Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753347AbbGOVvK (ORCPT ); Wed, 15 Jul 2015 17:51:10 -0400 Received: from mail-lb0-f169.google.com ([209.85.217.169]:34754 "EHLO mail-lb0-f169.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751661AbbGOVvI (ORCPT ); Wed, 15 Jul 2015 17:51:08 -0400 MIME-Version: 1.0 In-Reply-To: <20150715214848.GA24204@mail.hallyn.com> References: <1436989569-69582-1-git-send-email-seth.forshee@canonical.com> <1436989569-69582-4-git-send-email-seth.forshee@canonical.com> <20150715214848.GA24204@mail.hallyn.com> From: Andy Lutomirski Date: Wed, 15 Jul 2015 14:50:46 -0700 Message-ID: Subject: Re: [PATCH 3/7] fs: Ignore file caps in mounts from other user namespaces To: "Serge E. Hallyn" Cc: Seth Forshee , "Eric W. Biederman" , Alexander Viro , Serge Hallyn , James Morris , Linux FS Devel , LSM List , SELinux-NSA , "linux-kernel@vger.kernel.org" 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: 2304 Lines: 50 On Wed, Jul 15, 2015 at 2:48 PM, Serge E. Hallyn wrote: > On Wed, Jul 15, 2015 at 02:46:04PM -0500, Seth Forshee wrote: >> Capability sets attached to files must be ignored except in the >> user namespaces where the mounter is privileged, i.e. s_user_ns >> and its descendants. Otherwise a vector exists for gaining >> privileges in namespaces where a user is not already privileged. >> >> Add a new helper function, in_user_ns(), to test whether a user >> namespace is the same as or a descendant of another namespace. >> Use this helper to determine whether a file's capability set >> should be applied to the caps constructed during exec. >> >> Signed-off-by: Seth Forshee > > Acked-by: Serge Hallyn > > I think it's an ok behavior, though let's just go over the > alternatives. > > It might actually be ok to simply require that the user_ns be > equal. If I unshare a new userns in which a different uid is > mapped to root, I may not want file capabilities to be granted > to tasks in that ns. (On the other hand, I might be creating > a new user_ns specifically to not have a uid 0 mapped into it > at all, and only have file capabilities grant privilege) > > Conversely, if I unshare one user_ns with a MS_SHARED mnt_ns, mount > an ext4fs, and then (from the parent shell) unshare another user_ns > with the same mapping, intending it to be a "peer" to the first one > I'd unshared and be able to use the ext4fs it mounted. This won't > work here. That's probably best - the appropriate thing to do was > to attach to the existing user_ns. But it could end up being > limiting in some special cases, so I'm bringing it up here. > > Again I think what you have here is the simplest and most sensible > choice, so ack. > I think I'm missing something. Why is this separate from mount_may_suid? I can see why it would make sense to check s_user_ns (or maybe s_user_ns *and* the vfsmount namespace) in mount_may_suid, but I don't see why we need separate checks. --Andy -- 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/