On 2019-07-16 19:30, Paul Moore wrote: > On Tue, Jul 16, 2019 at 6:03 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote: > > On 2019-07-15 17:04, Paul Moore wrote: > > > On Mon, Jul 8, 2019 at 2:06 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote: > > ... > > > > > If we can't trust ns_capable() then why are we passing on > > > > CAP_AUDIT_CONTROL? It is being passed down and not stripped purposely > > > > by the orchestrator/engine. If ns_capable() isn't inherited how is it > > > > gained otherwise? Can it be inserted by cotainer image? I think the > > > > answer is "no". Either we trust ns_capable() or we have audit > > > > namespaces (recommend based on user namespace) (or both). > > > > > > My thinking is that since ns_capable() checks the credentials with > > > respect to the current user namespace we can't rely on it to control > > > access since it would be possible for a privileged process running > > > inside an unprivileged container to manipulate the audit container ID > > > (containerized process has CAP_AUDIT_CONTROL, e.g. running as root in > > > the container, while the container itself does not). > > > > What makes an unprivileged container unprivileged? "root", or "CAP_*"? > > My understanding is that when most people refer to an unprivileged > container they are referring to a container run without capabilities > or a container run by a user other than root. I'm sure there are > better definitions out there, by folks much smarter than me on these > things, but that's my working definition. Close enough to my understanding... > > If CAP_AUDIT_CONTROL is granted, does "root" matter? > > Our discussions here have been about capabilities, not UIDs. The only > reason root might matter is that it generally has the full capability > set. Good, that's my understanding. > > Does it matter what user namespace it is in? > > What likely matters is what check is called: capable() or > ns_capable(). Those can yield very different results. Ok, I finally found what I was looking for to better understand the challenge with trusting ns_capable(). Sorry for being so dense and slow on this one. I thought I had gone through the code carefully enough, but this time I finally found it. set_cred_user_ns() sets a full set of capabilities rather than inheriting them from the parent user_ns, called from userns_install() or create_userns(). Even if the container orchestrator/engine restricts those capabilities on its own containers, they could easily unshare a userns and get a full set unless it also restricted CAP_SYS_ADMIN, which is used too many other places to be practical to restrict. > > I understand that root is *gained* in an > > unprivileged user namespace, but capabilities are inherited or permitted > > and that process either has it or it doesn't and an unprivileged user > > namespace can't gain a capability that has been rescinded. Different > > subsystems use the userid or capabilities or both to determine > > privileges. > > Once again, I believe the important thing to focus on here is > capable() vs ns_capable(). We can't safely rely on ns_capable() for > the audit container ID policy since that is easily met inside the > container regardless of the process' creds which started the > container. Agreed. > > In this case, is the userid relevant? > > We don't do UID checks, we do capability checks, so yes, the UID is irrelevant. Agreed. > > > > At this point I would say we are at an impasse unless we trust > > > > ns_capable() or we implement audit namespaces. > > > > > > I'm not sure how we can trust ns_capable(), but if you can think of a > > > way I would love to hear it. I'm also not sure how namespacing audit > > > is helpful (see my above comments), but if you think it is please > > > explain. > > > > So if we are not namespacing, why do we not trust capabilities? > > We can trust capable(CAP_AUDIT_CONTROL) for enforcing audit container > ID policy, we can not trust ns_capable(CAP_AUDIT_CONTROL). Ok. So does a process in a non-init user namespace have two (or more) sets of capabilities stored in creds, one in the init_user_ns, and one in current_user_ns? Or does it get stripped of all its capabilities in init_user_ns once it has its own set in current_user_ns? If the former, then we can use capable(). If the latter, we need another mechanism, as you have suggested might be needed. If some random unprivileged user wants to fire up a container orchestrator/engine in his own user namespace, then audit needs to be namespaced. Can we safely discard this scenario for now? That user can use a VM. > paul moore - RGB -- Richard Guy Briggs <rgb@xxxxxxxxxx> Sr. S/W Engineer, Kernel Security, Base Operating Systems Remote, Ottawa, Red Hat Canada IRC: rgb, SunRaycer Voice: +1.647.777.2635, Internal: (81) 32635
