[PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2)

[Kees Cook]

On Tue, Mar 16, 2021 at 06:01:35PM +0100, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic at linux.microsoft.com>
> 
> Being able to easily change root directories enables 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.).
> 
> 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).
> 
> Unprivileged chroot is especially interesting for userspace developers
> wishing to harden their applications.  For instance, chroot(2) and Yama
> enable to build a capability-based security (i.e. remove filesystem
> ambient accesses) by calling chroot/chdir with an empty directory and
> accessing data through dedicated file descriptors obtained with
> openat2(2) and RESOLVE_BENEATH/RESOLVE_IN_ROOT/RESOLVE_NO_MAGICLINKS.
> 
> 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, or chroot/chdir), an
> unprivileged chroot is only allowed if the calling process is not
> already chrooted.  This limitation is the same as for creating user
> namespaces.
> 
> 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:
> https://lore.kernel.org/lkml/0e2f0f54e19bff53a3739ecfddb4ffa9a6dbde4d.1327858005.git.luto@amacapital.net/
> 
> Cc: Al Viro <viro at zeniv.linux.org.uk>
> Cc: Andy Lutomirski <luto at amacapital.net>
> Cc: Christian Brauner <christian.brauner at ubuntu.com>
> Cc: Christoph Hellwig <hch at lst.de>
> Cc: David Howells <dhowells at redhat.com>
> Cc: Dominik Brodowski <linux at dominikbrodowski.net>
> Cc: Eric W. Biederman <ebiederm at xmission.com>
> Cc: James Morris <jmorris at namei.org>
> Cc: John Johansen <john.johansen at canonical.com>
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Kentaro Takeda <takedakn at nttdata.co.jp>
> Cc: Serge Hallyn <serge at hallyn.com>
> Cc: Tetsuo Handa <penguin-kernel at i-love.sakura.ne.jp>
> Signed-off-by: Mickaël Salaün <mic at linux.microsoft.com>

Thanks for the updates! I find this version much easier to read. :)

Reviewed-by: Kees Cook <keescook at chromium.org>

-- 
Kees Cook