From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753641AbZHJLrn (ORCPT ); Mon, 10 Aug 2009 07:47:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753417AbZHJLrm (ORCPT ); Mon, 10 Aug 2009 07:47:42 -0400 Received: from msux-gh1-uea02.nsa.gov ([63.239.67.2]:61682 "EHLO msux-gh1-uea02.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752827AbZHJLrl (ORCPT ); Mon, 10 Aug 2009 07:47:41 -0400 Subject: Re: [Patch v3] vfs: allow file truncations when both suid and write permissions set From: Stephen Smalley To: Eric Paris Cc: OGAWA Hirofumi , Amerigo Wang , linux-kernel@vger.kernel.org, esandeen@redhat.com, eteo@redhat.com, linux-fsdevel@vger.kernel.org, akpm@linux-foundation.org, viro@zeniv.linux.org.uk, linux-security-module@vger.kernel.org In-Reply-To: <1249677481.2694.22.camel@dhcp231-106.rdu.redhat.com> References: <20090807100743.5822.90612.sendpatchset@localhost.localdomain> <1249675025.2694.15.camel@dhcp231-106.rdu.redhat.com> <87prb7v0dr.fsf@devron.myhome.or.jp> <1249677481.2694.22.camel@dhcp231-106.rdu.redhat.com> Content-Type: text/plain Organization: National Security Agency Date: Mon, 10 Aug 2009 07:49:24 -0400 Message-Id: <1249904964.2422.21.camel@moss-pluto.epoch.ncsc.mil> Mime-Version: 1.0 X-Mailer: Evolution 2.26.3 (2.26.3-1.fc11) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2009-08-07 at 16:38 -0400, Eric Paris wrote: > On Sat, 2009-08-08 at 05:23 +0900, OGAWA Hirofumi wrote: > > Eric Paris writes: > > > > > On Fri, 2009-08-07 at 06:05 -0400, Amerigo Wang wrote: > > >> V2 -> V3: > > >> Call notify_change() before clearing suid/sgid. > > >> Thanks to OGAWA Hirofumi. > > >> > > >> V1 -> V2: > > >> Introduce dentry_remove_suid(), and use it in do_truncate(). > > >> Thanks to Eric Paris. > > >> > > >> > > >> When suid is set and the non-owner user has write permission, > > >> any writing into this file should be allowed and suid should be > > >> removed after that. > > >> > > >> However, current kernel only allows writing without truncations, > > >> when we do truncations on that file, we get EPERM. This is a bug. > > >> > > >> Steps to reproduce this bug: > > >> > > >> % ls -l rootdir/file1 > > >> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1 > > >> % echo h > rootdir/file1 > > >> zsh: operation not permitted: rootdir/file1 > > >> % ls -l rootdir/file1 > > >> -rwsrwsrwx 1 root root 3 Jun 25 15:42 rootdir/file1 > > >> % echo h >> rootdir/file1 > > >> % ls -l rootdir/file1 > > >> -rwxrwxrwx 1 root root 5 Jun 25 16:34 rootdir/file1 > > >> > > >> This patch fixes it. > > >> > > >> Signed-off-by: WANG Cong > > >> Cc: Eric Sandeen > > >> Cc: Eric Paris > > >> Cc: Eugene Teo > > >> Cc: Al Viro > > >> Cc: hirofumi@mail.parknet.co.jp > > > > > > I was thinking about this and kept telling myself I was going to test v2 > > > before I ack/nak. Clearly we shouldn't for the dropping of SUID if the > > > process didn't have permission to change the ATTR_SIZE. > > > > > > Acked-by: Eric Paris > > > > BTW, Do you know why doesn't security modules fix the handling of > > do_truncate() (i.e. ATTR_MODE | ATTR_SIZE). And why doesn't it allow to > > pass ATTR_FORCE for it? > > I'm not sure what you mean. I understood ATTR_FORCE to mean 'I am magic > and get to override all security checks." Which is why nothing should > ever be using ATTR_FORCE with things other than SUID. > > I guess we could somehow force logic into the LSM to make it only apply > to SUID and friends but I'm not sure it buys us anything. SELinux shouldn't apply a permission check for the clearing of the suid bit on write or truncate. It should only apply a permission check for the actual truncate or write operation, and then the clearing of the suid bit should always be forced if that check passed. -- Stephen Smalley National Security Agency