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

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-10-21 14:53, Andy Lutomirski wrote:
> On Oct 19, 2015 7:25 AM, "Austin S Hemmelgarn" <ahferroin7@gmail.com> wrote:
>>
>> 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.
>
> Then you'd have to come up with some argument that it could possibly
> be safe.  You'd need *at least* no_new_privs forced on.  You would
> also have fun defining the privilege to own such a namespace once
> created.
Excellent point about the privileges, although wouldn't that also apply 
to just using a capability for non-root/CAP_SYS_ADMIN access to userns?


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