Security Engineering. Ross Anderson

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СКАЧАТЬ there in the last 90 days’. In early systems based on traditional ACLs, each access control decision required a reference to administrative systems to find out which nurses and which patients were on which ward, when – but this made both the HR system and the patient administration system safety-critical, which hammered reliability. Matters were fixed by giving nurses ID cards with certificates that entitle them to access the files associated with a number of wards or hospital departments [535, 536]. If you can make the trust relationships in systems mirror the trust relationships in that part of the world you're trying to automate, you should. Working with the grain can bring advantages at all levels in the stack, making things more usable, supporting safer defaults, cutting errors, reducing engineering effort and saving money too.

6.2.5 DAC and MAC

In the old days, anyone with physical access to a computer controlled all of it: you could load whatever software you liked, inspect everything in memory or on disk and change anything you wanted to. This is the model behind discretionary access control (DAC): you start your computer in supervisor mode and then, as the administrator, you can make less-privileged accounts available for less-trusted tasks – such as running apps written by companies you don't entirely trust, or giving remote logon access to others. But this can make things hard to manage at scale, and in the 1970s the US military started a huge computer-security research program whose goal was to protect classified information: to ensure that a file marked ‘Top Secret’ would never be made available to a user with only a ‘Secret’ clearance, regardless of the actions of any ordinary user or even of the supervisor. In such a multilevel secure (MLS) system, the sysadmin is no longer the boss: ultimate control rests with a remote government authority that sets security policy. The mechanisms started to be described as mandatory access control (MAC). The supervisor, or root access if you will, is under remote control. This drove development of technology for mandatory access control – a fascinating story, which I tell in Part 2 of the book.

      From the 1980s, safety engineers also worked on the idea of safety integrity levels; roughly, that a more dependable system must not rely on a less dependable one. They started to realise they needed something similar to multilevel security, but for safety. Military system people also came to realise that the tamper-resistance of the protection mechanisms themselves was of central importance. In the 1990s, as computers and networks became fast enough to handle audio and video, the creative industries lobbied for digital rights management (DRM) in the hope of preventing people undermining their business models by sharing music and video. This is also a form of mandatory access control – stopping a subscriber sharing a song with a non-subscriber is in many ways like stopping a Top Secret user sharing an intelligence report with a Secret user.

      In the early 2000s, these ideas came together as a number of operating-system vendors started to incorporate ideas and mechanisms from the MAC research programme into their products. The catalyst was an initiative by Microsoft and Intel to introduce cryptography into the PC platform to support DRM. Intel believed the business market for PCs was saturated, so growth would come from home sales where, they believed, DRM would be a requirement. Microsoft started with DRM and then realised that offering rights management for documents too might be a way of locking customers tightly into Windows and Office. They set up an industry alliance, now called the Trusted Computing Group, to introduce cryptography and MAC mechanisms into the PC platform. To do this, the operating system had to be made tamper-resistant, and this is achieved by means of a separate processor, the Trusted Platform Module (TPM), basically a smartcard chip mounted on the PC motherboard to support trusted boot and hard disk encryption. The TPM monitors the boot process, and at each stage a hash of everything loaded so far is needed to retrieve the key needed to decrypt the next stage. The real supervisor on the system is now no longer you, the machine owner – it's the operating-system vendor.

      MAC, based on TPMs and trusted boot, was used in Windows 6 (Vista) from 2006 as a defence against persistent malware¹. The TPM standards and architecture were adapted by other operating-system vendors and device OEMs, and there is now even a project for an open-source TPM chip, OpenTitan, based on Google's product. However the main purpose of such a design, whether the design itself is open or closed, is to lock a hardware device to using specific software.

      6.2.6 Apple's macOS

      Apple's macOS operating system (formerly called OS/X or Mac OS X) is based on the FreeBSD version of Unix running on top of the Mach kernel. The BSD layer provides memory protection; applications cannot access system memory (or each others') unless running with advanced permissions. This means, for example, that you can kill a wedged application using the ‘Force Quit’ command without having to reboot the system. On top of this Unix core are a number of graphics components, including OpenGL, Quartz, QuickTime and Carbon, while at the surface the Aqua user interface provides an elegant and coherent view to the user.

      At the file system level, macOS is almost a standard Unix. The default installation has the root account disabled, but users who may administer the system are in a group ‘wheel’ that allows them to su to root. If you are such a user, you can install programs (you are asked for the root password when you do so). Since version 10.5 (Leopard), it has been based on TrustedBSD, a variant of BSD that incorporates mandatory access control mechanisms, which are used to protect core system components against tampering by malware.

      6.2.7 iOS

Since 2008, Apple has led the smartphone revolution with the iPhone, which (along with other devices like the iPad) uses the iOS operating system – which is now (in 2020) the second-most popular. iOS is based on Unix; Apple took the Mach kernel from CMU and fused it with the FreeBSD version of Unix, making a number of changes for performance and robustness. For example, in vanilla Unix a filename can have multiple pathnames that lead to an inode representing a file object, which is what the operating system sees; in iOS, this has been simplified so that files have unique pathnames, which in turn are the subject of the file-level access controls. Again, there is a MAC component, where mechanisms from Domain and Type Enforcement (DTE) are used to tamper-proof core system components (we'll discuss DTE in more detail in chapter 9). Apple introduced this because they were worried that apps would brick the iPhone, leading to warranty claims.

Apps also have permissions, which are capabilities; they request a capability to access device services such as the mobile network, the phone, SMSes, the camera, and the first time the app attempts to use such a service. This is granted if the user consents². The many device services open up possible side-channel attacks; for example, an app that's denied access to the keyboard could deduce keypresses using the accelerometer and gyro. We'll discuss side channels in Part 2, in the chapter on that subject.

      The Apple ecosystem is closed in the sense that an iPhone will only run apps that Apple has signed³. This enables the company to extract a share of app revenue, and also to screen apps for malware or other undesirable behaviour, such as the exploitation of side channels to defeat access controls.

      The iPhone 5S introduced a fingerprint biometric and payments, adding a secure enclave (SE) to the A7 processor to give them separate protection. Apple decided to trust neither iOS nor TrustZone with such sensitive data, since vulnerabilities give transient access until they're patched. Its engineers also worried that an unpatchable exploit might be found in the ROM (this eventually happened, with Checkm8). While iOS has access to the system partition, the user's personal data are encrypted, with the keys managed by the SE. Key management is bootstrapped by a unique 256-bit AES key burned into fusible links on the system-on-chip. When the device is powered up, the user has ten tries to enter a passcode; only then are file keys derived from the master key and made available⁴. When the device is locked, some keys are still СКАЧАТЬ