Pre-deprecate setting Git.USE_SHELL by EliahKagan · Pull Request #1782 · gitpython-developers/GitPython (original) (raw)

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These are "non-reified docstrings" as described in gitpython-developers#1734. They are not made part of the data model, but most editors will display them, including on symbols present in code of other projects that use the GitPython library. A number of these have already been added, but this adds what will probably be most of the remaining ones.

For the most part, this doesn't create documentation where there wasn't any, but instead converts existing comments to "docstrings." In a handful of cases, they are expanded, reworded, or a docstring added.

This also fixes some small style inconsistencies that were missed in gitpython-developers#1725, and moves a comment that had become inadvertently displaced due to autoformatting to the item it was meant for.

The major omission here is HIDE_WINDOWS_KNOWN_ERRORS and HIDE_WINDOWS_FREEZE_ERRORS. This doesn't convert the comments above them to "docstrings," for a few reasons. They are not specific to either of the symbols, they are oriented toward considerations that are not really relevant except when developing GitPython itself, and they are out of date. Also, because HIDE_WINDOWS_KNOWN_ERRORS is listed in all, increasing the level of documentation for it might be taken as a committment to preserve some aspect of its current behavior, which could interfere with progress on gitpython-developers#790. So I've kept those comments as comments, and unchanged, for now.

This expands the "docstring" associated with the Git.USE_SHELL attribute to mention the dangers of setting it to True and explain the old purpose it once served for graphical Windows applications and why it is no longer required for that since 2.0.8. (See gitpython-developers#1781.)

Although setting Git.USE_SHELL = True or passing shell=True should rarely if ever be done and is no longer necessary even in the specific scenario for which it was once recommended, I have deliberately avoided claiming USE_SHELL is deprecated at this time.

Whether GitPython should formally deprecate it (documenting it as such and issuing DeprecationWarning on some or all uses) may hinge on whether it is possible for GitPython to incorporate enhancements that account for and suppress at least some unintended shell expansions when shell=True is passed through dynamic methods that indirectly call Git.execute. The decision may also benefit from examination of existing common uses, if any, of USE_SHELL = True.

EliahKagan added a commit to EliahKagan/GitPython that referenced this pull request

Dec 25, 2023

• Fix spacing in deprecation messages.
• Update versions in deprecation messages to reflect SemVer.
• Add and fix up docstrings for methods with some deprecated uses.
• Add a couple missing docstrings on nearby attributes and methods.
• Convert is_ deprecation comment to three docstrings.
• Avoid wrongly claiming is_darwin still uses os.name (see gitpython-developers#1732).
• Convert some other comments to docstrings.
• Make disembodied """-strings docstrings or comments, per intent.
• Move some comments that were misplaced during auto-formatting.

This commit builds on the changes in 94a85d1 (gitpython-developers#1782).

EliahKagan added a commit to EliahKagan/GitPython that referenced this pull request

Dec 25, 2023

This builds on 106bbe6 (gitpython-developers#1782) to actually deprecate Git.USE_SHELL. Its docstring is updated to state this, but no DeprecationWarning is issued for its use at this time.

The documentation for the shell parameter of Git.execute is also updated to caution against passing shell=True, but the deprecation is not currently extended to that parameter.

Some information is duplicated across the two docstrings. I have done this because the the USE_SHELL attribute's docstring, while it is displayed by many editors, doesn't currently appear in generated HTML documentation (see gitpython-developers#1734).

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This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
GitPython
==3.1.40 -> ==3.1.41
](https://docs.renovatebot.com/merge-confidence/)
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[!WARNING] Some dependencies could not be looked up. Check the Dependency Dashboard for more information.

GitHub Vulnerability Alerts

CVE-2024-22190

Summary

This issue exists because of an incomplete fix for CVE-2023-40590. On Windows, GitPython uses an untrusted search path if it uses a shell to run git, as well as when it runs bash.exe to interpret hooks. If either of those features are used on Windows, a malicious git.exe or bash.exe may be run from an untrusted repository.

