Protecting Android with more Linux kernel defenses

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Posted by Jeff Vander Stoep, Android Security team

Android relies heavily on the Linux kernel for enforcement of its security
model. To better protect the kernel, we’ve enabled a number of mechanisms within
Android. At a high level these protections are grouped into two
categories—memory protections and attack surface reduction.

Memory protections

One of the major security features provided by the kernel is memory protection
for userspace processes in the form of address space separation. Unlike
userspace processes, the kernel’s various tasks live within one address space
and a vulnerability anywhere in the kernel can potentially impact unrelated
portions of the system’s memory. Kernel memory protections are designed to
maintain the integrity of the kernel in spite of vulnerabilities.

Mark memory as read-only/no-execute

This feature segments kernel memory into logical sections and sets restrictive
page access permissions on each section. Code is marked as read only + execute.
Data sections are marked as no-execute and further segmented into read-only and
read-write sections. This feature is enabled with config option
CONFIG_DEBUG_RODATA. It was put together by Kees Cook and is based on a subset
of Grsecurity’s KERNEXEC feature by Brad
Spengler and Qualcomm’s CONFIG_STRICT_MEMORY_RWX feature by Larry Bassel and
Laura Abbott. CONFIG_DEBUG_RODATA landed in the upstream kernel for arm/arm64
and has been backported to Android’s 3.18+ arm/arm64 common
kernel.

Restrict kernel access to userspace

This feature improves protection of the kernel by preventing it from directly
accessing userspace memory. This can make a number of attacks more difficult
because attackers have significantly less control over kernel memory
that is executable, particularly with CONFIG_DEBUG_RODATA enabled. Similar
features were already in existence, the earliest being Grsecurity’s UDEREF. This
feature is enabled with config option CONFIG_CPU_SW_DOMAIN_PAN and was
implemented by Russell King for ARMv7 and backported to Android’s
4.1 kernel by Kees Cook.

Improve protection against stack buffer overflows

Much like its predecessor, stack-protector, stack-protector-strong protects
against stack
buffer overflows, but additionally provides coverage for more
array types, as the original only protected character arrays.
Stack-protector-strong was implemented by Han Shan and added to the gcc
4.9 compiler.


Attack surface reduction

Attack surface reduction attempts to expose fewer entry points to the kernel
without breaking legitimate functionality. Reducing attack surface can include
removing code, removing access to entry points, or selectively exposing
features.

Remove default access to debug features

The kernel’s perf system provides infrastructure for performance measurement and
can be used for analyzing both the kernel and userspace applications. Perf is a
valuable tool for developers, but adds unnecessary attack surface for the vast
majority of Android users. In Android Nougat, access to perf will be blocked by
default. Developers may still access perf by enabling developer settings and
using adb to set a property: “adb shell setprop security.perf_harden 0”.

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The patchset for blocking access to perf may be broken down into kernel and
userspace sections. The kernel patch is
by Ben Hutchings and is
derived from Grsecurity’s CONFIG_GRKERNSEC_PERF_HARDEN by Brad Spengler. The
userspace changes were contributed
by Daniel Micay. Thanks to Wish
Wu and others for responsibly disclosing security vulnerabilities in perf.

Restrict app access to ioctl commands

Much of Android security model is described and enforced by SELinux. The ioctl()
syscall represented a major gap in the granularity of enforcement via SELinux.
Ioctl command
whitelisting with SELinux was added as a means to provide per-command
control over the ioctl syscall by SELinux.

Most of the kernel vulnerabilities reported on Android occur in drivers and are
reached using the ioctl syscall, for example CVE-2016-0820.
Some ioctl commands are needed by third-party applications, however most are not
and access can be restricted without breaking legitimate functionality. In
Android Nougat, only a small whitelist of socket ioctl commands are available to
applications. For select devices, applications’ access to GPU ioctls has been
similarly restricted.

Require seccomp-bpf

Seccomp provides an additional sandboxing mechanism allowing a process to
restrict the syscalls and syscall arguments available using a configurable
filter. Restricting the availability of syscalls can dramatically cut down on
the exposed attack surface of the kernel. Since seccomp was first introduced on
Nexus devices in Lollipop, its availability across the Android ecosystem has
steadily improved. With Android Nougat, seccomp support is a requirement for all
devices. On Android Nougat we are using seccomp on the mediaextractor and
mediacodec processes as part of the media
hardening effort.

Ongoing efforts

There are other projects underway aimed at protecting the kernel:

  • The Kernel
    Self Protection Project is developing runtime and compiler defenses for the
    upstream kernel.

  • Further sandbox tightening and attack surface reduction with SELinux is
    ongoing in AOSP.

  • Minijail
    provides a convenient mechanism for applying many containment and sandboxing
    features offered by the kernel, including seccomp filters and namespaces.

  • Projects like kasan and kcov help fuzzers
    discover the root cause of crashes and to intelligently construct test cases
    that increase code coverage—ultimately resulting in a more efficient bug hunting
    process.

Due to these efforts and others, we expect the security of the kernel to
continue improving. As always, we appreciate feedback on our work and welcome
suggestions for how we can improve Android. Contact us at security@android.com.


Android Developers Blog

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