On Tue, Jul 16, 2019 at 7:03 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote: > > On Fri, Jul 12, 2019 at 12:54:02PM -0700, Casey Schaufler wrote: > > On 7/12/2019 11:25 AM, Stephen Smalley wrote: > > > On 7/12/19 1:58 PM, Casey Schaufler wrote: > > >> On 7/12/2019 10:34 AM, Nicholas Franck wrote: > > >>> At present security_capable does not pass any object information > > >>> and therefore can neither audit the particular object nor take it > > >>> into account. Augment the security_capable interface to support > > >>> passing supplementary data. Use this facility initially to convey > > >>> the inode for capability checks relevant to inodes. This only > > >>> addresses capable_wrt_inode_uidgid calls; other capability checks > > >>> relevant to inodes will be addressed in subsequent changes. In the > > >>> future, this will be further extended to pass object information for > > >>> other capability checks such as the target task for CAP_KILL. > > >> > > >> This seems wrong to me. The capability system has nothing to do > > >> with objects. Passing object information through security_capable() > > >> may be convenient, but isn't relevant to the purpose of the interface. > > >> It appears that there are very few places where the object information > > >> is actually useful. > > > > > > A fair number of capabilities are checked upon some attempted object access (often right after comparing UIDs or other per-object state), and the particular object can be very helpful in both audit and in access control. More below. > > > > I'm not disagreeing with that. What I'm saying is that the capability > > check interface is not the right place to pass that information. The > > capability check has no use for the object information. I would much > > I've had to argue this before while doing the namespaced file > capabilities implementation. Perhaps this would be worth writing something > more formal about. My main argument is, even if we want to claim that the > capabilities model is and should be object agnostic, the implementation > of user namespaces (currently) is such that the whole view of the user's > privilege must include information which is stored with the object. > > There are various user namespaces. > > The Linux capabilities ( :-) ) model is user namespaced. It must be, in > order to be useful. If we're going to use file capabilities in distros, > and distros are going to run in containers, then the capabilities must > be namespaced. Otherwise, capabilities will not be used, and heck, should > just be dropped. > > The only way to find out which user namespace has privilege over an inode > is to look at the inode. > > Therefore, object information is needed. Agreed. The concept in the kernel is "capability over a namespace." That being said, sticking a flexible object type into ns_capable() seems prematurely general to me. How about adding security_capable_wrt_inode_uidgid() and allowing LSMs to hook that? The current implementation would go into commoncap. The obvious extensions I can think of are security_dac_read_search(..., inode, ...) and security_dac_override(..., inode, ...). (Or dentry or whatever is appropriate.) If this patch were restructured like that, the semantics would be obvious, and it would arguably be a genuine cleanup instead of a whole new mechanism of unknown scope.
