Going Inside an Arbitrary Kernel Write Vulnerability in the Nexus 9 (CVE-2016-3873)
The IBM X-Force Application Security Research Team recently discovered an arbitrary write vulnerability in Nexus 9’s kernel (the Tegra kernel branch). Google’s Android Security Team acknowledged the vulnerability, which allows a privileged attacker to arbitrary write values within kernel space, and assigned it a high severity rating.
Kernel arbitrary write primitives can be used to achieve kernel code execution, which completely compromises the security of the device, not including TrustZone. It increases the TrustZone attack surface and allows attackers to access application data and override the Security-Enhanced Linux (SELinux) policy.
The vulnerability has existed since Nexus 9’s inception back in November 2014. It was reported to the Android Security Team on June 20, 2016, and fixed on Sept. 5, 2016. This flaw was verified on what were then the latest Nexus 9 images:
Vulnerable Code in the Nexus 9
The registers debugfs file node is initialized with the following write file operation:
That is, on write system call, cl_register_write() securely copies a user space buffer and parses its contents as two numeric values: val, a value to be written, and offs, an offset from a constant address. Cl_dvfs_writel() is then fed with val and offs (cast into a four-byte aligned address).
Eventually, __raw_writel() is used to write value val at offs+, which results in an arbitrary kernel write.
The vulnerability is reminiscent of an issue previously disclosed by security researcher Marco Grassi.
Attack Surface Analysis
We analyzed the Discretionary Access Control (DAC) and Mandatory Access Control (MAC; SELinux on Android) to find out which active processes can trigger the vulnerability.
DAC-wise, who can write to the file?
The attacker must execute code under root within the debugfs SELinux context:
SELinux-wise, what contexts can write to a debugfs file?
To find out which SELinux contexts can trigger the vulnerability, we analyzed Nexus 9’s (MOB30M) sepolicy. We need to find SELinux domains with allow rules that have target type debugfs with the open and write permissions on the file class.
Analyzing Nexus 9’s sepolicy (MOB30M) yields:
That is, SELinux-wise, any domain can open, write and append to any file with the debugfs context.
What active processes can trigger the vulnerability? Analyzing active processes on the device yields the following:
Code execution within any of the processes above can trigger and exploit the vulnerability.
To exploit the vulnerability from an untrusted application security context, start by escalating privileges from an untrusted app to one of the contexts of the aforementioned processes. For instance, CVE-2016-0807, disclosed by Zach Riggle, may be used, since it allows an untrusted app to execute code within debuggerd.
The commit that fixed the vulnerability indicates that Google simply removed the registers file from the debug file system. Clearly, the registers file node was not needed on production builds.