Details

Although GitPython often avoids executing programs found in an untrusted search path since 3.1.33, two situations remain where this still occurs. Either can allow arbitrary code execution under some circumstances.

When a shell is used

GitPython can be told to run git commands through a shell rather than as direct subprocesses, by passing shell=True to any method that accepts it, or by both setting Git.USE_SHELL = True and not passing shell=False. Then the Windows cmd.exe shell process performs the path search, and GitPython does not prevent that shell from finding and running git in the current directory.

When GitPython runs git directly rather than through a shell, the GitPython process performs the path search, and currently omits the current directory by setting NoDefaultCurrentDirectoryInExePath in its own environment during the Popen call. Although the cmd.exe shell will honor this environment variable when present, GitPython does not currently pass it into the shell subprocess's environment.

Furthermore, because GitPython sets the subprocess CWD to the root of a repository's working tree, using a shell will run a malicious git.exe in an untrusted repository even if GitPython itself is run from a trusted location.

This also applies if Git.execute is called directly with shell=True (or after Git.USE_SHELL = True) to run any command.

When hook scripts are run

On Windows, GitPython uses bash.exe to run hooks that appear to be scripts. However, unlike when running git, no steps are taken to avoid finding and running bash.exe in the current directory.

This allows the author of an untrusted fork or branch to cause a malicious bash.exe to be run in some otherwise safe workflows. An example of such a scenario is if the user installs a trusted hook while on a trusted branch, then switches to an untrusted feature branch (possibly from a fork) to review proposed changes. If the untrusted feature branch contains a malicious bash.exe and the user's current working directory is the working tree, and the user performs an action that runs the hook, then although the hook itself is uncorrupted, it runs with the malicious bash.exe.

Note that, while bash.exe is a shell, this is a separate scenario from when git is run using the unrelated Windows cmd.exe shell.

PoC

On Windows, create a git.exe file in a repository. Then create a Repo object, and call any method through it (directly or indirectly) that supports the shell keyword argument with shell=True:

mkdir testrepo
git init testrepo
cp ... testrepo git.exe # Replace "..." with any executable of choice.
python -c "import git; print(git.Repo('testrepo').git.version(shell=True))"

The git.exe executable in the repository directory will be run.

Or use no Repo object, but do it from the location with the git.exe:

cd testrepo
python -c "import git; print(git.Git().version(shell=True))"

The git.exe executable in the current directory will be run.

For the scenario with hooks, install a hook in a repository, create a bash.exe file in the current directory, and perform an operation that causes GitPython to attempt to run the hook:

mkdir testrepo
cd testrepo
git init
mv .git/hooks/pre-commit.sample .git/hooks/pre-commit
cp ... bash.exe # Replace "..." with any executable of choice.
echo "Some text" >file.txt
git add file.txt
python -c "import git; git.Repo().index.commit('Some message')"

The bash.exe executable in the current directory will be run.

Impact

The greatest impact is probably in applications that set Git.USE_SHELL = True for historical reasons. (Undesired console windows had, in the past, been created in some kinds of applications, when it was not used.) Such an application may be vulnerable to arbitrary code execution from a malicious repository, even with no other exacerbating conditions. This is to say that, if a shell is used to run git, the full effect of CVE-2023-40590 is still present. Furthermore, as noted above, running the application itself from a trusted directory is not a sufficient mitigation.

An application that does not direct GitPython to use a shell to run git subprocesses thus avoids most of the risk. However, there is no such straightforward way to prevent GitPython from running bash.exe to interpret hooks. So while the conditions needed for that to be exploited are more involved, it may be harder to mitigate decisively prior to patching.

Possible solutions

A straightforward approach would be to address each bug directly:

• When a shell is used, pass NoDefaultCurrentDirectoryInExePath into the subprocess environment, because in that scenario the subprocess is the cmd.exe shell that itself performs the path search.
• Set NoDefaultCurrentDirectoryInExePath in the GitPython process environment during the Popen call made to run hooks with a bash.exe subprocess.

These need only be done on Windows.

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