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Date: Wed, 10 Mar 2021 10:56:04 -0600
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Mickaël Salaün <mic@...ikod.net>
Cc: Al Viro <viro@...iv.linux.org.uk>, James Morris <jmorris@...ei.org>,
Serge Hallyn <serge@...lyn.com>, Andy Lutomirski <luto@...capital.net>,
Christian Brauner <christian.brauner@...ntu.com>, Christoph Hellwig
<hch@....de>, David Howells <dhowells@...hat.com>, Dominik Brodowski
<linux@...inikbrodowski.net>, John Johansen
<john.johansen@...onical.com>, Kees Cook <keescook@...omium.org>,
Kentaro Takeda <takedakn@...data.co.jp>, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp>,
kernel-hardening@...ts.openwall.com, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org,
Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2)
Mickaël Salaün <mic@...ikod.net> writes:
> From: Mickaël Salaün <mic@...ux.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
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