From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1423353AbXDYR4S (ORCPT ); Wed, 25 Apr 2007 13:56:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1423351AbXDYR4S (ORCPT ); Wed, 25 Apr 2007 13:56:18 -0400 Received: from smtp113.sbc.mail.mud.yahoo.com ([68.142.198.212]:24581 "HELO smtp113.sbc.mail.mud.yahoo.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1423354AbXDYR4R (ORCPT ); Wed, 25 Apr 2007 13:56:17 -0400 X-YMail-OSG: fXPVq6QVM1mCA2zpFEIfb5TwfumuSijpRR54Q3BdjVnXvYaTZ_QLlmMu34jkrtmbMvIG3pj5zA-- Date: Wed, 25 Apr 2007 12:56:09 -0500 From: "Serge E. Hallyn" To: "Eric W. Biederman" Cc: "Serge E. Hallyn" , "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 Message-ID: <20070425175609.GB20165@vino.hallyn.com> References: <462F87EA.1000002@zytor.com> <20070425172012.GA20336@sergelap.austin.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Quoting Eric W. Biederman (ebiederm@xmission.com): > "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. Not mount itself, but in looking up /share/fa/root/home/fa, user fa doesn't have the rights to read /share, and by setting fsuid to fa and dropping CAP_DAC_READ_SEARCH the mount action fails. But the solution you outlined in your previous post would work around this perfectly. > >> (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. Not if we've done prctl(PR_SET_KEEPCAPS, 1) > >> 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? See above. > You are not trying to test this using a non-default security model are you? Nope, at the moment CONFIG_SECURITY=n so I'm running with capabilities only. thanks, -serge