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Self-healing FreeRTOS

@author:       Michael Denzel
@license:      GNU General Public License 2.0 or later

A proof-of-concept implementation of an ARM TrustZone port of FreeRTOS V9.0.0rc1 which can automatically self-heal from certain attacks and runs on top of ARM TrustZone.

Source code includes code from:

• FreeRTOS V9.0.0rc1 -- www.freertos.org
• Dongli Zhang -- https://github.com/finallyjustice/imx53qsb-code "trustzone-smc"
• James Walmsley -- https://github.com/jameswalmsley/RaspberryPi-FreeRTOS
• Francesco Balducci -- https://balau82.wordpress.com/2010/12/16/using-newlib-in-arm-bare-metal-program

Quickstart

Compile the tool using:

$ make
$ ./setupsd.sh /dev/sdX

/dev/sdX is the path to the SD-card which should be flashed with the image.

Test system: Freescale i.MX53 Quick Start Board ARM Cortex-A8

Required tools:

• arm-none-eabi-gcc (worked with version 5.2.0, arm-none-eabi-as/-ld 2.27)
• arm-none-eabi-binutils
• arm-none-eabi-newlib
• uboot-utils (for mkimage, worked with version 2016.05)
• (minicom or similar)

To connect to the board:

$ minicom -s -c on

goto Serial port setup and set the following:
Serial Device:          /dev/ttyUSB0 (or where mounted)
Callin Program:         (empty)
Callout Program:        (empty)
Bps/Par/Bits:           115200 8N1
Hardware Flow Control:  No (important!)
Software Flow Control:  No

(start)

Makefile Options

There are a couple of Makefile options:

Cleanup

Use make clean to clean the compiled files.

Logging and Printing

• make info - print some logging information
• make smc - print also the secure monitor calls (includes info)
• make debug - very verbose logging level (includes info and smc)

Timing analysis

• make timingTz - will compile a special image used for timing TrustZone
• make timingNoTz - will compile a special image used for timing the system without TrustZone

TrustZone

Normally the operating system will always be compiled for TrustZone. If that is not desired, there is the option to compile using make noTz, even though we recommend to compile with TrustZone.

World Switches

TrustZone is split into two worlds (normal world and secure world) which are separated by the so-called monitor. The following drawings demonstrate the switches in the implemented Trustzone FreeRTOS:

Attack and Self-healing

Remark: by default the buffer overflow was turned off. To turn it on, one has to supply the compilation flag BUFFEROVERFLOW. See also file ./Demo/Drivers/IO.c. Note that there is no Makefile option to supply this flag directly to prevent accidentally compiling a vulnerable image.

Background

To test the self-healing capabilities of our adjusted FreeRTOS operating system, we simulated a simple temperature sensor and included a buffer overflow using the vulnerable C function strcpy (see ./Demo/Drivers/IO.c).

The function set_config_temperature can update the struct of the temperature sensor with a new "info" string. However, the buffer is only 16 bytes long and can be overflowed. This enables an attacker to change the max and min temperature fields (and of course various other memory attacks).

Our self-healing capability (see ./Demo/TrustZone/selfhealing.c::detection()) checks that the max temperature is below 100 degrees for task3. Note: this is just a proof-of-concept implementation, a full implementation would have to check also the min temperature, the variables of task1 and task2, that no further tasks exists, and so on. But, this should be enough to demonstrate the basic idea of self-healing

Example Run

The following picture shows an example run of ./attack.sh which overwrites the max temperature with 150 degrees. User inputs are highlighted in green, these are in this case the actual attack input.

Explanations:

After all three tasks are started, the attacker updates the "info" string to 1234561234567890 plus the binary number for 150. On can see three lines later that FreeRTOS reports back the temperature as [0; 150]. Immediately afterwards, self-healing takes place which can be seen at the reset task3 (it says again "task3 start" because task3 was deleted and restarted). When the attacker sends the next status command, the system already reset the temperature to the initial value [0; 100] and "info" to the initial value "init".

This implementation is not bulletproof of course. An adversary could still attack the self-healing functionality directly or circumvent it another way. However, since the self-healing functionality is part of the ARM TrustZone secure world, this is considerably more effort.

Known Issues

• bug in the interrupt system