Re: [PATCH 7/7] SLIM: documentation

From: Randy.Dunlap
Date: Wed Aug 23 2006 - 15:42:15 EST

On Wed, 23 Aug 2006 12:05:56 -0700 Kylene Jo Hall wrote:

> Documentation.
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxx>
> Signed-off-by: Kylene Hall <kjhall@xxxxxxxxxx>
> ---
> Documentation/slim.txt | 136 +++++++++++++++++++++++++++++++++++++++
> 1 files changed, 136 insertions(+)
> --- linux-2.6.18/Documentation/slim.txt 1969-12-31 16:00:00.000000000 -0800
> +++ linux-2.6.18-rc4/Documentation/slim.txt 2006-08-22 14:48:12.000000000 -0700
> @@ -0,0 +1,136 @@
> +Simple Linux Integrity Model (SLIM)
> +
> +SLIM is an LSM module which provides an enhanced low water-mark
> +integrity and high water-mark secrecy mandatory access control
> +model. It also is a consumer of the new integrity subsystem,
> +using the integrity_verify_data(), integrity_verify_metadata(),
> +and integrity_measure() calls to base mandatory access control
> +decisions on the verified integrity status of the involved objects.
> +SLIM is an extension of several prior models, including Biba[1],
> +Lowmac[2], and Caernarvon[3], which provide excellent background.
> +
> +SLIM's specific model is:
> +
> + All objects (files) are labeled with extended attributes to indicate:
> + Integrity Access Class (IAC)
> + Secrecy Access Class (SAC)
> +
> + All processes inherit from their parents:
> + Integrity Read Access Class (IRAC)
> + Integrity Write/Execute Access Class (IWXAC)
> + Secrecy Write Access Class (SWAC)
> + Secrecy Read/Execute Access Class (SRXAC)
> +
> + SLIM enforces the following Mandatory Access Control Rules:
> + Read:
> + IRAC(process) <= IAC(object)
> + SRXAC(process) >= SAC(object)
> + Write:
> + IWXAC(process) >= IAC(object)
> + SWAC(process) <= SAC(process)
> + Execute:
> + IWXAC(process) <= IAC(object)
> + SRXAC(process) >= SAC(object)
> +
> +In the low water-mark model, rather than blocking attempted
> +reads of lower integrity objects, the reading process is demoted
> +to the integrity level of the object, so that the read is allowed.
> +In a Linux client, this provides a much more usable environment,
> +in which applications run more transparently, while being demoted
> +as needed to protect the integrity of the system.
> +
> +When the process is demoted, it may have objects open for write
> +of now higher integrity level, and these objects have to have their
> +write access revoked. This revocation of write privilege must
> +occur for normal and mmap'ed file writes. Similarly, when reading
> +an object of higher secrecy, the process is promoted to the higher
> +secrecy level, and write access to now lower secrecy objects is revoked.
> +
> +SLIM performs a generic revocation operation, including revoking
> +mmap and shared memory access. Note that during demotion or promotion
> +of a process, SLIM needs only revoke write access to files with higher
> +integrity, or lower secrecy.
> +
> +SLIM inherently deals with dynamic task labels, which is a feature
> +not currently available in selinux. While it might be possible to
> +add support for this to selinux, it would not appear to be simple,
> +and it is not clear if the added complexity would be desirable
> +just to support this one model.
> +
> +Comments on the model:
> +
> +Some of the prior comments questioned the usefulness of the
> +low water-mark model itself. Two major questions raised concerned
> +a potential progression of the entire system to a fully demoted
> +state, and the security issues surrounding the guard processes.
> +
> +In normal operation, the system seems to stabilize with a roughly
> +equal mixture of SYSTEM, USER, and UNTRUSTED processes. Most
> +applications seem to do a fixed set of operations in a fixed domain,
> +and stabilize at their appropriate level. Some applications, like
> +firefox and evolution, which inherently deal with untrusted data,
> +immediately go to the UNTRUSTED level, which is where they belong.
> +In a couple of cases, including cups and Notes, the applications
> +did not handle their demotions well, as they occured well into their

same as my previous comments: "occurred"

> +startup. For these applications, we simply force them to start up
> +as UNTRUSTED, so demotion is not an issue. The one application

s/application/application area/ or /application type/ ?

> +that does tend to get demoted over time are shells, such as bash.
> +These are not problems, as new ones can be created with the
> +windowing system, or with su, as needed. To help with the associated
> +user interface issue, the user space package[4] README shows how to
> +display the SLIM level in window titles, so it is always clear at
> +what level the process is currently running.
> +
> +As for the issue of guard processes, SLIM defines three types of
> +guard processes: Unlimited Guards, Limited Guards, and Untrusted
> +Guards. Unlimited Guards are the most security sensitive, as they
> +allow less trusted process to acquire a higher level of trust.
> +On my current system there are two unlimited guards, passwd and
> +userhelper. These two applications inherently have to be trusted
> +this way regardless of the MAC model used. In SLIM, the policy
> +clearly and simply labels them as having this level of trust.
> +
> +Limited Guards are programs which cannot give away higher
> +trust, but which can keep their existing level despite reading
> +less trusted data. On my system I have seven limited guards:
> +yum, which is trusted to verify the signature on an (untrusted)
> +downloaded RPM file, and to install it, login and sshd, which read
> +untrusted user supplied login data, for authentication, dhclient
> +which reads untrusted network data, and updates they system
> +file /etc/resolv.conf, dbus-daemon, which accepts data from
> +potentially untrusted processes, Xorg, which has to accept data
> +from all Xwindow clients, regardless of level, and postfix which
> +delivers untrusted mail. Again, these applications inherently
> +must cross trust levels, and SLIM properly identifies them.
> +
> +As mentioned earlier, cupsd and notes are applications which are

Notes (as used earlier)

> +always run directly in untrusted mode, regardless of the level of
> +the invoking process.
> +
> +The bottom line is that SLIM guard programs inherently do security
> +sensitive things, and have to be trusted. There are only a small
> +number of them, and they are clearly identified by their labels.
> +
> +Userspace Tools:
> +
> +Papers and slides on SLIM, along with source code for the needed
> +userspace tools, and installation instructions are available at:
> +
> +[4]
> +
> +References:
> +
> +[1 Biba]: K. J. Biba. âIntegrity Considerations for Secure Computer Systemsâ
> +Technical Report ESD-TR-76-372, USAF Electronic Systems Division, Hanscom Air
> +Force Base, Bedford, Massachusetts, April 1977.
> +
> +[2 Lomac]: T. Fraser, "LOMAC: Low Water-Mark Integrity Protection for COTS
> +Environments," Proceedings of the 2000 IEEE Symposium on Security and
> +Privacy, Oakland, California, USA, 2000.
> +
> +[3 Caernarvon]: P. Karger, V. Austel, and D. Toll. âUsing a Mandatory Secrecy
> +and Integrity Policy on Smart Cards and Mobile Devicesâ EUROSMART Security
> +Conference. 13-15 June 2000, Marseilles, France p. 134-148.

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