# Minijail

The Minijail homepage is
https://google.github.io/minijail/.

The main source repo is
https://chromium.googlesource.com/chromiumos/platform/minijail.

There might be other copies floating around, but this is the official one!

[TOC]

## What is it?

Minijail is a sandboxing and containment tool used in ChromeOS and Android.
It provides an executable that can be used to launch and sandbox other programs,
and a library that can be used by code to sandbox itself.

## Purpose and threat model

Minijail is intended for sandboxing known binaries on a system. It is intended
to mitigate the risk if a service is compromised through a bug, or a
confused-deputy scenario.

It is not designed for safely running malicious code, including
attacker-controlled binaries or binaries that may use attacker-controlled
shared object libraries.

## Getting the code

You're one `git clone` away from happiness.

```
$ git clone https://chromium.googlesource.com/chromiumos/platform/minijail
$ cd minijail
```

Releases are tagged as `linux-vXX`:
https://chromium.googlesource.com/chromiumos/platform/minijail/+refs

## Building

See the [HACKING.md](./HACKING.md) document for more details.

## Release process

See the [RELEASE.md](./RELEASE.md) document for more details.

## Additional tools

See the [tools/README.md](./tools/README.md) document for more details.

## Contact

We've got a couple of contact points.

* [minijail@chromium.org]: Public user & developer mailing list.
* [minijail-users@google.com]: Internal Google user mailing list.
* [minijail-dev@google.com]: Internal Google developer mailing list.
* [crbug.com/list]: Existing bug reports & feature requests.
* [crbug.com/new]: File new bug reports & feature requests.
* [Chromium Gerrit]: Code reviews.

[minijail@chromium.org]: https://groups.google.com/a/chromium.org/forum/#!forum/minijail
[minijail-users@google.com]: https://groups.google.com/a/google.com/forum/#!forum/minijail-users
[minijail-dev@google.com]: https://groups.google.com/a/google.com/forum/#!forum/minijail-dev
[crbug.com/list]: https://crbug.com/?q=component:OS>Systems>Minijail
[crbug.com/new]: https://bugs.chromium.org/p/chromium/issues/entry?components=OS>Systems>Minijail
[Chromium Gerrit]: https://chromium-review.googlesource.com/q/project:chromiumos/platform/minijail

## GitHub Pages homepage

The https://google.github.io/minijail/ homepage is maintained in the `gh-pages`
branch, not in the `main` branch.
Changes to it can be sent via Gerrit, but requires a little extra work.

```shell
# Make sure you have all the branches, and not only the "main" one.
$ git fetch

# Create a new local branch tracking the remote "gh-pages".
# Git should automatically detect the remote and track it for you.
$ git checkout gh-pages
# If git can't auto-detect the remote, try one of these.
$ git checkout -b gh-pages origin/gh-pages
$ git checkout -b gh-pages cros/gh-pages

# Make your changes like normal, then push them to Gerrit for review.
# Here's a couple of different ways to post changes; you only need one!
$ repo upload -D gh-pages
$ git push origin HEAD:refs/for/gh-pages
$ git push cros HEAD:refs/for/gh-pages

# Now review your changes via Gerrit like normal.
```

Once a change merges into the `gh-pages` branch, there is no additional work for
you other than waiting -- GitHub periodically syncs with our host, and then it
will automatically regenerate the homepage when the `gh-pages` branch updates.

## Talks and presentations

The following talk serves as a good introduction to Minijail and how it can be used.

[Video](https://drive.google.com/file/d/0BwPS_JpKyELWZTFBcTVsa1hhYjA/preview),
[slides](https://docs.google.com/presentation/d/e/2PACX-1vRBqpin5xR9sng6lIBPjG0XQtu-uWWgr0ds-M3zW13XpDO-bTcMERLwoHUEB9078p1yqr9L-su9n5dk/pub).

## Example usage

The ChromiumOS project has a comprehensive
[sandboxing](https://www.chromium.org/chromium-os/developer-library/guides/development/sandboxing)
document that is largely based on Minijail.

After you play with the simple examples below, you should check that out.

### Change root to any user

```
# id
uid=0(root) gid=0(root) groups=0(root),128(pkcs11)
# minijail0 -u jorgelo -g 5000 /usr/bin/id
uid=72178(jorgelo) gid=5000(eng) groups=5000(eng)
```

### Drop root while keeping some capabilities

```
# minijail0 -u jorgelo -c 3000 -- /bin/cat /proc/self/status
Name: cat
...
CapInh: 0000000000003000
CapPrm: 0000000000003000
CapEff: 0000000000003000
CapBnd: 0000000000003000
```

## Historical notes

Q. "Why is it called minijail0?"

A. It is minijail0 because it was a rewrite of an earlier program named
minijail, which was considerably less mini, and in particular had a dependency
on libchrome (the ChromeOS packaged version of Chromium's //base).  We needed a
new name to not collide with the deprecated one.

We didn't want to call it minijail2 or something that would make people
start using it before we were ready, and it was also concretely _less_ since it
dropped libbase, etc.  Technically, we needed to be able to fork/preload with
minimal extra syscall noise which was too hard with libbase at the time (onexit
handlers, etc that called syscalls we didn't want to allow).  Also, Elly made a
strong case that C would be the right choice for this for linking and ease of
controlled surprise system call use.

https://crrev.com/c/4585/ added the original implementation.

Source: Conversations with original authors, ellyjones@ and wad@.
