GitHub Security Advisory: GHSA-fqmf-w4xh-33rh
Package: gix-worktree-state (RUST)
Severity: MODERATE (CVSS 5.0)
CWE: CWE-281: Improper Preservation of Permissions, CWE-687: Function Call With Incorrectly Specified Argument Value
Vulnerable versions: < 0.17.0
Patched in: 0.17.0

Summary: gix-worktree-state nonexclusive checkout sets executable files world-writable

Details:
### Summary

`gix-worktree-state` specifies 0777 permissions when checking out executable files, intending that the umask will restrict them appropriately. But one of the strategies it uses to set permissions is not subject to the umask. This causes files in a repository to be world-writable in some situations.

### Details

Git repositories track executable bits for regular files. In tree objects and the index, regular file modes are stored as 0644 if not executable, or 0755 if executable. But this is independent of how the permissions are set in the filesystem (where supported).

[`gix_worktree_state::checkout`](https://github.com/GitoxideLabs/gitoxide/blob/8d84818240d44e1f5fe78a231b5d9bffd0283918/gix-worktree-state/src/checkout/function.rs#L8-L35) has two strategies for checking out a file and marking it executable on a Unix-like operating system, one of which is vulnerable:

- If the file is created by assuming it does not already exist, correct permissions are applied, because permissions specified when opening a file are subject to the umask.
- If the file is considered possibly already to exist—even in a clean checkout if the application does not specify the option to treat the destination directory as empty—then permissions conferring unrestricted access to any user account on the system are wrongly applied, because permissions specified when calling chmod on an existing file are not subject to the umask. 

Specifically, [`checkout::entry::checkout`](https://github.com/GitoxideLabs/gitoxide/blob/8d84818240d44e1f5fe78a231b5d9bffd0283918/gix-worktree-state/src/checkout/entry.rs#L56-L191) chooses the strategy for each file. The same strategy is usually chosen for each executable file, if no [process](https://github.com/git/git/blob/a60673e9252b08d4eca90543b3729f4798b9aafd/Documentation/RelNotes/2.11.0.txt#L149-L154) (i.e. [long running](https://github.com/GitoxideLabs/gitoxide/discussions/996)) smudge filter is in use. The strategy depends on the [`checkout::Options::destination_is_initially_empty`](https://github.com/GitoxideLabs/gitoxide/blob/8d84818240d44e1f5fe78a231b5d9bffd0283918/gix-worktree-state/src/checkout/mod.rs#L50-L53) value, which is passed along to [`checkout::entry::open_file`](https://github.com/GitoxideLabs/gitoxide/blob/8d84818240d44e1f5fe78a231b5d9bffd0283918/gix-worktree-state/src/checkout/entry.rs#L253-L277), whose return value includes a flag indicating whether permissions still need to be set:

- With `destination_is_initially_empty: true`, executable permissions are specified when opening the file, via [`OpenOptionsEx::mode`](https://doc.rust-lang.org/std/os/unix/fs/trait.OpenOptionsExt.html#tymethod.mode), by its effect on the behavior of [`OpenOptions::open`](https://doc.rust-lang.org/std/fs/struct.OpenOptions.html#method.open). A mode of 0777 is safe here, for the same reason the default mode of 0666 is safe. When creating a file, the applied mode is the specified mode with any bits unset from it that are set in t