Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752059AbcDTPeg (ORCPT ); Wed, 20 Apr 2016 11:34:36 -0400 Received: from mail-wm0-f53.google.com ([74.125.82.53]:33900 "EHLO mail-wm0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751899AbcDTPed (ORCPT ); Wed, 20 Apr 2016 11:34:33 -0400 MIME-Version: 1.0 In-Reply-To: <87mvoo8h3d.fsf@x220.int.ebiederm.org> References: <878u0s3orx.fsf_-_@x220.int.ebiederm.org> <20160409140909.42315e6d@lxorguk.ukuu.org.uk> <83FE8CD2-C0A2-4ADB-AEBD-8DD89AD4F88A@zytor.com> <87bn5ij0x1.fsf@x220.int.ebiederm.org> <78205895-E11D-417F-91DC-4BCA0B61A122@zytor.com> <570D4781.3070600@zytor.com> <877ffyzy1j.fsf_-_@x220.int.ebiederm.org> <87twixgsnq.fsf@x220.int.ebiederm.org> <87oa95gevf.fsf_-_@x220.int.ebiederm.org> <87mvoo8h3d.fsf@x220.int.ebiederm.org> Date: Wed, 20 Apr 2016 18:34:31 +0300 Message-ID: Subject: Re: [PATCH] devpts: Make each mount of devpts an independent filesystem. From: Konstantin Khlebnikov To: "Eric W. Biederman" Cc: Linus Torvalds , "H. Peter Anvin" , Andy Lutomirski , security@debian.org, "security@kernel.org" , Al Viro , "security@ubuntu.com >> security" , Peter Hurley , Serge Hallyn , Willy Tarreau , Aurelien Jarno , One Thousand Gnomes , Jann Horn , Greg KH , Linux Kernel Mailing List , Jiri Slaby , Florian Weimer Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2147 Lines: 50 On Wed, Apr 20, 2016 at 5:55 PM, Eric W. Biederman wrote: > Linus Torvalds writes: > >> On Tue, Apr 19, 2016 at 9:36 PM, Konstantin Khlebnikov wrote: >>> On Wed, Apr 20, 2016 at 6:04 AM, Eric W. Biederman >>>> >>>> The kernel.pty.reserve sysctl is neutered with no way currently >>>> implemented to be able to use the reserved ptys. >>> >>> I think we could convert this into reserve for init user namespace, >>> ssh in host will work even if containers eaten all ptys. >> >> Yes. That's basically how it effectively worked before (ie everything >> but the initial non-newinstance devpts mount would be limited to the >> non-reserved numbers). >> >> We required the non-init namespaces to do a newinstance mount, so the >> whole test for "newinstance" was effectively the same thing as just >> checking for the init namespace from a security standpoint. >> >> And in fact, rewriting it in that form (ie checking for init_ns) would >> just make it much more obvious what the intent it. > > How does this sound. > > When mounting a devpts filesystem. We look at the caller (aka current) > and if we are in the initial mount namespace set a flag in fsi that > allows that instance of devpts to draw into the reserve pool. Maybe just check current user namespace when task opens /dev/ptmx? IIRR now check looks like: count < limit - (newinstance ? reserved : 0). So, it will be count < limit - (current_in_init_userns ? 0 : newinstance). > > That will still allow crazy pieces of code like xen-create-instance run > by root that mount a devpts filesystem in a chroot environment to draw > into the reserved pool, but any sane users that set up their own mount > namespace won't be able to user the reserve pool. > > I believe that will give an almost identical policy to what we have > today, and it certainly makes a good default test for a container. Just > for cleanliness containers (of anyone's definition) almost always use > mount namespaces instead of chroots. > > Sigh one last past through all of the distros, to confirm that this > works in practice. > > Eric