[Replicant] [PATCH 6/9] freedom-privacy-security-issues: Improve hardware freedom state description.
Denis 'GNUtoo' Carikli
GNUtoo at no-log.org
Wed Mar 2 19:28:53 UTC 2016
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo at no-log.org>
---
freedom-privacy-security-issues.php | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
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<p>Regarding the software side of things on mobile devices, the main CPU (inside the SoC) starts by executing code located inside the silicon. If this is an ARM CPU, that code will be ARM instructions. This code is known as the botrom. It will look up various places such as NAND, eMMC or MMC (sd/micro sd card) storage, depending on the hardware configuration, to load a bootloader. The bootloader, which is in fact often split in different stages, is in charge of bringing up and configuring various aspects of the hardware and eventually starting the operating system by loading and running its kernel.<br /><a href="images/freedom-privacy-security-issues/software.png" data-lightbox="overview" data-title="Software-side overview"><img src="images/freedom-privacy-security-issues/software.png" alt="Software-side overview" style="width: 250px; float: right;"/></a>The kernel itself, among other things, deals with the hardware directly and provides ways for other programs (running i
n user-s
pace) to access it. In user-space, hardware abstraction layers are programs specific to each device that know how to properly drive the hardware. They use the kernel to communicate back and forth with the hardware and implement the proper protocols for it.<br /><br />The actual knowledge of how to drive the hardware is split between the kernel and the hardware abstraction layer libraries: both are needed to make it work properly. Hardware abstraction layers provide a generic interface for the framework to use. The framework itself provides an interface for applications that is independent of the device and the hardware. That way, applications can access hardware features through the generic framework interface, which will call the hardware abstraction layer libraries, ending up with the kernel communicating with the hardware. For instance, when making a call, the dialer application will communicate with the framework, which in turn will communicate with the hardware abstract
ion laye
r. That hardware abstraction layer will implement the protocol to speak with the modem, and the Linux kernel will be responsible of permitting the communication between the hardware abstraction layer and the modem.</p>
<p>Many other components of a mobile device also run software in different forms. The various integrated circuits run small pieces of dedicated software that are called firmwares. When the device is telephony-enabled, there is also software running on the modem. Modern modems are complex and run full operating systems.</p>
<h3>The current situation of freedom and privacy/security on mobile devices</h3>
- <p>A mobile device respecting the users' freedom would have:<ul><li>Free hardware</li><li>Free firmwares</li><li>Free modem system</li><li>Free bootrom and bootloader</li><li>Free system and applications</li></ul>Regarding <a href="#free-hardware">free hardware</a>, it barely exist as of today. Modifying hardware is nearly impossible: new versions of the hardware have to be produced, and those are expensive. While producing printed circuit boards (PCBs) costs a lot of money, producing integrated circuits is out of reach. A few devices come with schematics for the PCB, but that's usually as far as it gets. Hence, totally-free hardware doesn't exist yet.</p>
+ <p>A mobile device respecting the users' freedom would have:<ul><li>Free hardware</li><li>Free firmwares</li><li>Free modem system</li><li>Free bootrom and bootloader</li><li>Free system and applications</li></ul>Regarding <a href="#free-hardware">free hardware</a>, it barely exist as of today. The ways of modifying existing hardware are very limited. Because of that, new versions of the hardware have to be produced to carry the modifications, and this is expensive. While producing printed circuit boards (PCBs) costs a lot of money, producing integrated circuits is out of reach. A few devices come with schematics, or full design files for the PCB, but that's usually as far as it gets. Hence, totally-free hardware doesn't exist yet. While design for FPGAs do exist in free software licenses, FPGAs are not practical enough to be used to replace ASICs in smartphones, and most of them even proprietary software tools.</p>
<p>Firmwares running inside integrated circuits are most of the time proprietary. While free firmwares are hard to write, some exist for very specific hardware (e.g. <a href="//www.arduino.cc/">Arduino</a>, <a href="//dangerousprototypes.com/docs/Bus_Pirate">Bus Pirate</a>) and sometimes, manufacturers can liberate firmwares running in their integrated circuits (e.g. <a href="//github.com/qca/open-ath9k-htc-firmware">ath9k_htc</a>). However, it is not always possible to even replace those firmwares: some are loaded to the integrated circuit by the main CPU but some others are pre-installed in the circuit (in that case, they almost seem to behave like hardware) and cannot be updated to a free replacement.</p>
<p><a href="images/freedom-privacy-security-issues/bad-modem-isolation.png" data-lightbox="current-situation" data-title="Bad modem isolation"><img src="images/freedom-privacy-security-issues/bad-modem-isolation.png" alt="Bad modem isolation" style="width: 250px; float: left;"/></a>The modem system on telephony-enabled mobile devices is always proprietary. While <a href="//bb.osmocom.org/">OsmocomBB</a>, a free software GSM stack exists, it only runs on old feature phones, currently requires a host computer to operate and is not certified to run on public networks. Despite this situation, the modem remains a crucial part for privacy/security: it is nearly always connected to the GSM network, allowing for <a href="//www.gnu.org/philosophy/malware-mobiles.html">remote control</a>. The modem can be more or less damaging to privacy/security depending on what hardware it has access to and can control. That is to say, how isolated it is from the rest of the device.<br /><br />A
device
with bad modem isolation would allow the modem to access and control key parts of the hardware, such as the RAM, storage, GPS, camera, user I/O and microphone. This situation is terrible for privacy/security as it provides plenty of ways to efficiently spy on the user, triggered remotely over the mobile telephony network. Those are accessible to the mobile telephony operator, but also to attackers setting up fake base stations for that purpose. <a href="images/freedom-privacy-security-issues/good-modem-isolation.png" data-lightbox="current-situation" data-title="Good modem isolation"><img src="images/freedom-privacy-security-issues/good-modem-isolation.png" alt="Good modem isolation" style="width: 250px; float: right;"/></a>On the other hand, when the modem is well-isolated from the rest of the device, it is limited to communicating directly with the SoC and can only access the device's microphone when allowed by the SoC. It is then strictly limited to accessing what it real
ly needs
, which considerably reduces its opportunities to spy on the user. While it doesn't solve any of the freedom issues, having an isolated modem is a big step forward for privacy/security. However, it is nearly impossible to be entirely sure that the modem is actually isolated, as any documentation about the device cannot be trusted, due to the lack of effective hardware freedom. On the other hand, it is possible to know that the modem is not isolated, when there is proof that it can access hardware that could be used to spy on the user.</p>
<p>Looking at the software that runs early on the SoC, the first component is the bootrom. It is always proprietary and is stored in read-only memory, so it cannot be changed (in that case, it almost seems to behave like hardware). However, regarding the bootloader, the situation is different for each platform. There are actually multiple stages of bootloaders, some of which can be free. However, it also occurs that the bootloaders are cryptographically signed with a private key. In that case, the bootrom will check the signature against a public key that cannot be replaced and only run the bootloader if the signature matches. That sort of tivoization prevents replacing pre-installed bootloaders, even when their sources are released as free software. There are some good platforms that don't perform such signature checks and can run free bootloaders (e.g. Allwinner Ax, TI OMAP General-Purpose).</p>
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