Shut the HAL Up

Posted by Jeff Vander Stoep, Senior Software Engineer, Android Security

Updates are essential for security, but they can be difficult and expensive for
device manufacturers. href="">Project
Treble is making updates easier by separating the underlying vendor
implementation from the core Android framework. This modularization allows
platform and vendor-provided components to be updated independently of each
other. While easier and faster updates are awesome, Treble's increased
modularity is also designed to improve security.

Isolating HALs

A Hardware
Abstraction Layer
(HAL) provides an interface between device-agnostic code
and device-specific hardware implementations. HALs are commonly packaged as
shared libraries loaded directly into the process that requires hardware
interaction. Security boundaries are enforced at the process level. Therefore,
loading the HAL into a process means that the HAL is running in the same
security context as the process it's loaded into.

The traditional method of running HALs in-process means that the process needs
all the permissions required by each in-process HAL, including direct access to
kernel drivers. Likewise, all HALs in a process have access to the same set of
permissions as the rest of the process, including permissions required by other
in-process HALs. This results in over-privileged processes and HALs that have
access to permissions and hardware that they shouldn't.

Figure 1. Traditional method of multiple HALs in one process.

Moving HALs into their own processes better adheres to the href="">principle of
least privilege. This provides two distinct advantages:

  1. Each HAL runs in its own sandbox and is permitted access to only the
    hardware driver it controls and the permissions granted to the process are
    limited to the permissions required to do its job.
  2. Similarly, the process loses access to hardware drivers and other
    permissions and capabilities needed by the HALs.

Figure 2. Each HAL runs in its own process.

Moving HALs into their own processes is great for security, but it comes at the
cost of increased IPC overhead between the client process and the HAL. href="">Improvements to the binder
driver made IPC between HALs and clients practical. Introducing
scatter-gather into binder improves the performance of each transaction by
removing the need for the serialization/deserialization steps and reducing the
number of copy operations performed on data from three down to one. Android O
also introduces binder domains to provide separate communication streams for
vendor and platform components. Apps and the Android frameworks continue to use
/dev/binder, but vendor-provided components now use /dev/vndbinder.
Communication between the platform and vendor components must use /dev/hwbinder.
Other means of IPC between platform and vendor are disallowed.

Case study: System Server

Many of the services offered to apps by the core Android OS are provided by the
system server. As Android has grown, so has system server's responsibilities and
permissions, making it an attractive target for an href="">attacker.
As part of project Treble, approximately 20 HALs were moved out of system
server, including the HALs for sensors, GPS, fingerprint, Wi-Fi, and more.
Previously, a compromise in any of those HALs would gain privileged system
permissions, but in Android O, permissions are restricted to the subset needed
by the specific HAL.

Case study: media frameworks

Efforts to href="">harden
the media stack in Android Nougat continued in Android O. In Nougat,
mediaserver was split into multiple components to better adhere to the principle
of least privilege, with audio hardware access restricted to audioserver, camera
hardware access restricted to cameraserver, and so on. In Android O, most direct
hardware access has been entirely removed from the media frameworks. For example
HALs for audio, camera, and DRM have been moved out of audioserver,
cameraserver, and drmserver respectively.

Reducing and isolating the attack surface of the kernel

The Linux kernel is the primary enforcer of the security model on Android.
Attempts to escape sandboxing mechanisms often involve attacking the kernel. An
of kernel vulnerabilities on Android showed that they overwhelmingly occurred in
and were reached through hardware drivers.

De-privileging system server and the media frameworks is important because they
interact directly with installed apps. Removing direct access to hardware
drivers makes bugs difficult to reach and adds another layer of defense to
Android's security model.

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