From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423346AbXDYRsQ (ORCPT ); Wed, 25 Apr 2007 13:48:16 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1423343AbXDYRsQ (ORCPT ); Wed, 25 Apr 2007 13:48:16 -0400 Received: from ebiederm.dsl.xmission.com ([166.70.28.69]:38877 "EHLO ebiederm.dsl.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1423334AbXDYRsO (ORCPT ); Wed, 25 Apr 2007 13:48:14 -0400 From: ebiederm@xmission.com (Eric W. Biederman) To: "Serge E. Hallyn" Cc: "H. Peter Anvin" , Miklos Szeredi , akpm@linux-foundation.org, viro@ftp.linux.org.uk, linuxram@us.ibm.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, containers@lists.osdl.org, linux-security-module@vger.kernel.org Subject: Re: [patch] unprivileged mounts update References: <462F87EA.1000002@zytor.com> <20070425172012.GA20336@sergelap.austin.ibm.com> Date: Wed, 25 Apr 2007 11:46:15 -0600 In-Reply-To: <20070425172012.GA20336@sergelap.austin.ibm.com> (Serge E. Hallyn's message of "Wed, 25 Apr 2007 12:20:12 -0500") Message-ID: User-Agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org "Serge E. Hallyn" writes: > Quoting H. Peter Anvin (hpa@zytor.com): >> Miklos Szeredi wrote: >> > >> > Andrew, please skip this patch, for now. >> > >> > Serge found a problem with the fsuid approach: setfsuid(nonzero) will >> > remove filesystem related capabilities. So even if root is trying to >> > set the "user=UID" flag on a mount, access to the target (and in case >> > of bind, the source) is checked with user privileges. >> > >> > Root should be able to set this flag on any mountpoint, _regardless_ >> > of permissions. >> > >> >> Right, if you're using fsuid != 0, you're not running as root > > Sure, but what I'm not clear on is why, if I've done a > prctl(PR_SET_KEEPCAPS, 1) before the setfsuid, I still lose the > CAP_FS_MASK perms. I see the special case handling in > cap_task_post_setuid(). I'm sure there was a reason for it, but > this is a piece of the capability implementation I don't understand > right now. So we drop CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH, CAP_FOWNER, and CAP_FSETID Since we are checking CAP_SETUID or CAP_SYS_ADMIN how is that a problem? Are there other permission checks that mount is doing that we care about. >> (fsuid is >> the equivalent to euid for the filesystem.) > > If it were really the equivalent then I could keep my capabilities :) > after changing it. We drop all capabilities after we change the euid. >> I fail to see how ruid should have *any* impact on mount(2). That seems >> to be a design flaw. > > May be, but just using fsuid at this point stops me from enabling user > mounts under /share if /share is chmod 000 (which it is). I'm dense today. If we can't work out the details we can always use a flag. But what is the problem with fsuid? You are not trying to test this using a non-default security model are you? Eric