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From: ebiederm@xmission.com (Eric W. Biederman)
To: "Mickaël Salaün" <mic@digikod.net>
Cc: "Al Viro" <viro@zeniv.linux.org.uk>,
	"James Morris" <jmorris@namei.org>,
	"Serge Hallyn" <serge@hallyn.com>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Christoph Hellwig" <hch@lst.de>,
	"David Howells" <dhowells@redhat.com>,
	"Dominik Brodowski" <linux@dominikbrodowski.net>,
	"John Johansen" <john.johansen@canonical.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Kentaro Takeda" <takedakn@nttdata.co.jp>,
	"Tetsuo Handa" <penguin-kernel@i-love.sakura.ne.jp>,
	kernel-hardening@lists.openwall.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	"Mickaël Salaün" <mic@linux.microsoft.com>
Subject: Re: [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2)
Date: Wed, 10 Mar 2021 10:56:04 -0600	[thread overview]
Message-ID: <m1lfavt0bf.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20210310161000.382796-2-mic@digikod.net> (=?utf-8?Q?=22Micka?= =?utf-8?Q?=C3=ABl_Sala=C3=BCn=22's?= message of "Wed, 10 Mar 2021 17:10:00 +0100")

Mickaël Salaün <mic@digikod.net> writes:

> From: Mickaël Salaün <mic@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


  reply	other threads:[~2021-03-10 16:57 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-10 16:09 [PATCH v1 0/1] Unprivileged chroot Mickaël Salaün
2021-03-10 16:10 ` [PATCH v1 1/1] fs: Allow no_new_privs tasks to call chroot(2) Mickaël Salaün
2021-03-10 16:56   ` Eric W. Biederman [this message]
2021-03-10 18:13     ` Mickaël Salaün
2021-03-10 17:22 ` [PATCH v1 0/1] Unprivileged chroot Casey Schaufler
2021-03-10 18:17   ` Mickaël Salaün
2021-03-10 20:59     ` Casey Schaufler
2021-03-11 10:42       ` Mickaël Salaün

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