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

[Austin S Hemmelgarn <ahferroin7-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

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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.


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