Towards Secure System Graphics: Arcan and OpenBSD

Let me preface this by saying that this is a (very) long and medium-rare technical article about the security considerations and minutiae of porting (most of) the Arcan ecosystem to work under OpenBSD. The main point of this article is not so much flirting with the OpenBSD crowd or adding further noise to software engineering topics, but to go through the special considerations that had to be taken, as notes to anyone else that decides to go down this overgrown and lonesome trail, or are curious about some less than obvious differences between how these things “work” on Linux vs. other parts of the world.

A disclaimer is also that most of this have been discovered by experimentation and combining bits and pieces scattered in everything from Xorg code to man pages, there may be smarter ways to solve some of the problems mentioned – this is just the best I could find within the time allotted. I’d be happy to be corrected, in patch/pull request form that is 😉

Each section will start with a short rant-like explanation of how it works in Linux, and what the translation to OpenBSD involved or, in the cases that are still partly or fully missing, will require. The topics that will be covered this time are:

Motivation

One of the many lofty goals behind Arcan has been to not only reduce system-wide desktop complexity, to push the envelope in terms of system graphics features, quality and performance – but also to enable experimentation with security sensitive workflows and interaction schemes, with the One Night in Rio: Vacation Photos from Plan9 article covering one such experiment. Working off the classic Confidentiality- Integrity- Availability- information security staple, the article on Crash-Resilient Wayland Compositing also showed the effort being put into the Availability aspect.

The bigger picture of how much this actually entails will be saved for a different article, but the running notes on the current state of things, as well as higher level experiments are kept in [Engine Security] and [Durden-Security/Safety].

Outside of attack surface reduction, exploit mitigations and safety features, there is a more mundane, yet important aspect of security – and that is of software quality itself. This is an area where it is easy to succumb to fad language “fanboyism” or to drift in the Software Homeopathy direction of some bastardised form of code autism, such as counting the number of lines of code per function, indentation style, comment:code ratio and many other ridiculous metrics.

While there are many parts to reaching a lofty goal of supposed ‘quality software’, the one that is the implicit target of this post is the value of a portable codebase as a way of weening out bugs (where #ifdef hell or ‘stick it in a VM masked as a language runtime’ does not count).

The road to- and value of- a portable codebase is built on the ability to swap out system integration layers as a way of questioning the existing codebase about hidden assumptions and reliance on non-standard, non-portable and possibly non-robust behaviours and interfaces.

This does not always have to be on the OS level, but in other layers in the stack as well. For instance, in the domain that Arcan targets, the OpenGL implementation can be switched out for one that uses GLES, though their respective feature sets manifest many painful yet subtle differences.

Similarly, the accelerated buffer sharing mechanism can be swapped between DMA-Buf and EGLStreams. Low-level DRI and evdev based access for graphics and input can be swapped out for high-level LibSDL based. Engine builds are tested with multiple libc:s to avoid “glibc-rot” and so on. Some of these cases are not without friction, and even painful at times, but the net contribution of being able to swap out key culprits as a means of finding and resolving bugs have, in my experience, been a positive one.

Arcan has been ported to quite a few platforms by now – even if not all of them are publicly accessible; there are certain ecosystems (e.g. android, iOS) we have very little interest in publicly supporting in a normal way. The latest addition to the list of actually supported platforms is OpenBSD, added about a year after we had support for FreeBSD.  So, enough with the high-level banter, time to dive into the technical meat of the matter.

Graphics Device Access Differences

Recall that in the BSDs, most of the DRM+GBM+Mesa (DRI) stack (or as Ilja van Sprundel poignantly and accurately puts it – the shit sandwich) is mostly lifted straight from Linux, hence why some of the interfaces are decidedly “non-BSD” in their look and feel.

Device Nodes and drmMaster

A normal (e)udev setup provides device nodes on linux as /dev/dri/Card + RenderD. In OpenBSD, these nodes are located at /dev/drm, but merely changing the corresponding ‘open’ calls won’t yield much of a success, unless you are running as root. The reason is a piece of ugly known as drmMaster.

As with most things linux these days, if something previously had permissions – there is now likely both a permission, some hidden additional permission layer and some kind of service manager that shift things around if you ever get too comfortable. Something that once looked like a file actually has the characteristics and behaviour of a file system; file permissions and ownership gets “extended” with hidden attributes, capabilities, and so on.

This is the case here as well. A user may well have R/W access to the device node, but only a user with CAP_SYS_ROOT/ADMIN is allowed to become “drmMaster”, a singleton role that determines who is allowed to configure display outputs (modeset) or send data (scanout) to a display, and who that is allowed to only use the device node to access certain buffers and setup accelerated graphics. As a note, this will get more complicated with the ongoing addition of ‘leases’ – tokenised outsourced slicing of display outputs connectors.

