6f054f9fb3
When cloning a repository with `--local`, Git relies on either making a
hardlink or copy to every file in the "objects" directory of the source
repository. This is done through the callpath `cmd_clone()` ->
`clone_local()` -> `copy_or_link_directory()`.
The way this optimization works is by enumerating every file and
directory recursively in the source repository's `$GIT_DIR/objects`
directory, and then either making a copy or hardlink of each file. The
only exception to this rule is when copying the "alternates" file, in
which case paths are rewritten to be absolute before writing a new
"alternates" file in the destination repo.
One quirk of this implementation is that it dereferences symlinks when
cloning. This behavior was most recently modified in 36596fd2df (clone:
better handle symlinked files at .git/objects/, 2019-07-10), which
attempted to support `--local` clones of repositories with symlinks in
their objects directory in a platform-independent way.
Unfortunately, this behavior of dereferencing symlinks (that is,
creating a hardlink or copy of the source's link target in the
destination repository) can be used as a component in attacking a
victim by inadvertently exposing the contents of file stored outside of
the repository.
Take, for example, a repository that stores a Dockerfile and is used to
build Docker images. When building an image, Docker copies the directory
contents into the VM, and then instructs the VM to execute the
Dockerfile at the root of the copied directory. This protects against
directory traversal attacks by copying symbolic links as-is without
dereferencing them.
That is, if a user has a symlink pointing at their private key material
(where the symlink is present in the same directory as the Dockerfile,
but the key itself is present outside of that directory), the key is
unreadable to a Docker image, since the link will appear broken from the
container's point of view.
This behavior enables an attack whereby a victim is convinced to clone a
repository containing an embedded submodule (with a URL like
"file:///proc/self/cwd/path/to/submodule") which has a symlink pointing
at a path containing sensitive information on the victim's machine. If a
user is tricked into doing this, the contents at the destination of
those symbolic links are exposed to the Docker image at runtime.
One approach to preventing this behavior is to recreate symlinks in the
destination repository. But this is problematic, since symlinking the
objects directory are not well-supported. (One potential problem is that
when sharing, e.g. a "pack" directory via symlinks, different writers
performing garbage collection may consider different sets of objects to
be reachable, enabling a situation whereby garbage collecting one
repository may remove reachable objects in another repository).
Instead, prohibit the local clone optimization when any symlinks are
present in the `$GIT_DIR/objects` directory of the source repository.
Users may clone the repository again by prepending the "file://" scheme
to their clone URL, or by adding the `--no-local` option to their `git
clone` invocation.
The directory iterator used by `copy_or_link_directory()` must no longer
dereference symlinks (i.e., it *must* call `lstat()` instead of `stat()`
in order to discover whether or not there are symlinks present). This has
no bearing on the overall behavior, since we will immediately `die()` on
encounter a symlink.
Note that t5604.33 suggests that we do support local clones with
symbolic links in the source repository's objects directory, but this
was likely unintentional, or at least did not take into consideration
the problem with sharing parts of the objects directory with symbolic
links at the time. Update this test to reflect which options are and
aren't supported.
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission). To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks