SELinux and extended permissions

One of the features present in the August release of the SELinux user space is its support for ioctl xperm rules in modular policies. In the past, this was only possible in monolithic ones (and CIL). Through this, allow rules can be extended to not only cover source (domain) and target (resource) identifiers, but also a specific number on which it applies. And ioctl's are the first (and currently only) permission on which this is implemented.

Note that ioctl-level permission controls isn't a new feature by itself, but the fact that it can be used in modular policies is.

What is ioctl?

Many interactions on a Linux system are done through system calls. From a security perspective, most system calls can be properly categorized based on who is executing the call and what the target of the call is. For instance, the unlink() system call has the following prototype:

int unlink(const char *pathname);

Considering that a process (source) is executing unlink (system call) against a target (path) is sufficient for most security implementations. Either the source has the permission to unlink that file or directory, or it hasn't. SELinux maps this to the unlink permission within the file or directory classes:

allow <domain> <resource> : { file dir }  unlink;

Now, ioctl() is somewhat different. It is a system call that allows device-specific operations which cannot be expressed by regular system calls. Devices can have multiple functions/capabilities, and with ioctl() these capabilities can be interrogated or updated. It has the following interface:

int ioctl(int fd, unsigned long request, ...);

The file descriptor is the target device on which an operation is launched. The second argument is the request, which is an integer whose value identifiers what kind of operation the ioctl() call is trying to execute. So unlike regular system calls, where the operation itself is the system call, ioctl() actually has a parameter that identifies this.

A list of possible parameter values on a socket for instance is available in the Linux kernel source code, under include/uapi/linnux/sockios.h.

SELinux allowxperm

For SELinux, having the purpose of the call as part of a parameter means that a regular mapping isn't sufficient. Allowing ioctl() commands for a domain against a resource is expressed as follows:

allow <domain> <resource> : <class> ioctl;

This of course does not allow policy developers to differentiate between harmless or informative calls (like SIOCGIFHWADDR to obtain the hardware address associated with a network device) and impactful calls (like SIOCADDRT to add a routing table entry).

To allow for a fine-grained policy approach, the SELinux developers introduced an extended allow permission, which is capable of differentiating based on an integer value.

For instance, to allow a domain to get a hardware address (SIOCGIFHWADDR, which is 0x8927) from a TCP socket:

allowxperm <domain> <resource> : tcp_socket ioctl 0x8927;

This additional parameter can also be ranged:

allowxperm <domain> <resource> : <class> ioctl 0x8910-0x8927;

And of course, it can also be used to complement (i.e. allow all ioctl parameters except a certain value):

allowxperm <domain> <resource> : <class> ioctl ~0x8927;

Small or negligible performance hit

According to a presentation given by Jeff Vander Stoep on the Linux Security Summit in 2015, the performance impact of this addition in SELinux is well under control, which helped in the introduction of this capability in the Android SELinux implementation.

As a result, interested readers can find examples of allowxperm invocations in the SELinux policy in Android, such as in the app.te file:

# only allow unprivileged socket ioctl commands
allowxperm { appdomain -bluetooth } self:{ rawip_socket tcp_socket udp_socket } ioctl { unpriv_sock_ioctls unpriv_tty_ioctls };

And with that, we again show how fine-grained the SELinux access controls can be.

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SELinux Userspace 2.7

A few days ago, Jason "perfinion" Zaman stabilized the 2.7 SELinux userspace on Gentoo. This release has quite a few new features, which I'll cover in later posts, but for distribution packagers the main change is that the userspace now has many more components to package. The project has split up the policycoreutils package in separate packages so that deployments can be made more specific.

Let's take a look at all the various userspace packages again, learn what their purpose is, so that you can decide if they're needed or not on a system. Also, when I cover the contents of a package, be aware that it is based on the deployment on my system, which might or might not be a complete installation (as with Gentoo, different USE flags can trigger different package deployments).

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Using nVidia with SELinux

Yesterday I've switched to the gentoo-sources kernel package on Gentoo Linux. And with that, I also attempted (succesfully) to use the propriatary nvidia drivers so that I can enjoy both a smoother 3D experience while playing minecraft, as well as use the CUDA support so I don't need to use cloud-based services for small exercises.

The move to nvidia was quite simple, as the nvidia-drivers wiki article on the Gentoo wiki was quite easy to follow.

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SELinux System Administration, 2nd Edition

While still working on a few other projects, one of the time consumers of the past half year (haven't you noticed? my blog was quite silent) has come to an end: the SELinux System Administration - Second Edition book is now available. With almost double the amount of pages and a serious update of the content, the book can now be bought either through Packt Publishing itself, or the various online bookstores such as Amazon.

With the holidays now approaching, I hope to be able to execute a few tasks within the Gentoo community (and of the Gentoo Foundation) and get back on track. Luckily, my absence was not jeopardizing the state of SELinux in Gentoo thanks to the efforts of Jason Zaman.

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Getting su to work in init scripts

While developing an init script which has to switch user, I got a couple of errors from SELinux and the system itself:

~# rc-service hadoop-namenode format
Authenticating root.
 * Formatting HDFS ...
su: Authentication service cannot retrieve authentication info
(Ignored)
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Doing away with interfaces

CIL is SELinux' Common Intermediate Language, which brings on a whole new set of possibilities with policy development. I hardly know CIL but am (slowly) learning. Of course, the best way to learn is to try and do lots of things with it, but real-life work and time-to-market for now forces me to stick with the M4-based refpolicy one.

Still, I do try out some things here and there, and one of the things I wanted to look into was how CIL policies would deal with interfaces.

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Why we do confine Firefox

If you're a bit following the SELinux development community you will know Dan Walsh, a Red Hat security engineer. Today he blogged about CVE-2015-4495 and SELinux, or why doesn't SELinux confine Firefox. He should've asked why the reference policy or Red Hat/Fedora policy does not confine Firefox, because SELinux is, as I've mentioned before, not the same as its policy.

In effect, Gentoo's SELinux policy does confine Firefox by default. One of the principles we focus on in Gentoo Hardened is to develop desktop policies in order to reduce exposure and information leakage of user documents. We might not have the manpower to confine all desktop applications, but I do think it is worthwhile to at least attempt to do this, even though what Dan Walsh mentioned is also correct: desktops are notoriously difficult to use a mandatory access control system on.

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Can SELinux substitute DAC?

A nice twitter discussion with Erling Hellenäs caught my full attention later when I was heading home: Can SELinux substitute DAC? I know it can't and doesn't in the current implementation, but why not and what would be needed?

SELinux is implemented through the Linux Security Modules framework which allows for different security systems to be implemented and integrated in the Linux kernel. Through LSM, various security-sensitive operations can be secured further through additional access checks. This criteria was made to have LSM be as minimally invasive as possible.

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Filtering network access per application

Iptables (and the successor nftables) is a powerful packet filtering system in the Linux kernel, able to create advanced firewall capabilities. One of the features that it cannot provide is per-application filtering. Together with SELinux however, it is possible to implement this on a per domain basis.

SELinux does not know applications, but it knows domains. If we ensure that each application runs in its own domain, then we can leverage the firewall capabilities with SELinux to only allow those domains access that we need.

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