Possible resource exhaustation of `expandvars` in `Tools/freeze/checkextensions.py`

Bug Description:
The code will re-scan the newly generated content after replacing the variable. If a variable is defined to contain itself (i.e., a recursive definition), this process will fall into infinite recursion.

This recursion can lead to two forms of attacks: one is to cause an infinite loop and deplete CPU resources, and the other is to cause the string to grow infinitely and deplete memory resources. The final result will all lead to the application crashing or being unresponsive.

Vulnerability Locations :
1.https://github.com/python/cpython/blob/e64395e8eb8d3a9e35e3e534e87d427ff27ab0a5/Tools/freeze/checkextensions.py#L72

Repair Status:
None

Common Information:

• CPython Version: main branch
• Operating System: Linux
• Credits: Finder is kexinoh (Xiangfan Wu) from QI-ANXIN Technology Research Institute.

Linked PRs

• gh-136093

從 Tools/freeze/checkextensions.py 約第 72 行開始，這裡 expandvars 會在變數替換後重新掃描產生的內容。重現一個自我參照的變數定義，並檢查連結的 gh-136093 工作；當遞迴展開不再耗盡 CPU 或記憶體，且現有行為維持不變時，即表示完成。