From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman) Subject: Re: [PATCH 1/1] simplified security.nscapability xattr Date: Tue, 03 May 2016 00:54:40 -0500 Message-ID: <87bn4nhejj.fsf@x220.int.ebiederm.org> References: <1461345993-17526-1-git-send-email-serge.hallyn@ubuntu.com> <1461345993-17526-2-git-send-email-serge.hallyn@ubuntu.com> <20160426222627.GA19307@mail.hallyn.com> <20160502035452.GA31837@mail.hallyn.com> <87h9egp2oq.fsf@x220.int.ebiederm.org> <20160503051921.GA31551@mail.hallyn.com> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20160503051921.GA31551-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org> (Serge E. Hallyn's message of "Tue, 3 May 2016 00:19:21 -0500") Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Serge E. Hallyn" Cc: "Andrew G. Morgan" , Kees Cook , Jann Horn , Michael Kerrisk-manpages , "Serge E. Hallyn" , Linux API , Andy Lutomirski , Linux Containers , LKML List-Id: linux-api@vger.kernel.org "Serge E. Hallyn" writes: > Quoting Andrew G. Morgan (morgan-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org): >> >> I guess I'm confused how we have strayed so far that this isn't an obvious >> requirement. Uid=0 as being the root of privilege was the basic problem >> that capabilities were designed to change. > > The task executing the file can be any uid mapped into the namespace. The > file only has to be owned by the root of the user_ns. Which I agree is > unfortunate. We can work around it by putting the root uid into the xattr > itself (which still isn't orthogonal but allows the file to at least by > owned by non-root), but the problem then is that a task needs to know its > global root k_uid just to write the xattr. The root kuid is just make_kuids(user_ns, 0) so it is easy to find. It might be a hair better to use the userns->owner instead of the root uid. That would allow user namespaces without a mapped root to still use file capabilities. >> Uid is an acl concept. Capabilities are supposed to be independent of that. >> >> If we want to support NS file capabilities I would look at replacing the >> xattr syscall with a dedicated file capabilities modification syscall. Then > > That was one ofthe possibilities I'd mentioned in my earlier proposal, > fwiw. The problem is if we want tar to still work unmodified then > simple xattr operations still have to work. > > Maybe there's workable semantics there though. Worth thinking about. If the problem is compatibilty please look at posix_acl_fix_xattr_from_user. With something similar for the security.capability attribute we can perform whatever transformation makes sense. I admit adding 4 bytes is a bit of a pain in that context but not a big one. Eric