[3.13] gh-117657: Fix QSBR race condition (GH-118843) (#118905)

`_Py_qsbr_unregister` is called when the PyThreadState is already
detached, so the access to `tstate->qsbr` isn't safe without locking the
shared mutex. Grab the `struct _qsbr_shared` from the interpreter
instead.
(cherry picked from commit 33d20199af65c741bdc908a968edd8dc179b6974)

Co-authored-by: Alex Turner <alexturner@meta.com>

2 files changed, 7 insertions, 6 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index de6a768..0832b37 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1794,7 +1794,7 @@ tstate_delete_common(PyThreadState *tstate)
     HEAD_UNLOCK(runtime);
 
 #ifdef Py_GIL_DISABLED
-    _Py_qsbr_unregister((_PyThreadStateImpl *)tstate);
+    _Py_qsbr_unregister(tstate);
 #endif
 
     // XXX Unbind in PyThreadState_Clear(), or earlier
diff --git a/Python/qsbr.c b/Python/qsbr.c
index d7ac8f4..1e02ff9 100644
--- a/Python/qsbr.c
+++ b/Python/qsbr.c
@@ -231,20 +231,21 @@ _Py_qsbr_register(_PyThreadStateImpl *tstate, PyInterpreterState *interp,
 }
 
 void
-_Py_qsbr_unregister(_PyThreadStateImpl *tstate)
+_Py_qsbr_unregister(PyThreadState *tstate)
 {
-    struct _qsbr_shared *shared = tstate->qsbr->shared;
+    struct _qsbr_shared *shared = &tstate->interp->qsbr;
+    struct _PyThreadStateImpl *tstate_imp = (_PyThreadStateImpl*) tstate;
 
     PyMutex_Lock(&shared->mutex);
     // NOTE: we must load (or reload) the thread state's qbsr inside the mutex
     // because the array may have been resized (changing tstate->qsbr) while
     // we waited to acquire the mutex.
-    struct _qsbr_thread_state *qsbr = tstate->qsbr;
+    struct _qsbr_thread_state *qsbr = tstate_imp->qsbr;
 
     assert(qsbr->seq == 0 && "thread state must be detached");
-    assert(qsbr->allocated && qsbr->tstate == (PyThreadState *)tstate);
+    assert(qsbr->allocated && qsbr->tstate == tstate);
 
-    tstate->qsbr = NULL;
+    tstate_imp->qsbr = NULL;
     qsbr->tstate = NULL;
     qsbr->allocated = false;
     qsbr->freelist_next = shared->freelist;