In Linux, there are about 2.5 solutions to this problem. The trivial one is weston-launch, have it installed suid and there you go. The second is an ugly monster that we can call a “YAFDOBF” (Yet Another FreeDesktop.Org Brain Fart). It involves an unholy union between Logind, D-Bus, udev and the ‘session’ and ‘s+[ea | hi]+t’ abstractions that ultimately leads to a permanent state of not being able to trust file descriptors.

The last .5 is the one we have been using up until recently, and involves taking advantage of a corner case in that you do not have to set drmMaster – if no-one else has tried to; buffer scanout and modeset actually work anyway, just ignore the failed call. Set normal user permissions on the card node and there we go. This has the risk that an untrusted process running as the same user can do the same and break display state, which leads us to a case of “avoid mixing trust domains within the same uid/gid”. If you really desire to run software that you don’t trust, well, give such miscreants another uid/gid that has permission on the ‘RenderD’ node for privilege separation and the problem would be solved.

Alas, neither the Master-less scanout nor the render node feature currently exist on OpenBSD,  leaving the no-option of running as root or the remaining one of doing what weston-launch did, but in a saner way. This means taking a cue from Xenocara. Make the binary suid, and the first thing you do, no argument parsing, no hidden __constructor nonsense, just setup a socketpair, fork and drop privileges. Implement a trivial open/close protocol over the socketpair, and have a whitelist of permitted devices.

Since we need dynamic and reactive access to a number of other devices anyhow, this tactic works well enough for that as well. There is a caveat here in that the user running the thing can successfully send kill -KILL to the supposedly root process, breaking certain operations like VT switching (see the Missing section).

Input

One of the absolutely murkiest corner in any user – kernel space interface has to be that of input devices: keyboards, trackpads, joysticks, mice and so on. This is one of the rare few areas where I can sit and skim Android documentation and silently whisper “if only” before going back to alternating between screaming into a pillow and tracing through an ever expanding tree of “works 100% – 95% of the time” translation tables, where each new branch manages to be just a tad bit dumber than the ones that came before it.

Starting in Linux land again, first open the documentation. Now, you can close it again – it won’t be useful – I just wanted you to get a glimpse of its horrors. The only interface that can really be used, is evdev, written in cursive just to indicate that it should be pronounced with a hiss. It is a black lump of code that will never turn into a diamond. Instead of being replaced, it gets an ever increasing pile of “helper” libraries (libevdev, libudev, libinput, libratbag, libkilljoy, libreadabuttonpress, libvaluebetween0and1, …) that tries to hide how shitty this layer is, a decay pattern which matches the DLL creep in Windows – without trying to address the core of the matter. The reason for this pattern is, I assume, that no one in their right mind wants to actually work on such a thing within the Linux kernel ecosystem – if it can at all be avoided.

For OpenBSD, the better source of information is probably the PDF (Input Handling in wscons and X) along with the related manpages on wscons, uhid, wsmouse, wskbd. For the most part, these are obvious and simple, with the caveat that most of the low level controls are left to other tools, or that they work as multiplexer abstract devices without any ‘stream identifier’ for demultiplexation. Then there’s the case of the keyboard layout format. The underlying “problem”, depending on your point of view, is the legacy that exist in both Linux and BSDs in that tradition has meant going from BIOS -> boot loader(s) -> “text” console -> (display manager) -> Xorg and we at least need the option of being able to boot straight into UI.

Hotplug

There are two sides to the hotplug coin: input devices and output devices. The normal developer facing Linux strategy uses udev, the source code of which is recommended reading for anyone still looking for more reasons to get away from linux systems programming. If you want to have extra fun, take a look at how udev distinguishes a keyboard from a joystick.

Udev is an abomination that has been on my blacklist for a number of years. It has all the hallmark traits of a parasitic dependency and is a hard one to ignore. For input devices, the choice was simply to go with inotify on a user provided folder (default, /dev/input), and let whatever automatic or manual process he has populating that folder with the device nodes he thinks that Arcan should try and use.

For output, one might be easily fooled to think that the card device node that is used for everything (almost, see the backlight section) that relate to displays like mode setting, buffer scanout and synch triggers, should also be used to detect when a display is added or removed to this card. Welcome to Linux. How about, instead, we send an event with data over a socket (Netlink) in a comically shitty format, to a daemon (udev) which maps to a path in a filesystem (sysfs best pronounced as Sisyphus) which gets scraped and correlated to a database and then broadcasted to a set of listeners over D-Bus. Oh goodie. One might be so bold to call this an odd design, given that all we need is a signal to trigger a rescan of the CRTCs (“Cathode Ray ConTrollers” for a fun legacy word) as the related API basically forces us to do our own tracking of connector card set deltas anyhow.

In OpenBSD (>= 6.3) you add the file descriptor of the card device node to a kqueue (❤️) set and wait for it to signal.

