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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Authenticating with U2F

In order to further secure access to my workstation, after the switch to Gentoo sources, I now enabled two-factor authentication through my Yubico U2F USB device. Well, at least for local access - remote access through SSH requires both userid/password as well as the correct SSH key, by chaining authentication methods in OpenSSH.

Enabling U2F on (Gentoo) Linux is fairly easy. The various guides online which talk about the pam_u2f setup are indeed correct that it is fairly simple. For completeness sake, I've documented what I know on the Gentoo Wiki, as the pam_u2f article.

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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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Switch to Gentoo sources

You've might already read it on the Gentoo news site, the Hardened Linux kernel sources are removed from the tree due to the grsecurity change where the grsecurity Linux kernel patches are no longer provided for free. The decision was made due to supportability and maintainability reasons.

That doesn't mean that users who want to stick with the grsecurity related hardening features are left alone. Agostino Sarubbo has started providing sys-kernel/grsecurity-sources for the users who want to stick with it, as it is based on minipli's unofficial patchset. I seriously hope that the patchset will continue to be maintained and, who knows, even evolve further.

Personally though, I'm switching to the Gentoo sources, and stick with SELinux as one of the protection measures. And with that, I might even start using my NVidia graphics card a bit more, as that one hasn't been touched in several years (I have an Optimus-capable setup with both an Intel integrated graphics card and an NVidia one, but all attempts to use nouveau for the one game I like to play - minecraft - didn't work out that well).

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Project prioritization

This is a long read, skip to “Prioritizing the projects and changes” for the approach details...

Organizations and companies generally have an IT workload (dare I say, backlog?) which needs to be properly assessed, prioritized and taken up. Sometimes, the IT team(s) get an amount of budget and HR resources to "do their thing", while others need to continuously ask for approval to launch a new project or instantiate a change.

Sizeable organizations even require engineering and development effort on IT projects which are not readily available: specialized teams exist, but they are governance-wise assigned to projects. And as everyone thinks their project is the top-most priority one, many will be disappointed when they hear there are no resources available for their pet project.

So... how should organizations prioritize such projects?

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Structuring infrastructural deployments

Many organizations struggle with the all-time increase in IP address allocation and the accompanying need for segmentation. In the past, governing the segments within the organization means keeping close control over the service deployments, firewall rules, etc.

Lately, the idea of micro-segmentation, supported through software-defined networking solutions, seems to defy the need for a segmentation governance. However, I think that that is a very short-sighted sales proposition. Even with micro-segmentation, or even pure point-to-point / peer2peer communication flow control, you'll still be needing a high level overview of the services within your scope.

In this blog post, I'll give some insights in how we are approaching this in the company I work for. In short, it starts with requirements gathering, creating labels to assign to deployments, creating groups based on one or two labels in a layered approach, and finally fixating the resulting schema and start mapping guidance documents (policies) toward the presented architecture.

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Matching MD5 SSH fingerprint

Today I was attempting to update a local repository, when SSH complained about a changed fingerprint, something like the following:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
SHA256:p4ZGs+YjsBAw26tn2a+HPkga1dPWWAWX+NEm4Cv4I9s.
Please contact your system administrator.
Add correct host key in /home/user/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /home/user/.ssh/known_hosts:9
ECDSA host key for 192.168.56.101 has changed and you have requested strict checking.
Host key verification failed.
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Switched to Lineage OS

I have been a long time user of Cyanogenmod, which discontinued its services end of 2016. Due to lack of (continuous) time, I was not able to switch over toward a different ROM. Also, I wasn't sure if LineageOS would remain the best choice for me or not. I wanted to review other ROMs for my Samsung Galaxy SIII (the i9300 model) phone.

Today, I made my choice and installed LineageOS.

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