From: Andreas Dilger Subject: Re: [PATCH v11 21/48] ext4: Add richacl feature flag Date: Mon, 19 Oct 2015 10:39:15 -0600 Message-ID: <5DE0621B-7308-41C9-A147-32E0270B28A8@dilger.ca> References: <1445008706-15115-1-git-send-email-agruenba@redhat.com> <1445008706-15115-22-git-send-email-agruenba@redhat.com> <5621346E.5000500@gmail.com> <5624ED40.7040206@gmail.com> <5625182C.3050007@gmail.com> Mime-Version: 1.0 (Mac OS X Mail 8.2 \(2104\)) Content-Type: multipart/signed; boundary="Apple-Mail=_3404C98A-6D6D-47D4-B89F-1E37AFFC17BB"; protocol="application/pgp-signature"; micalg=pgp-sha256 Cc: Andreas Gruenbacher , Alexander Viro , Theodore Ts'o , "J. Bruce Fields" , Jeff Layton , Trond Myklebust , Anna Schumaker , Dave Chinner , linux-ext4 , XFS Developers , LKML , linux-fsdevel , Linux NFS Mailing List , linux-cifs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Linux API , "Aneesh Kumar K.V" To: Austin S Hemmelgarn Return-path: In-Reply-To: <5625182C.3050007-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-ext4.vger.kernel.org --Apple-Mail=_3404C98A-6D6D-47D4-B89F-1E37AFFC17BB Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii On Oct 19, 2015, at 10:19 AM, Austin S Hemmelgarn = wrote: >=20 > On 2015-10-19 11:34, Andreas Gruenbacher wrote: >> On Mon, Oct 19, 2015 at 3:16 PM, Austin S Hemmelgarn >> wrote: >>> On 2015-10-16 13:41, Andreas Gruenbacher wrote: >>>>=20 >>>> On Fri, Oct 16, 2015 at 7:31 PM, Austin S Hemmelgarn >>>> wrote: >>>>>=20 >>>>> I would like to re-iterate, on both XFS and ext4, I _really_ think = this >>>>> should be a ro_compat flag, and not an incompat one. If a person = has the >>>>> ability to mount the FS (even if it's a read-only mount), then = they by >>>>> definition have read access to the file or partition that the = filesystem >>>>> is contained in, which means that any ACL's stored on the = filesystem are >>>>> functionally irrelevant, >>>>=20 >>>> It is unfortunately not safe to make such a file system accessible = to >>>> other users, so the feature is not strictly read-only compatible. >>>>=20 >>> OK, seeing as I wasn't particularly clear as to why I object to this = in my >>> other e-mail, let's try this again. >>>=20 >>> Can you please explain exactly why it isn't safe to make such a = filesystem >>> accessible to other users? >>=20 >> See here: http://www.spinics.net/lists/linux-ext4/msg49541.html > OK, so to clarify, this isn't 'safe' because: > 1. The richacls that exist on the filesystem won't be enforced. > 2. Newly created files will have no ACL's set. >=20 > It is worth noting that these are also issues with any kind of access = control mechanism. Using your logic, all LSM's need to set separate = incompat feature flags in filesystems they are being used on, as should = POSIX ACLs, and for that matter so should Samba in many circumstances, = and any NFS system not using idmapping or synchronized/centralized user = databases. Now, if the SELinux (or SMACK, or TOMOYO) people had taken = this approach, then I might be inclined to not complain (at least not to = you, I'd be complaining to them about this rather poor design choice), = but that is not the case, because (I assume) they realized that all this = provides is a false sense of security. I would tend to agree here. Anyone who can mount the filesystem on a = kernel without RichACL support can do whatever they want, so at most having a RO_COMPAT flag would serve as a reminder for accidental problems by a sysadmin not in the know. Using an INCOMPAT flag is just asking for = major headaches when someone needs to recover their filesystem on an old = kernel but doesn't provide any added safety. Cheers, Andreas > Issue 1, as I have said before, is functionally irrelevant for anyone = who actually knows what they are doing; all you need for ext* is one of = the myriad of programs for un-deleting files on such a filesystem (such = as ext4magic or extundelete, and good luck convincing them to not allow = being used when this flag is set), for BTRFS you just need the regular = filesystem administration utilities ('btrfs restore' works wonders, and = that one will _never_ honor any kind of permissions, because it's for = disaster recovery), and while I don't know of any way to do this with = XFS, that is only because I don't use XFS myself and have not had the = need to provide tech support for anyone who does. If somebody = absolutely _needs_ a guarantee that the acls will be enforced, they need = to be using whole disk encryption, not just acls, and even that can't = provide such a guarantee. >=20 > As for issue 2, that can be solved by making it a read-only compatible = flag, which is what I was suggesting be done in the first place. The = only situation I can think of that this would cause an issue for is if = the filesystem was not cleanly unmounted, and the log-replay doesn't set = the ACL's, but mounting an uncleanly unmounted filesystem that has = richacls on a kernel without support should fall into one of the = following 2 cases more than 99% of the time: > 1. The system crashed hard, and the regular kernel is un-bootable for = some reason, in this case you're at the point of disaster recovery, = should not be exposing _anything_ to a multi-user environment, and = probably care a lot more about being able to get the system running = again than about not accidentally creating a file with a missing ACL. > 2. The filesystem was maliciously stolen in some way (either the = hardware was acquired, or more likely, someone got an image of a still = mounted filesystem), in which case all of my statements above regarding = issue 1 apply. >=20 Cheers, Andreas --Apple-Mail=_3404C98A-6D6D-47D4-B89F-1E37AFFC17BB Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP using GPGMail -----BEGIN PGP SIGNATURE----- Comment: GPGTools - http://gpgtools.org iQIVAwUBViUctXKl2rkXzB/gAQiVFg//WlCjN9eSwE5gEBtGcuYLlNHlyjvM5EoU UAheHBujW+3QS6JFGykQuZdY1qNTgl5TeSFUXCnNprMd6Jp0XvKDtY600iXwwAmC l8PQoYP7v3qwgtbfNUj7la9kcAYZY9u+GDmh1e32aLI0bP+KrBwXj9o4POWtdL6F Z6TL7P/wBJCJj9UWVbintU8sIiMyPmnHsz52izdNoO1LOSIhfAy5XAjn9d8TED9Z ZCwCbAP4c6GU/DhMceQMGuTR0BPUJyqzhoSAwEd+QxBm3D2N1BUhHbehiHq9r+uG MSa49iOdJ6dj6KzkzFlroIX2xuPtofcxC8qYKC905rO5eQiYaU+P4raLfZpZa2pc 2pQi09rPOM0+8LnjyFIdKyYslhf8AwcEwLQimGoBJoWPwgnjTIUCU4xmADU0WeOG IHV+dX3UNcI5wdXpI+vZlFHNyub4wBvty1JIuK1QkK94V2iuV0eb7PZIZwpxf5/9 LJQexMPTikhOvXqYd4u+8ZmGNrcq7iuOrt2+LOtxaRszRV9qRIZWctZ6n7lUSYAP v9lNZ11QKldKQhpTaYR7ESQSlMf4+ecBPc0TumBfKW9QE0vWSTJ9vmnNLAsM1KVd PDV8lw/OC6hg7DlQ0fWcv8pO17t0brZ+XpW9T3ZQ4KjktW5taYDzXcjDN430+FnC bNhL6QX+4mQ= =GnvZ -----END PGP SIGNATURE----- --Apple-Mail=_3404C98A-6D6D-47D4-B89F-1E37AFFC17BB--