Backlight

Backlight is typically reserved for the laptop use case where it is an important one, as not only can modern screens be murdering bright, but also bat away at your battery. This subsystem is actually quite involved in Arcan as it maps as yet another LED controller, though one that gets paired to ‘display added’ events. Hence why we can do things like in  this (youtube video).

The approach we used for Linux is lifted from libbacklight before it became a part of the katamari damacy game that is systemd. Again one might be tempted to think that the backlight control could somehow be tied to the display that it regulates the brightness of, but that is not how this works. Just like with hotplug, there’s scraping around in sysfs.

In OpenBSD, you grab the console tty and send an ioctl to it.

Xorg

As touched upon in the section on graphics device access, OpenBSD maintains its own Xorg fork known as Xenocara in order to maintain their specific brand of privilege separation. Arcan meanwhile maintains its own Xarcan fork to get more flexibility and security options than what XWayland can offer, at the cost of legacy application “transparency” (though that will be introduced via the Wayland bridge at some point).

The only real difference here right now, in addition to the much superior DRI3(000) rendering and data passing model is not working (see the Missing section further below) – was adding translation tables to go from SDL1.2 keysyms (the unfortunate format that we have to work with, no one, especially not me, is without sin here).

Pledging

A large part of the work behind Arcan has been to resection the entire desktop side of user space to permit much more fine grained privilege separation, particularly when it comes to parsers working on untrusted data – but also for server to client sharing modes. Thus we split out as much of the data transformations and specialised data providers as possible into their own processes – and the same goes for data parsing.

One of the reasons for this is, of course, that these are error prone tasks that should not be allowed to damage the rest of the system in any way, segmenting these out and monitoring for crashes is a decent passive way of finding new less than exciting vulnerabilities in FFmpeg, something that still happens much too often some ten years later.

The other goal is, of course, to be able to apply more precise sandboxing as one process gets assigned one task, like a good Mr. Meeseeks. An indirect beneficial side effect to this is that we get a context for experimenting with the different sandboxing experiences that the various OSes provide, such as the case of the supplementary tool ‘aloadimage‘ (a quite paranoid version of xloadimage).

Missing

The OpenBSD port of DRI current lacks two important features – render nodes and DMA buffers. This means that we have no sane way of sending accelerated buffers between a client using accelerated graphics and Arcan, and we put little effort in providing support for the older way (GEM/…). This breaks the nested ‘arcan-in-arcan’ mode with the arcan_lwa binary, it breaks wayland support and it breaks Xarcan supporting DRI3 as these all rely on that feature.

While it is technically possibly to explicitly enable old GEM style GPU sharing (durden exposes these as config/system/GPU delegation) where clients to be trusted authenticate via a challenge response scheme to the GPU then gets the keys to the kingdom, it is far from a recommended way as this puts the client on just about the same privilege terms as the display server itself. That being said, it is murderously foolish to think that a client who is allowed any form of raw GPU access these days doesn’t have multiple ways of privilege escalation waiting to be discovered; the order of priority is getting them to do what they are told without crashing the system, then to account for clients that rely on unspecified or buggy behaviour and maybe to get them to perform under these conditions. Security is a distant figure somewhere on the horizon.

Last and incidentally, my least favourite part of the entire ecosystem, “libwayland-server” still lacks a working port – which comes as no surprise as this thread unsafe, typecasting crazy, faux-vtable loving, use-after free factory is littered with no-nos; the least of which being its pointless reliance on ‘epoll‘.

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2 Responses to Towards Secure System Graphics: Arcan and OpenBSD

  1. Trek says:

    wow, this is a very interesting reading!

  2. N. Prince says:

    Interesting read, indeed! There is something about the details, here. I said to myself, there is actually someone who slipped through the cracks and somehow avoided the trapdoor spider. It is so exciting the read the truth about udev and other poor quality code standards in Linux. All this wholeheartedly slavish Linux worshipping went into the direction of accepted poor industry standards. The industry and the blind worshippers allows Linux to lead with support when there was already better unix types like Plan 9, OpenSolaris, and BSDs.

    Why throw away 40 years of proven reliable accomplishments just to reinvent the wheel with a broken implementation? For bragging rights? To attract hordes of worshippers who are clueless and precarious about the truth in regards to quality coding, standards, and implementation?

    That’s like killing the most advanced culture ever known to man just to replace them with the experts in plagiarism setting back man’s advancement for thousands of years!

    How counterintuitive can man be?
    When he is desperate for power and control, …without limits.

    Nowadays, worshipping and stardom appears to have more higher priority over the truth.

    That’s not dissimilar to allowing Microsoft and Intel take the lead in industry standards. Hostile takerovers, and strongarming legislators aside, these 2 corporations are the biggest setbacks to progress in the computer world.

    What else is crazy? The founder of Linux, codes Linux on a Mac (Darwin BSD type) computer.

    I like the direction of OpenBSD and Arcan.

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