Using the Kernel Security Module Interface

Greg shows how to create a simple kernel module that uses the LSM framework.
But Is It Secure?

This example shows how powerful and simple the LSM interface can be. With one hook, any program with the root group ID is prevented from running unless a device is physically present in the system.

Using this code, if the device is not present, users are not allowed to log in to the console, as mingetty traditionally runs as root. But users can log in through SSH as normal users, as sshd already was running before the device was removed. Web pages also can be served, and other services that do not run as root (your mail server, database server, etc.) also will function properly. If one of these server programs were broken into, and they tried to spawn a root shell, that root shell would not be allowed to run.

This module does not prevent any program already running as root from cloning itself, or keep a program from trying to change the privileges that are currently assigned to it. To check for these things, the task_* functions in the security_operations structure should be used. The implementation of these functions will be much like the bprm_check_security function, but the parameters passed to the function will be different, so the egid will need to be determined differently.

There are probably other methods of taking an existing running program and spawning a root process that this module does not catch. Please do not use it in a production environment, but rather as a learning exercise for how to create other LSM example code.

Acknowledgements

I would like to thank Chris Wright, Stephen Smalley, James Morris and all of the other programmers who helped create the LSM interface and get it accepted into the main kernel tree. Due to their hard work, Linux now has a flexible security model that will give everyday users the ability to have access to different security models with little effort. I also would like to thank Alan Cox for the initial idea that spawned this example.

For more information about the LSM Project, the development mailing list, documentation and patches for different kernel versions, please see the web site at lsm.immunix.org.

Greg Kroah-Hartman is currently the Linux USB and PCI Hot Plug kernel maintainer. He works for IBM, doing various Linux kernel-related things, and can be reached at greg@kroah.com.

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modules should support multiple devices

Warren Crossing's picture

I know your magazine and buy/read it occasionally, generally it is a good read. This article provided good information in a clean uncomplicated style. I think LJ has the right to write/solicit articles and charge access for them in print. I think LJ should be committed to maintaining this document and allowing access now that kernel code refer to it.

I also think people shouldn't jump to conclusions and demand software information without any fiscal cost.

We are talking free as in freedom! Money doesn't grow on trees and there is no obligation for module developers to produce/provide their new cool code to the community.

Please be grateful, for SO many have given SO much before you!!

Huh

Anonymous's picture

WTF... i want to read this

very poor that we cannot view this

Anonymous's picture

i came here based on comments in the kernel which referred to this article. i was very disappointed to find that i have to be a financial member in order to read the article.

i will be making a recommendation for your site to be removed from any kernel comments.

Article Referenced in Kernel Docs...

cjsutton's picture

...yet you decide to deny access to it unless I pay you money.

"This still leaves more articles available for public viewing than are reserved for subscribers -- as of last count over 2000!"

Interesting that this isn't one of those.

Does Linus know about this?

Anonymous's picture

This is BS, open up the article.

I wonder how Linus would feel about this non-free scheme, of pay for docs, that are referenced in his kernel.

came here because of referenc

Anonymous's picture

came here because of reference in the kernel docs
looked interesting

Rest assured that, because of

Anonymous's picture

Rest assured that, because of this, I will NEVER subscribe to your magazine. This is complete and utter bull$hit!! Docs shouldn't be proprietary!

Settle down. It's not LJ's f

Anonymous's picture

Settle down. It's not LJ's fault that some kernel dev couldn't be bothered to write real documentation for the rootplug module...

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