On 2015-10-21 14:53, Andy Lutomirski wrote:
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?On Oct 19, 2015 7:25 AM, "Austin S Hemmelgarn" <ahferroin7@xxxxxxxxx> 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.
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