Segfault: Py_DECREF(NULL) in PyContextVar_Set (context.c:367)

[prakashsellathurai]

Crash report

What happened?

Segfault: Py_DECREF(NULL) in PyContextVar_Set (context.c:367)

PyContextVar_Set doesn't NULL-check token_new(); under OOM the contextvar_set() error path then Py_DECREFs the NULL token.

AI Disclaimer: this gist was drafted by Claude Code, which also generated the reduced reproducer.

Crash report

PyContextVar_Set does not NULL-check the result of token_new(). Under memory pressure token_new() returns NULL; if the following contextvar_set() also fails, the error path runs Py_DECREF(tok) on the NULL token and segfaults.

Reproducer

import contextvars
from _testcapi import set_nomemory, remove_mem_hooks
cv = contextvars.ContextVar("x")
for start in range(16):            # crashes at start=2
    set_nomemory(start, 0)
    try:
        try:
            cv.set(object())
        finally:
            remove_mem_hooks()
    except MemoryError:
        pass

A single set_nomemory(0, 0); cv.set(object()) does not crash — PyContextVar_Set's own early context_get() fails first and returns cleanly. The crash needs a budget where the early allocations succeed but token_new() and contextvar_set() then fail, hence the short sweep.

Backtrace

#0 _Py_atomic_load_uint32_relaxed
#1 Py_DECREF              Include/refcount.h:345    (op == 0x0)
#2 PyContextVar_Set       Python/context.c:367      (Py_DECREF(tok), tok == NULL)

Root cause

Python/context.c, PyContextVar_Set (L346):

    PyContextToken *tok = token_new(ctx, var, old_val);  /* L363: may return NULL, UNCHECKED */
    Py_XDECREF(old_val);
    if (contextvar_set(var, val)) {                      /* L366: fails under OOM (_PyHamt_Assoc -> NULL) */
        Py_DECREF(tok);                                  /* L367: tok == NULL -> SIGSEGV */
        return NULL;
    }
    return (PyObject *)tok;

token_new() allocates (PyObject_GC_New) and can return NULL on failure, but tok is not checked before the Py_DECREF(tok) on the contextvar_set() failure path. (On the success path the function also returns tok unchecked — harmless NULL-with-exception, but the modification still happened.)

Suggested fix

    PyContextToken *tok = token_new(ctx, var, old_val);
    Py_XDECREF(old_val);
    if (tok == NULL) {
        return NULL;
    }

(or Py_XDECREF(tok) on the error path).

Notes

Found by OOM-injection fuzzing (set_nomemory). Same pattern as gh-146080 (Py_DECREF(NULL) in an _ssl.c error label). Not free-threading-specific: reproduces on free-threaded and default (GIL) builds, debug and release. (A _py_warnings/ContextVar vehicle reaches this only when context_aware_warnings is enabled — the default on free-threaded builds — but the C bug is build-agnostic.)

Versions

• main (3.16.0a0); reproduced on free-threaded debug+ASan, free-threaded release, and GIL release builds.

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Part of python/cpython#151763 — an umbrella tracking 35 OOM-related crash findings.

CPython versions tested on:

3.16, CPython main branch

Operating systems tested on:

Windows

Output from running 'python -VV' on the command line:

Python 3.9.23 (main, Jun 4 2025, 08:55:39) [GCC 13.3.0]