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Hardware Access Control Exercise

If you haven't already, please go to the 'building the exercises' section to see how the exercises are built.

In this exercise we utilise the compartmentalisation available in CHERIoT RTOS to control access to a hardware peripheral: the LEDs.

For this exercise, when the xmake.lua build file is mentioned exercises/hardware_access_control/xmake.lua is being referred to.

Part 1

Let's start with the firmware image called hardware_access_part_1 in the xmake.lua file. This image has two threads running two compartments: blinky_raw and led_walk_raw. blinky_raw simply toggles an LED and led_walk_raw walks through all the LEDs toggling them as it goes. The sources of these compartments can be found in exercises/hardware_access_control/part_1/.

Let's look inside blinky_raw. It uses the RTOS' MMIO_CAPABILITY macro to get the capability that grants it access to the GPIO MMIO region. This magic macro will handle adding the MMIO region to the compartment's imports and mapping it to a type, in this case SonataGpioBoard (from platform-gpio.hh). For more information on this macro, see the drivers section of CHERIoT programmers guide.

This is great! If you build and load the hardware_access_part_1 firmware on the FPGA, you have flashing LEDs! What more could one want?

Well maybe some level of access control. Currently both blinky_raw and led_walk_raw have access to all of the GPIO ports, and neither can trust the other compartment isn't toggling the LED when they are not looking. The keen-eyed among you will have noticed that this is already happening with both toggling user LED 7.

Part 2

Let's introduce some access control for the LEDs. To do this, we can create a new compartment gpio_access with sole access to the GPIO MMIO region. This compartment will arbitrate access to the LED outputs by making use of CHERIoT's sealing mechanism. When a compartment seals a capability, it can no longer be dereferenced or modified until it is unsealed by a compartment with the capability to do so. The gpio_access compartment creates these sealing capabilities as LED handles that it can give to other compartments. These other compartments can't use the handles directly, but can only pass them to gpio_access which can unseal them and use them. In this case, they only point to a LedHandle structure that only holds the index of a LED. They are purely used as a proof of LED ownership. For more information on sealing, see the cheriot-rtos/examples/05.sealing/.

blinky_raw and led_walk_raw have been adapted to use this new compartment and renamed blinky_dynamic and led_walk_dynamic. You'll notice these compartments use add_deps in the xmake.lua file to declare that they depend on gpio_access. Take a moment to look at the sources for these compartments in exercises/hardware_access_control/part_2/.

If you now run the hardware_access_part_2 firmware on the FPGA, you'll notice only blinky_dynamic is toggling it's LED. Looking at the UART console from the FPGA, the following message will pop up.

hardware_access_control/part_2/led_walk_dynamic.cc:19 Assertion failure in start_walking
LED 0x7 couldn't be acquired

led_walk_dynamic was run after blinky_dynamic because it's thread was given a lower priority in the xmake.lua. So when it asked for access to user LED 7, it was denied by gpio_access because this LED had already been allocated to blinky_dynamic.

Now change NumLeds in led_walk_dynamic.cc from 8 to 7, then rebuild. Both compartments should run happily again. Not only will both compartments run happily, but led_walk_dynamic will output the following over the console.

Led Walk Dynamic:           LED 3 Handle: 0x1087d0 (v:1 0x1087d0-0x1087e0 l:0x10 o:0xc p: G RWcgm- -- ---)
Led Walk Dynamic: Destroyed LED 3 Handle: 0x1087d0 (v:1 0x1087d0-0x1087e0 l:0x10 o:0xc p: G RWcgm- -- ---)
Led Walk Dynamic:       New LED 3 Handle: 0x108878 (v:1 0x108878-0x108888 l:0x10 o:0xc p: G RWcgm- -- ---)

These come from some superfluous lines in led_walk_dynamic.cc, which release ownership of user LED 3 only to later reacquire ownership. You can comment out the line that reacquires the LED:

	leds[3] = acquire_led(3).value();

When run led_walk_dynamic will now fail to toggle user LED 3 because it has relinquished ownership of the LED.

hardware_access_control/part_2/led_walk_dynamic.cc:34 Assertion failure in start_walking
Failed to toggle an LED

Part 3

This is great and all, but how do we stop a compartment bypassing gpio_access and using MMIO_CAPABILITY directly? In other words, how do we ensure that only gpio_access has access to the GPIOs?

Luckily the linker has all the information needed to check which compartments can access the GPIO MMIOs. It outputs this information in a JSON report with the rest of the build artefacts. To automate checking this report, we can use cheriot-audit which should already be in your path.

cheriot-audit allows you to query the JSON report and assert certain rules are followed. You do this with a language called Rego, but don't worry you won't have to learn it for this exercise. There are some pre-written rules in the gpio_access.rego module. Let's first look at only_gpio_access_has_access. It uses mmio_allow_list from the compartment package included in cheriot-audit to check that only the gpio_access compartment has access to the GPIO MMIOs. If we run this on the part 2 firmware image's JSON report, it will return true. However, when run against the part 1 firmware image's report it will return false, because the blinky_raw and led_walk_raw are not in the allow list.

# This should return true
cheriot-audit \
    --board cheriot-rtos/sdk/boards/sonata-prerelease.json \
    --module exercises/hardware_access_control/part_3/gpio_access.rego \
    --query "data.gpio_access.only_gpio_access_has_access" \
    --firmware-report "build/cheriot/cheriot/release/hardware_access_part_2.json"
# This should return false
cheriot-audit \
    --board cheriot-rtos/sdk/boards/sonata-prerelease.json \
    --module exercises/hardware_access_control/part_3/gpio_access.rego \
    --query "data.gpio_access.only_gpio_access_has_access" \
    --firmware-report "build/cheriot/cheriot/release/hardware_access_part_1.json"

There's a second rule, whitelisted_compartments_only, which adds an additional condition that only led_walk_dynamic and blinky_dynamic can use gpio_access. We can use this to restrict which compartments have access to the GPIO via gpio_access.

cheriot-audit \
    --board cheriot-rtos/sdk/boards/sonata-prerelease.json \
    --module exercises/hardware_access_control/part_3/gpio_access.rego \
    --query "data.gpio_access.whitelisted_compartments_only" \
    --firmware-report "build/cheriot/cheriot/release/hardware_access_part_2.json"

The above should return true as both compartments are in the allow list. Try removing one of the compartments from the allow list given to compartment_allow_list in gpio_access.rego and check the result of the above command is no longer true.

One can browse the other functions available as part of the compartment package in cheriot-audit's readme.

Part ∞

Where to go from here...

  • There are input devices available through SonataGpioBoard. You could have a go at adding these to the gpio_access compartment.
  • The interactions with ledTaken global in the gpio_access compartment aren't thread safe. You could take a look at cheriot-rtos/examples/06.producer-consumer/ to learn how to use a futex to make it thread safe.
  • There is a technical interest group for Sunburst and a technology access programme run by UKRI that lowRISC is helping to adjudicate. If you are interested in either of these please reach out to [email protected].