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

[Mickaël Salaün]

On 10/03/2021 17:56, Eric W. Biederman wrote:
> 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("../../../../../../../../..").

Not with this patch.

> 
> 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.

chroot(2) is simple.

> 
> 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.

Could you please provide a new attack scenario?

> 
> 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.

They are explained in this commit message.

> 
> Eric
>