Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752520Ab3CCBSX (ORCPT ); Sat, 2 Mar 2013 20:18:23 -0500 Received: from mail-ob0-f176.google.com ([209.85.214.176]:54230 "EHLO mail-ob0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751418Ab3CCBSV (ORCPT ); Sat, 2 Mar 2013 20:18:21 -0500 MIME-Version: 1.0 In-Reply-To: <20130303005700.GA32213@austin.hallyn.com> References: <20130303005700.GA32213@austin.hallyn.com> Date: Sat, 2 Mar 2013 17:18:17 -0800 Message-ID: Subject: Re: user ns: arbitrary module loading From: Kees Cook To: "Serge E. Hallyn" Cc: "Eric W. Biederman" , LKML , Serge Hallyn , Brad Spengler , Al Viro , Eric Paris , Rusty Russell Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2857 Lines: 73 On Sat, Mar 2, 2013 at 4:57 PM, Serge E. Hallyn wrote: > Quoting Kees Cook (keescook@google.com): >> The rearranging done for user ns has resulted in allowing arbitrary >> kernel module loading[1] (i.e. re-introducing a form of CVE-2011-1019) >> by what is assumed to be an unprivileged process. >> >> At present, it does look to require at least CAP_SETUID along the way >> to set up the uidmap (but things like the setuid helper newuidmap >> might soon start providing such a thing by default). >> >> It might be worth examining GRKERNSEC_MODHARDEN in grsecurity, which >> examines module symbols to verify that request_module() for a >> filesystem only loads a module that defines "register_filesystem" >> (among other things). >> >> -Kees >> >> [1] https://twitter.com/grsecurity/status/307473816672665600 > > So the concern is root in a child user namespace doing > > mount -t randomfs <...> > > in which case do_new_mount() checks ns_capable(), not capable(), > before trying to load a module for randomfs. Well, not just randomfs. Any module that modprobe in the init ns can find. > As well as (secondly) the fact that there is no enforcement on > the format of the module names (i.e. fs-*). > > Kees, from what I've seen the GRKERNSEC_MODHARDEN won't be acceptable. > At least Eric Paris is strongly against it. I'd be curious to hear the objections. It seems pretty nice to me to add a new argument to every request_module() that specifies the "subsystem" it expects a module to load from. Maybe pass "request_module=filesystem" or "...=netdev" to the modprobe call. And then in init_module(), check the userargs for which subsystem was requested and look up in a table for the entry point module symbol for that subsystem to require. e.g. for "request_module=filesystem", require that the module contains the "register_filesystem" symbol, etc. > But how about if we > add a check for 'current_user_ns() == &init_user_ns' at that place > instead? Well, we'd need to mostly revert 57eccb830f1cc93d4b506ba306d8dfa685e0c88f ("mount: consolidate permission checks") since get_fs_type() is being called before may_mount() right now. (And then, as you suggest, we should strengthen the test.) I think this will require either more plumbing into get_fs_type (something like "bool load_module_if_missing") or the subsystem verification stuff in request_module. I think the latter is MUCH nicer as it covers this problem in all places, not just this "mount" case. > Eric Biederman, do you have any objections to that? > > thanks, > -serge -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/