Fix TemporaryFileSwap regression where file_path could not be Path by EliahKagan · Pull Request #1776 · gitpython-developers/GitPython (original) (raw)

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This is a general test for TemporaryFileSwap, but by being parametrized by the type of file_path, it reveals a regression introduced in 9e86053 (gitpython-developers#1770). TemporaryFileSwap still works when file_path is a string, but is now broken when it is a Path. That worked before, and the type annotations document that it should be able to work. This is at least a bug because TemporaryFileSwap is public. (I am unsure whether, in practice, GitPython itself uses it in a way that sometimes passes a Path object as file_path. But code that uses GitPython may call it directly and pass Path.)

This fixes the regression introduced in 9e86053 (gitpython-developers#1770) where the file_path argument to TemporaryFileSwap.init could no longer be a Path object.

The change also makes this truer to the code from before gitpython-developers#1770, still without the race condition fixed there, in that str was called on file_path then as well. However, it is not clear that this is a good thing, because this is not an idiomatic use of mkstemp. The reason the prefix cannot be a Path is that it is expected to be a filename prefix, with leading directories given in the dir argument.

This changes the mkstemp call TemporaryFileSwap uses to atomically create (and thus reserve) the temporary file, so that it passes only a filename (i.e., basename) prefix as prefix, and passes all other path components (i.e., the directory) as dir. (If the original path has no directory, then dir is "" as before.)

This makes the mkstemp call slightly more idiomatic. This also makes it clearer, as it is no longer using prefix for something that feels like it should be possible to pass as a Path object.

Although mkstemp does not accept a Path as prefix, it does (as expected) accept one as dir. However, to keep the code simple, I'm passing str for both. The os.path.split function accepts both str and Path (since Python 3.6), and returns str objects, which are now used for the dir and prefix arguments to mkstemp.

For unusual cases, this may technically not be a refactoring. For example, a file_path of "a/b//c" will be split into "a/b" and "c". If the automatically generated temporary file suffix is "xyz", then that results in a tmp_file_path of "a/b/cxyz" where "a/b//cxyz" would have been used before. The tmp_file_path attribute of a TemporaryFileSwap object is public (and used in filesystem calls).

However, no guarantee has ever been given that the temporary file path have the original path as an exact string prefix. I believe the slightly weaker relationship I expressed in the recently introduced test_temporary_file_swap -- another file in the same directory, named with the original filename with more characters, consisting of equivalent path components in the same order -- has always been the intended one.

Note that this slight possible variation does not apply to the file_path attribute. That attribute is always kept exactly as it was, both in its type and its value, and it always used unmodified in calls that access the filesystem.

This removes the "fmt: off" / "fmt: on" directives around the @pytest.mark.parametrize decoration on test_blob_filter, and reformats it with black, for consistency with other such decorations.

The style used there, if it could be specified as a rule and thus used without "fmt:" directives, may be nicer than how black formats multi-line mark decorations. However, since that decoration was written, there have been a number of other such decorations, which have been in black style.

This also removes the only (or only remaining?) "fmt:" directive in the codebase. As such, it should possibly have been done in gitpython-developers#1760.

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This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
GitPython
==3.1.40 -> ==3.1.41
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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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