bytearray.strip() reads freed memory if the argument's __release_buffer__ resizes the bytearray

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難度
4/5
預估耗時
3-5 天
新手友好度
25/100
Issue 類型
缺陷
描述清晰度
描述清楚
活躍度
停滯
技術堆疊
c, python
領域
backend

研究方向

Read the strip implementation at Objects/bytearrayobject.c:2460-2462 and inspect the existing bytearray strip tests. Check how buffer release can run Python code and resize the bytearray before the result is copied. Done when strip(), lstrip(), and rstrip() avoid the reported crash and return bytes consistent with the bounds they computed.

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描述

interpreter-core type-crash
Crash report

bytearray.strip(chars) (and lstrip/rstrip) takes a pointer into the bytearray, finds the bounds, then releases the argument's buffer, and only then copies the bytes. If the argument's class defines __release_buffer__ (PEP 688), that release runs Python code, which can resize the bytearray and free the storage the pointer refers to.

ba = bytearray(b"x" * (1 << 22))

class S:
    def __buffer__(self, flags):
        return memoryview(b" ")
    def __release_buffer__(self, view):
        ba.clear()

ba.strip(S())   # segfault

Output: Segmentation fault, the same for lstrip and rstrip. With a small bytearray and an in-place write instead of clear(), the result is built from the modified bytes, not the ones the bounds were computed on.

Cause: Objects/bytearrayobject.c:2460-2462 calls PyBuffer_Release(&vbytes) before PyByteArray_FromStringAndSize(myptr + left, right - left). Copying first and releasing after would fix it.

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

Linux

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

Python 3.15.2 (tags/v3.15.2:5e58eaa, Nov 5 2026, 20:20:36) [GCC 11.4.0]

Found while building a Lean 4 model of CPython (https://broyojo.com/python-formalization/). AI tools (Claude and Codex) helped investigate the findings and prepare this report. The examples were rerun on CPython 3.15.2 and main, and the report received technical review. Record: bytesobj-bytearray-strip-release-uaf

Linked PRs
  • gh-161701
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