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[liberationtech] The second operating system hiding in every mobile phone

Eugen Leitl eugen at
Wed Nov 13 00:54:09 PST 2013

The second operating system hiding in every mobile phone

posted by Thom Holwerda	 on Tue 12th Nov 2013 23:06 UTC

I've always known this, and I'm sure most of you do too, but we never really
talk about it. Every smartphone or other device with mobile communications
capability (e.g. 3G or LTE) actually runs not one, but two operating systems.
Aside from the operating system that we as end-users see (Android, iOS,
PalmOS), it also runs a small operating system that manages everything
related to radio. Since this functionality is highly timing-dependent, a
real-time operating system is required.

This operating system is stored in firmware, and runs on the baseband
processor. As far as I know, this baseband RTOS is always entirely
proprietary. For instance, the RTOS inside Qualcomm baseband processors (in
this specific case, the MSM6280) is called AMSS, built upon their own
proprietary REX kernel, and is made up of 69 concurrent tasks, handling
everything from USB to GPS. It runs on an ARMv5 processor.

The problem here is clear: these baseband processors and the proprietary,
closed software they run are poorly understood, as there's no proper peer
review. This is actually kind of weird, considering just how important these
little bits of software are to the functioning of a modern communication
device. You may think these baseband RTOS' are safe and secure, but that's
not exactly the case. You may have the most secure mobile operating system in
the world, but you're still running a second operating system that is poorly
understood, poorly documented, proprietary, and all you have to go on are
Qualcomm's Infineon's, and others' blue eyes.

The insecurity of baseband software is not by error; it's by design. The
standards that govern how these baseband processors and radios work were
designed in the '80s, ending up with a complicated codebase written in the
'90s - complete with a '90s attitude towards security. For instance, there is
barely any exploit mitigation, so exploits are free to run amok. What makes
it even worse, is that every baseband processor inherently trusts whatever
data it receives from a base station (e.g. in a cell tower). Nothing is
checked, everything is automatically trusted. Lastly, the baseband processor
is usually the master processor, whereas the application processor (which
runs the mobile operating system) is the slave.

So, we have a complete operating system, running on an ARM processor, without
any exploit mitigation (or only very little of it), which automatically
trusts every instruction, piece of code, or data it receives from the base
station you're connected to. What could possibly go wrong?

With this in mind, security researcher Ralf-Philipp Weinmann of the
University of Luxembourg set out to reverse engineer the baseband processor
software of both Qualcomm and Infineon, and he easily spotted loads and loads
of bugs, scattered all over the place, each and every one of which could lead
to exploits - crashing the device, and even allowing the attacker to remotely
execute code. Remember: all over the air. One of the exploits he found
required nothing more but a 73 byte message to get remote code execution.
Over the air.

You can do some crazy things with these exploits. For instance, you can turn
on auto-answer, using the Hayes command set. This is a command language for
modems designed in 1981, and it still works on modern baseband processors
found in smartphones today (!). The auto-answer can be made silent and
invisible, too.

While we can sort-of assume that the base stations in cell towers operated by
large carriers are "safe", the fact of the matter is that base stations are
becoming a lot cheaper, and are being sold on eBay - and there are even open
source base station software packages. Such base stations can be used to
target phones. Put a compromised base station in a crowded area - or even a
financial district or some other sensitive area - and you can remotely turn
on microphones, cameras, place rootkits, place calls/send SMS messages to
expensive numbers, and so on. Yes, you can even brick phones permanently.

This is a pretty serious issue, but one that you rarely hear about. This is
such low-level, complex software that I would guess very few people in the
world actually understand everything that's going on here.

That complexity is exactly one of the reasons why it's not easy to write your
own baseband implementation. The list of standards that describe just GSM is
unimaginably long - and that's only GSM. Now you need to add UMTS, HSDPA, and
so on, and so forth. And, of course, everything is covered by a ridiculously
complex set of patents. To top it all off, communication authorities require
baseband software to be certified.

Add all this up, and it's easy to see why every cellphone manufacturer just
opts for an off-the-shelf baseband processor and associated software. This
does mean that each and every feature and smartphone has a piece of software
that always runs (when the device is on), but that is essentially a black
box. Whenever someone does dive into baseband software, many bugs and issues
are found, which raises the question just how long this rather dubious
situation can continue.

It's kind of a sobering thought that mobile communications, the cornerstone
of the modern world in both developed and developing regions, pivots around
software that is of dubious quality, poorly understood, entirely proprietary,
and wholly insecure by design.

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