Joshua Brindle

How to Win At Security

Misunderstanding UNIX security

I just got a comment on my post about path based access control that was fairly startling to me. The more I thought about it, though, the more I thought maybe others shared the beliefs so I'm going to respond to it here.

inode-based security has analogous problems to path-based security. Software opens paths, not inodes, so it very much matters what the permissions are for the file at /etc/shadow or /var/data/mydb, regardless of what the inode happens to be. With an inode-based system, security may go out the window when programs replace or manipulate inodes, as many of them do.

Software opens paths because paths are the exposed abstraction for userspace applications. Sure it matters what permissions are on /etc/shadow, but those permissions and the access control alike is always done at the inode level, as I will demonstrate below. Applications that manipulate inodes have always needed to set security attributes on those inodes, look at passwd source code and you will see that it creates the inode with mode 400 (r--------).

So, neither AppArmor nor SELinux provide bullet proof security or data integrity, although both may be useful in protecting against some problems.

This is blatantly wrong, SELinux provides controls to specify precisely how things can be labeled. If my policy does not allow an application that handles confidential information to relabel or create a file with a label that is accessible by other applications there are no circumstances where that application will be able to leak the data. Similarly if an application can't read low-integrity data and write to high-integrity files there are no circumstances where the application can harm the data integrity. I suggest you read some of the papers about SELinux policy analysis and information flow to understand the issues and how SELinux can address them. Many are available at the SELinux symposium website.

But arguments like the ones in this blog really make me doubt whether the SELinux people even understand the issues or have much of a notion of how files and security work on real-world UNIX systems.

As much as I'd like to ignore people using ad hominum attacks there may be others under this impression. Note the following demonstration as a counter example:

# ls -al /etc/shadow
-r-------- 1 root root 947 2007-06-06 03:40 /etc/shadow
# ln /etc/shadow /tmp/shadow
# ls -al /tmp/shadow
-r-------- 2 root root 947 2007-06-06 03:40 /tmp/shadow
# chmod 777 /tmp/shadow
# ls -al /etc/shadow
-rwxrwxrwx 2 root root 947 2007-06-06 03:40 /etc/shadow

It should be clear at this point that the UNIX permissions absolutely do not have anything to do with paths, only with inodes. This is how real-world UNIX systems have always worked, path based mechanisms break that convention and are subject to the issues I've noted in the path based article. The simple fact is that UNIX systems never ever use paths to determine access to anything, access control has always been centered around inodes and permissions set on inodes. Do not think that the existance of paths changes that, these are merely an abstraction to make userspace more useful.

The arguments against SELinux by the AppArmor people, and others, are purely functionality based arguments. They want to use paths because paths are easy to understand and paths have conventions that can be reflected in their policy (eg., that passwords are always stored in /etc/shadow and therefore that file should be protected). SELinux provides mechanisms to enforce many different security policies including protecting /etc/shadow and /var/data/mydb.