[PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2)

[Eric W. Biederman]

Mickaël Salaün <mic at digikod.net> writes:

> From: Mickaël Salaün <mic at linux.microsoft.com>
>
> Being able to easily change root directories enable to ease some
> development workflow and can be used as a tool to strengthen
> unprivileged security sandboxes.  chroot(2) is not an access-control
> mechanism per se, but it can be used to limit the absolute view of the
> filesystem, and then limit ways to access data and kernel interfaces
> (e.g. /proc, /sys, /dev, etc.).

Actually chroot does not so limit the view of things.  It only limits
the default view.

A process that is chrooted can always escape by something like
chroot("../../../../../../../../..").

So I don't see the point of allowing chroot once you are in your locked
down sandbox.

> Users may not wish to expose namespace complexity to potentially
> malicious processes, or limit their use because of limited resources.
> The chroot feature is much more simple (and limited) than the mount
> namespace, but can still be useful.  As for containers, users of
> chroot(2) should take care of file descriptors or data accessible by
> other means (e.g. current working directory, leaked FDs, passed FDs,
> devices, mount points, etc.).  There is a lot of literature that discuss
> the limitations of chroot, and users of this feature should be aware of
> the multiple ways to bypass it.  Using chroot(2) for security purposes
> can make sense if it is combined with other features (e.g. dedicated
> user, seccomp, LSM access-controls, etc.).
>
> One could argue that chroot(2) is useless without a properly populated
> root hierarchy (i.e. without /dev and /proc).  However, there are
> multiple use cases that don't require the chrooting process to create
> file hierarchies with special files nor mount points, e.g.:
> * A process sandboxing itself, once all its libraries are loaded, may
>   not need files other than regular files, or even no file at all.
> * Some pre-populated root hierarchies could be used to chroot into,
>   provided for instance by development environments or tailored
>   distributions.
> * Processes executed in a chroot may not require access to these special
>   files (e.g. with minimal runtimes, or by emulating some special files
>   with a LD_PRELOADed library or seccomp).
>
> Allowing a task to change its own root directory is not a threat to the
> system if we can prevent confused deputy attacks, which could be
> performed through execution of SUID-like binaries.  This can be
> prevented if the calling task sets PR_SET_NO_NEW_PRIVS on itself with
> prctl(2).  To only affect this task, its filesystem information must not
> be shared with other tasks, which can be achieved by not passing
> CLONE_FS to clone(2).  A similar no_new_privs check is already used by
> seccomp to avoid the same kind of security issues.  Furthermore, because
> of its security use and to avoid giving a new way for attackers to get
> out of a chroot (e.g. using /proc/<pid>/root), an unprivileged chroot is
> only allowed if the new root directory is the same or beneath the
> current one.  This still allows a process to use a subset of its
> legitimate filesystem to chroot into and then further reduce its view of
> the filesystem.
>
> This change may not impact systems relying on other permission models
> than POSIX capabilities (e.g. Tomoyo).  Being able to use chroot(2) on
> such systems may require to update their security policies.
>
> Only the chroot system call is relaxed with this no_new_privs check; the
> init_chroot() helper doesn't require such change.
>
> Allowing unprivileged users to use chroot(2) is one of the initial
> objectives of no_new_privs:
> https://www.kernel.org/doc/html/latest/userspace-api/no_new_privs.html
> This patch is a follow-up of a previous one sent by Andy Lutomirski, but
> with less limitations:
> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/

Last time I remember talking architecture we agreed that user namespaces
would be used for enabling features and that no_new_privs would just be
used to lock-down userspace.  That way no_new_privs could be kept simple
and trivial to audit and understand.

You can build your sandbox and use chroot if you use a user namespace at
the start.  A mount namespace would also help lock things down.  Still
allowing chroot after the sanbox has been built, a seccomp filter has
been installed and no_new_privs has been enabled seems like it is asking
for trouble and may weaken existing sandboxes.

So I think we need a pretty compelling use case to consider allowing
chroot(2).  You haven't even mentioned what your usecase is at this
point so I don't know why we would tackle that complexity.

Eric