Re: [PATCH] userns/capability: Add user namespace capability

From: Austin S Hemmelgarn
Date: Mon Oct 19 2015 - 10:25:16 EST

On 2015-10-17 11:58, Tobias Markus wrote:
Add capability CAP_SYS_USER_NS.
Tasks having CAP_SYS_USER_NS are allowed to create a new user namespace
when calling clone or unshare with CLONE_NEWUSER.

Rationale:

Linux 3.8 saw the introduction of unpriviledged user namespaces,
allowing unpriviledged users (without CAP_SYS_ADMIN) to be a "fake" root
inside a separate user namespace. Before that, any namespace creation
required CAP_SYS_ADMIN (or, in practice, the user had to be root).
Unfortunately, there have been some security-relevant bugs in the
meantime. Because of the fairly complex nature of user namespaces, it is
reasonable to say that future vulnerabilties can not be excluded. Some
distributions even wholly disable user namespaces because of this.

Both options, user namespaces with and without CAP_SYS_ADMIN, can be
said to represent the extreme end of the spectrum. In practice, there is
no reason for every process to have the abilitiy to create user
namespaces. Indeed, only very few and specialized programs require user
namespaces. This seems to be a perfect fit for the (file) capability
system: Priviledged users could manually allow only a certain executable
to be able to create user namespaces by setting a certain capability,
I'd suggest the name CAP_SYS_USER_NS. Executables completely unrelated
to user namespaces should and can not create them.

The capability should only be required in the "root" user namespace (the
user namespace with level 0) though, to allow nested user namespaces to
work as intended. If a user namespace has a level greater than 0, the
original process must have had CAP_SYS_USER_NS, so it is "trusted" anyway.

One question remains though: Does this break userspace executables that
expect being able to create user namespaces without priviledge? Since
creating user namespaces without CAP_SYS_ADMIN was not possible before
Linux 3.8, programs should already expect a potential EPERM upon calling
clone. Since creating a user namespace without CAP_SYS_USER_NS would
also cause EPERM, we should be on the safe side.

Potentially stupid counter proposal:
Make it CAP_SYS_NS, make it allow access to all namespace types for non-root/CAP_SYS_ADMIN users, and teach the stuff that's using userns just to get to mount/pid/net/ipc namespaces to use those instead when it's something that doesn't really need to think it's running as root.

While this would still add a new capability (which is arguably not a good thing), the resultant capability would be significantly more useful for many of the use cases.

Potentially more flame resistant counter proposal:
Write a simple LSM to allow selective usage of namespaces (IIRC, working LSM stacking is in mainline now). While this is more complicated than just adding a capability, it is also a lot more resilient from a long term prospective.

Attachment: smime.p7s
Description: S/MIME Cryptographic Signature