionVariable(cv);
                }
-               threads_lock->unlock(threads_lock);
+               condvar_lock->unlock(condvar_lock);
        }
 }
 
@@ -408,10 +409,10 @@ METHOD(thread_t, join, void*,
 static DWORD thread_cb(private_thread_t *this)
 {
        /* Enable cancelability once the thread starts. We must check for any
-        * pending cancellation request an queue the APC that gets executed
+        * pending cancellation request and queue the APC that gets executed
         * at the first cancellation point. */
-       this->cancelability = TRUE;
-       if (this->canceled)
+       atomic_set_bool(&this->cancelability, TRUE);
+       if (atomic_get_bool(&this->canceled))
        {
                cancel(this);
        }
@@ -579,10 +580,10 @@ bool thread_cancelability(bool enable)
        bool old;
 
        this = get_current_thread();
-       old = this->cancelability;
-       this->cancelability = enable;
+       old = atomic_get_bool(&this->cancelability);
+       atomic_set_bool(&this->cancelability, enable);
 
-       if (enable && !old && this->canceled)
+       if (enable && !old && atomic_get_bool(&this->canceled))
        {
                cancel(this);
        }
@@ -664,6 +665,7 @@ BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved)
 void threads_init()
 {
        threads_lock = spinlock_create();
+       condvar_lock = spinlock_create();
        threads = hashtable_create(hashtable_hash_ptr, hashtable_equals_ptr, 4);
 
        /* reset counter should we initialize more than once */
@@ -686,5 +688,7 @@ void threads_deinit()
 
        threads_lock->destroy(threads_lock);
        threads_lock = NULL;
+       condvar_lock->destroy(condvar_lock);
+       condvar_lock = NULL;
        threads->destroy(threads);
 }