[FD] Aleph Research: Google Nexus 9 SensorHub Firmware Downgrade Vulnerability (CVE-2017-0582)

[Roee Hay <roee.hay@xxxxxxx>]

Title:
====
Google Nexus 9 SensorHub Firmware Downgrade Vulnerability

Identifier:
========
CVE-2017-0582

Product:
=======
Google Nexus 9

Vulnerable Version:
================
Nexus 9 Android Builds before N4F27B - May 2017, i.e. before bootloader 
3.50.0.0143.

Mitigation:
=========
Install N4F27B or later (bootloader version 3.50.0.0143).

Severity:
=======
Moderate

Technical Details:
==============
The Nexus 9 device contains a SoC manufactured by Cywee which implements a 
"Sensor Hub". The SoC is an STM32F401B/C ARM Cortext-M4 MCU, managed by a 
driver available under drivers/i2c/chips/CwMcuSensor.c. The platform 
communicates with SensorHub via I2C bus #0 and 4 GPIO ports.

Upon the platform boot, the CwMcuSensor driver queries the firmware's version 
(I2C register 0x10). If it does not match the one found in the vendor's 
partition (/vendor/firmware/sensor_hub.img), it switches to the bootloader 
mode, and upgrades the firmware (again, via I2C). Please note that the firmware 
is not signed.

By issuing a proprietary fastboot oem command: 'sensorhubflash', a physical 
attacker / malicious charger / malicious headphones (via the UART interface, 
exposed by the headphones jack - see our blog post [2]) can downgrade the 
SensorHub firmware to an old version, saved under the 'SER' partition 
(/dev/block/mmcblk0p19). This version may contain vulnerabilities which may 
allow the attacker to compromise the MCU.
One may claim that it is not an issue because the platform would immediately 
upgrade the firmware upon boot (since its version is different from the one 
found in the vendor image), however, in Nexus 9, the I2C buses could be 
accessed via the fastboot interface, by using the fastboot oem {i2cr, i2cw, 
i2crNoAddr, i2cwNoAddr, i2cdetect} commands. (I2C could also be accessed via 
UART, in the HBOOT mode. [2]) Thus, the attacker could interact with the old 
firmware BEFORE it was replaced by the platform using I2C, and thus potentially 
exploit a security vulnerability which would allow him to return a bogus 
version identifier, bypassing the platform's check. Please note that the SoC's 
I2C code runs in privileged mode.

For more details and PoC, visit the Aleph Research Vulnerability Report [1].

Patch:
=====
Google patched the vulnerability on build N4F27B / bootloader 3.50.0.0143 by 
removing the 'sensorhubflash' bootloader command. In addition, Google 
restricted access to the I2C buses - The I2C related bootloader commands are no 
longer available.
Please note that although Google published the advisory on the April 2017 
Security Bulletin [3], the patch has been included only since the April 5 2017 
Security Patch Level, where the April Nexus 9 image (N4F26X) has the April 1 
2017 Security Patch Level, hence it does not contain the patched bootloader.

Credit:
=====
Roee Hay (@roeehay) of Aleph Research, HCL technologies.

References:
==========
[1] Aleph Research Vulnerability Report. 
https://alephsecurity.com/vulns/aleph-2017010
[2] Attacking Nexus 9 with Malicious Headphones. 
https://alephsecurity.com/2017/03/08/nexus9-fiq-debugger/
[3] Google's Security Bulletin (April 2017).  
https://source.android.com/security/bulletin/2017-04-01#eop-in-htc-oem-fastboot-command


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