gh-108987: Fix _thread.start_new_thread() race condition (#109135)

Fix _thread.start_new_thread() race condition. If a thread is created
during Python finalization, the newly spawned thread now exits
immediately instead of trying to access freed memory and lead to a
crash.

thread_run() calls PyEval_AcquireThread() which checks if the thread
must exit. The problem was that tstate was dereferenced earlier in
_PyThreadState_Bind() which leads to a crash most of the time.

Move _PyThreadState_CheckConsistency() from thread_run() to
_PyThreadState_Bind().

1 files changed, 3 insertions, 25 deletions

diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c
index cef5317..3b7e6cb 100644
--- a/Python/ceval_gil.c
+++ b/Python/ceval_gil.c
@@ -329,28 +329,6 @@ drop_gil(struct _ceval_state *ceval, PyThreadState *tstate)
 }
 
 
-/* Check if a Python thread must exit immediately, rather than taking the GIL
-   if Py_Finalize() has been called.
-
-   When this function is called by a daemon thread after Py_Finalize() has been
-   called, the GIL does no longer exist.
-
-   tstate must be non-NULL. */
-static inline int
-tstate_must_exit(PyThreadState *tstate)
-{
-    /* bpo-39877: Access _PyRuntime directly rather than using
-       tstate->interp->runtime to support calls from Python daemon threads.
-       After Py_Finalize() has been called, tstate can be a dangling pointer:
-       point to PyThreadState freed memory. */
-    PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(&_PyRuntime);
-    if (finalizing == NULL) {
-        finalizing = _PyInterpreterState_GetFinalizing(tstate->interp);
-    }
-    return (finalizing != NULL && finalizing != tstate);
-}
-
-
 /* Take the GIL.
 
    The function saves errno at entry and restores its value at exit.
@@ -366,7 +344,7 @@ take_gil(PyThreadState *tstate)
     // XXX It may be more correct to check tstate->_status.finalizing.
     // XXX assert(!tstate->_status.cleared);
 
-    if (tstate_must_exit(tstate)) {
+    if (_PyThreadState_MustExit(tstate)) {
         /* bpo-39877: If Py_Finalize() has been called and tstate is not the
            thread which called Py_Finalize(), exit immediately the thread.
 
@@ -404,7 +382,7 @@ take_gil(PyThreadState *tstate)
             _Py_atomic_load_relaxed(&gil->locked) &&
             gil->switch_number == saved_switchnum)
         {
-            if (tstate_must_exit(tstate)) {
+            if (_PyThreadState_MustExit(tstate)) {
                 MUTEX_UNLOCK(gil->mutex);
                 // gh-96387: If the loop requested a drop request in a previous
                 // iteration, reset the request. Otherwise, drop_gil() can
@@ -444,7 +422,7 @@ _ready:
     MUTEX_UNLOCK(gil->switch_mutex);
 #endif
 
-    if (tstate_must_exit(tstate)) {
+    if (_PyThreadState_MustExit(tstate)) {
         /* bpo-36475: If Py_Finalize() has been called and tstate is not
            the thread which called Py_Finalize(), exit immediately the
            thread.