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().

Include/internal/pycore_pystate.h

@@ -71,6 +71,8 @@ extern _Py_thread_local PyThreadState *_Py_tss_tstate;
 extern int _PyThreadState_CheckConsistency(PyThreadState *tstate);
 #endif
 
+int _PyThreadState_MustExit(PyThreadState *tstate);
+
 // Export for most shared extensions, used via _PyThreadState_GET() static
 // inline function.
 PyAPI_FUNC(PyThreadState *) _PyThreadState_GetCurrent(void);

Misc/NEWS.d/next/Library/2023-09-08-12-09-55.gh-issue-108987.x5AIG8.rst

@@ -0,0 +1,4 @@
+Fix :func:`_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. Patch by
+Victor Stinner.

Modules/_threadmodule.c

@@ -1051,21 +1051,21 @@ _localdummy_destroyed(PyObject *localweakref, PyObject *dummyweakref)
 /* Module functions */
 
 struct bootstate {
-    PyInterpreterState *interp;
+    PyThreadState *tstate;
     PyObject *func;
     PyObject *args;
     PyObject *kwargs;
-    PyThreadState *tstate;
-    _PyRuntimeState *runtime;
 };
 
 
 static void
-thread_bootstate_free(struct bootstate *boot)
+thread_bootstate_free(struct bootstate *boot, int decref)
 {
-    Py_DECREF(boot->func);
+    if (decref) {
-    Py_DECREF(boot->args);
+        Py_DECREF(boot->func);
-    Py_XDECREF(boot->kwargs);
+        Py_DECREF(boot->args);
+        Py_XDECREF(boot->kwargs);
+    }
     PyMem_Free(boot);
 }
 
@@ -1076,9 +1076,24 @@ thread_run(void *boot_raw)
     struct bootstate *boot = (struct bootstate *) boot_raw;
     PyThreadState *tstate = boot->tstate;
 
-    // gh-104690: If Python is being finalized and PyInterpreterState_Delete()
+    // gh-108987: If _thread.start_new_thread() is called before or while
-    // was called, tstate becomes a dangling pointer.
+    // Python is being finalized, thread_run() can called *after*.
-    assert(_PyThreadState_CheckConsistency(tstate));
+    // _PyRuntimeState_SetFinalizing() is called. At this point, all Python
+    // threads must exit, except of the thread calling Py_Finalize() whch holds
+    // the GIL and must not exit.
+    //
+    // At this stage, tstate can be a dangling pointer (point to freed memory),
+    // it's ok to call _PyThreadState_MustExit() with a dangling pointer.
+    if (_PyThreadState_MustExit(tstate)) {
+        // Don't call PyThreadState_Clear() nor _PyThreadState_DeleteCurrent().
+        // These functions are called on tstate indirectly by Py_Finalize()
+        // which calls _PyInterpreterState_Clear().
+        //
+        // Py_DECREF() cannot be called because the GIL is not held: leak
+        // references on purpose. Python is being finalized anyway.
+        thread_bootstate_free(boot, 0);
+        goto exit;
+    }
 
     _PyThreadState_Bind(tstate);
     PyEval_AcquireThread(tstate);
@@ -1097,14 +1112,17 @@ thread_run(void *boot_raw)
         Py_DECREF(res);
     }
 
-    thread_bootstate_free(boot);
+    thread_bootstate_free(boot, 1);
+
     tstate->interp->threads.count--;
     PyThreadState_Clear(tstate);
     _PyThreadState_DeleteCurrent(tstate);
 
+exit:
     // bpo-44434: Don't call explicitly PyThread_exit_thread(). On Linux with
     // the glibc, pthread_exit() can abort the whole process if dlopen() fails
     // to open the libgcc_s.so library (ex: EMFILE error).
+    return;
 }
 
 static PyObject *
@@ -1128,7 +1146,6 @@ and False otherwise.\n");
 static PyObject *
 thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
 {
-    _PyRuntimeState *runtime = &_PyRuntime;
     PyObject *func, *args, *kwargs = NULL;
 
     if (!PyArg_UnpackTuple(fargs, "start_new_thread", 2, 3,
@@ -1171,8 +1188,7 @@ thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
     if (boot == NULL) {
         return PyErr_NoMemory();
     }
-    boot->interp = _PyInterpreterState_GET();
+    boot->tstate = _PyThreadState_New(interp);
-    boot->tstate = _PyThreadState_New(boot->interp);
     if (boot->tstate == NULL) {
         PyMem_Free(boot);
         if (!PyErr_Occurred()) {
@@ -1180,7 +1196,6 @@ thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
         }
         return NULL;
     }
-    boot->runtime = runtime;
     boot->func = Py_NewRef(func);
     boot->args = Py_NewRef(args);
     boot->kwargs = Py_XNewRef(kwargs);
@@ -1189,7 +1204,7 @@ thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
     if (ident == PYTHREAD_INVALID_THREAD_ID) {
         PyErr_SetString(ThreadError, "can't start new thread");
         PyThreadState_Clear(boot->tstate);
-        thread_bootstate_free(boot);
+        thread_bootstate_free(boot, 1);
         return NULL;
     }
     return PyLong_FromUnsignedLong(ident);

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.

Python/pystate.c

@@ -1907,6 +1907,10 @@ PyThreadState_Swap(PyThreadState *newts)
 void
 _PyThreadState_Bind(PyThreadState *tstate)
 {
+    // gh-104690: If Python is being finalized and PyInterpreterState_Delete()
+    // was called, tstate becomes a dangling pointer.
+    assert(_PyThreadState_CheckConsistency(tstate));
+
     bind_tstate(tstate);
     // This makes sure there's a gilstate tstate bound
     // as soon as possible.
@@ -2908,6 +2912,31 @@ _PyThreadState_CheckConsistency(PyThreadState *tstate)
 #endif
 
 
+// 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 can be a dangling pointer (point to freed memory): only tstate value
+// is used, the pointer is not deferenced.
+//
+// tstate must be non-NULL.
+int
+_PyThreadState_MustExit(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);
+}
+
+
 #ifdef __cplusplus
 }
 #endif