Google beefs Linux up kernel defenses in Android

Android's security model relies heavily on the Linux kernel that sits at its core. As such, Android developers have always been interested in adding new security features that are intended to prevent potentially malicious code from reaching the kernel, which is the most privileged area of the operating system.

One older example is Security Enhancements for Android (SEAndroid), a set of kernel add-ons and tools that make exploitation of certain vulnerabilities harder by enforcing access controls.

SEAndroid, which is based on the NSA-developed SELinux project, started being used to enforce the application sandbox boundaries in Android 4.3 (Jelly Bean).

In a blog post Wednesday, Jeff Vander Stoep, a member of the Android Security team, revealed some of the more recent kernel security additions that will make the upcoming Android Nougat and future Android versions harder to compromise. These include various memory protections, as well as changes intended to reduce the kernel's attack surface.

One new configuration option called CONFIG_DEBUG_RODATA segments the kernel memory into multiple sections and limits how much of this memory is writeable and executable. Attackers need writeable and executable memory pages in order to inject malicious code into them via exploits, and then run that code with kernel privileges.

Another config option, called CONFIG_CPU_SW_DOMAIN_PAN, prevents the kernel from directly accessing user space memory, giving attackers even less control over where their exploits can execute code.

The existing protection against stack-based buffer overflows, a family of memory corruption vulnerabilities, has also been improved by providing coverage for more array types, Vander Stoep said.

The attack surface of the kernel has been reduced by blocking default access to debugging features like the perf system, which used for performance monitoring. The access to IOCTL commands has also been restricted for third-party applications as much as possible without breaking legitimate functionality.

"Most of the kernel vulnerabilities reported on Android occur in drivers and are reached using the ioctl syscall," Vander Stoep said.

Android developers have also hardened the OS's media-handling processes, which have been a source of many vulnerabilities over the past year. This has been done through a sandboxing mechanism called seccomp, which was first introduced on Nexus devices in Android Lollipop. With Android Nougat, seccomp support will be required for all devices.