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Message-ID: <202103161142.87100A8133@keescook>
Date: Tue, 16 Mar 2021 11:43:06 -0700
From: Kees Cook <keescook@...omium.org>
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>,
Casey Schaufler <casey@...aufler-ca.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Christoph Hellwig <hch@....de>, David Howells <dhowells@...hat.com>,
Dominik Brodowski <linux@...inikbrodowski.net>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
John Johansen <john.johansen@...onical.com>,
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 v4 1/1] fs: Allow no_new_privs tasks to call chroot(2)
On Tue, Mar 16, 2021 at 06:01:35PM +0100, Mickaël Salaün wrote:
> From: Mickaël Salaün <mic@...ux.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@...iv.linux.org.uk>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Christian Brauner <christian.brauner@...ntu.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: David Howells <dhowells@...hat.com>
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> Cc: Eric W. Biederman <ebiederm@...ssion.com>
> Cc: James Morris <jmorris@...ei.org>
> Cc: John Johansen <john.johansen@...onical.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Kentaro Takeda <takedakn@...data.co.jp>
> Cc: Serge Hallyn <serge@...lyn.com>
> Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
Thanks for the updates! I find this version much easier to read. :)
Reviewed-by: Kees Cook <keescook@...omium.org>
--
Kees Cook
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