shutil.copy2 race condition leading to local file disclosure

Bug report

shutil.copy2 first copies the contents (creating the file with default permissions), then sets the permissions. This allows a local attacker that can read the directory to quickly grab the contents of the file before the permissions are changed. (This was reported to security@ on 2022-02-04 and has now been publicly disclosed - the link has some extra detail. Creating the bug here as directed.)

Source code: https://github.com/python/cpython/blob/3.10/Lib/shutil.py#L434

Note that changing the permissions after file creation but before writing content is insufficient: if the file is created with permissions that allow the attacker to open it, the attacker can get a file handle for the (empty) file. Under POSIX, the handle remains valid and readable even if the permissions change, allowing the attacker to read the file contents when they're populated. (@tiran, please feel free to add the info from your security@ response.)

I've confirmed that this is reliably exploitable (if the file is being created in a directory that the attacker can read), proof of concept code is available in https://github.com/janschejbal/raceread/. This is relevant in practice due to questionable choices by some distributions, causing many home directories to be created with permissions 755, i.e. world-readable (Ubuntu only changed that in 21.04).

Your environment

I've tested this on Python 3.8.10 on Ubuntu 20.04.3 LTS, but I believe it to affect all versions on any POSIX-compliant platform.

首先檢查 Lib/shutil.py 第 434 行附近的程式碼以及連結的 raceread proof of concept，重點關注 POSIX 上複製和權限處理的順序。當回報的本機檔案洩漏在受影響的 POSIX 情況下不再能夠重現，並且已根據 issue 的情境驗證行為時，即表示完成。