`_decimal::CURRENT_CONTEXT`: UAF via borrowed reference across Python callbacks

Crash report

Originally reported by @PidSS as a security advisory

What happened?

The CURRENT_CONTEXT() macro in Modules/_decimal/_decimal.c:1951 obtains a new reference from PyContextVar_Get(), then immediately calls Py_DECREF, leaving a borrowed reference. If Python code executes before the context pointer is next used — via warning handlers, GC finalizers, instancecheck, etc. — and that code calls decimal.setcontext(), the old context is freed while C code still holds a dangling pointer. The macro is used at 20+ call sites covering virtually every decimal operation.

Original reproducer (I will try to make a shorter one):

import decimal, warnings, gc, sys, ctypes

gc.disable()
ctx = decimal.getcontext()
CTX_ID, CTX_SIZE = id(ctx), sys.getsizeof(ctx)
del ctx  # refcount = 1, held only by ContextVar HAMT

def evil_warning_handler(message, category, filename, lineno, file=None, line=None):
    if 'Format specifier' not in str(message):
        return
    ctx = decimal.getcontext()                       # refcount -> 2
    decimal.setcontext(decimal.Context(prec=999))    # HAMT ref dropped -> 1
    del ctx                                          # refcount -> 0, FREED
    spray = [bytearray(b'\xCC' * (CTX_SIZE - 32)) for _ in range(100000)]

warnings.showwarning = evil_warning_handler
warnings.filterwarnings('always', category=DeprecationWarning)
format(decimal.Decimal('123.456'), 'N')

FTR, I saw that we have this macro twice, but it depends on whether we have WITH_DECIMAL_CONTEXTVAR. If I build Python with --without-decimal-contextvar, I also have a crash so it's a bit weird.

Related but not with the same root cause: https://github.com/python/cpython/issues/146011.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

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

No response

Empieza en Modules/_decimal/_decimal.c, en la macro CURRENT_CONTEXT alrededor de la línea 1951; después, inspecciona sus más de 20 puntos de llamada y la segunda macro que se usa sin WITH_DECIMAL_CONTEXTVAR. Ejecuta el reproducer suministrado del warning handler en una build main de CPython. Se considera terminado cuando la ruta de callback reproducida ya no use un contexto liberado en ninguna de las dos configuraciones.