The uVisor provides sandboxed environments and resources protection for applications built for ARM Cortex-M3 and Cortex-M4 devices. We will show you how to enable the uVisor and configure a secure box to get hold of some exclusive resources (memory, peripherals, interrupts). For more information on the uVisor design philosophy, please check out our the uVisor [introductory document](../../README.md).
* A platform and a toolchain supported by uVisor on mbed OS. You can verify this on [the official list](../../README.md#supported-platforms). Please note that uVisor might support some platform internally, but not on mbed OS. Generally this means that the porting process is only partly complete. If you want to port your platform to uVisor and enable it on mbed OS, please follow the [uVisor Porting Guide for mbed OS](../core/PORTING.md).
* You are building the app for the [NXP FRDM-K64F](http://developer.mbed.org/platforms/FRDM-K64F/) target, with the [GNU ARM Embedded Toolchain](https://launchpad.net/gcc-arm-embedded) toolchain.
The mbed-cli tools will automatically fetch the mbed codebase for you. By default, git will be used to track your code changes, so your application will be ready to be pushed to a git server, if you want to.
Once the import process is finished, create a `source` folder:
Drag and drop it onto the USB device mounted on your computer in order to flash the device. When the flashing process is completed, press the reset button on the device. You should see the device LED blinking.
* How to [enable uVisor](#enable-uvisor) on the `uvisor-example` app.
* How to [add a secure box](#add-a-secure-box) to the `uvisor-example` app with exclusive access to a timer, to a push-button interrupt, and to static and dynamic memories.
1. Public box Access Control Lists (ACLs). Since with uVisor enabled everything runs in unprivileged mode, we need to make sure that peripherals that are accessed by the OS and the public box are allowed. These peripherals are specified using a list like the one in the snippet above. For the purpose of this example we provide you the list of all the ACLs that we know you will need. For other platforms or other applications you need to determine those ACLs following a process that is described in a [section](#the-main-box-acls) below.
1. App-specific uVisor configurations: `UVISOR_SET_MODE_ACL`. This macro sets the uVisor mode (enabled) and associates the list of ACLs we just created with the public box.
Before compiling, we need to override the original `K64F` target to enable the uVisor feature. To do so, add the file `~/code/uvisor-example/mbed_app.json` with the following content:
The macros `FEATURE_UVISOR` and `TARGET_UVISOR_SUPPORTED` in the configuration file above are automatically defined for C and C++ files, but not for assembly files. Because the uVisor relies on those symbols in some assembly code, we need to define them manually.
If you enable uVisor in the `blinky` app as it was written above, you will not get any particular security feature. All code and resources share the same security context, which we call the *public box*.
A lot happens unseen, though. All the user code now runs in unprivileged mode, and the systems services such as the `NVIC` APIs and the OS SVCalls are routed through the uVisor.
Now that uVisor is enabled, we can finally add a *secure box*.
A secure box is a special compartment that is granted exclusive access to peripherals, memories and interrupts. Private resources are only accessible when the *context* of the secure box is active. The uVisor is the only one that can enable a secure box context, for example upon thread switching or interrupt handling.
uVisor does not obfuscate code that belongs to a box, so it is still readable and executable from outside of the box. In addition, declaring an object in the same file that configures a secure box does not protect that object automatically.
Instead, we provide specific APIs to instruct the uVisor to protect a private resource. We will show how to use these APIs in the `uvisor-example` app.
For this example, we want to create a secure box called `private_button`. The `private_button` box has exclusive access to the push-button on the NXP FRDM-K64F board, which means that other boxes cannot access its corresponding peripheral.
Each secure box must have at least one thread, which we call the box's main thread. In our `private_button` box we only use this thread throughout the whole program. The thread runs every second and counts the number of times it has been called between 2 button presses. The thread count is saved in a variable private to the box. Whenever we press the `SW2` button on the board the current thread count is stored into a private buffer and is restarted. For debug purposes, the program prints the content of the buffer every time it is filled up.
Create a new source file, `~/code/uvisor-example/source/secure_box.cpp`. We will configure the secure box inside this file. The secure box name for this example is `private_button`.
In general, you can decide what to do in your box's main thread. You can run it once and then stop it, or use it to configure memories or peripherals or to create other threads. In this app, the box's main thread is the only thread for the `private_button` box, and it runs throughout the whole program.
The `private_button_main_thread` function configures the push-button to trigger an interrupt when pressed, allocates the dynamic buffer to hold the thread count values and initializes its private static memory, `PrivateButtonStaticMemory`. A spinning loop is used to update the counter value every second.
* If code is running in the context of `private_button`, then any object instantiated inside that code will belong to the `private_button` heap and stack. This means that in the example above, the `InterruptIn` object is private to the `private_button` box. The same applies to the dynamically allocated buffer `uvisor_ctx->buffer`.
* You can access the content of the private memory `PrivateButtonStaticMemory` using the `void * const __uvisor_ctx` pointer, which uVisor maintains. You need to cast this pointer to your own context type. In this example we used a pre-processor symbol to improve readability.
* The `InterruptIn` object triggers the registration of an interrupt slot. Because that code is run in the context of the `private_button` box, then the push-button IRQ belongs to that box. If you want to use the IRQ APIs directly, read the [section](#the-nvic-apis) below.
* Even if the `private_button_on_press` function runs in the context of `private_button`, we can still use the `printf` function, which accesses the `UART0` peripheral, owned by the public box. This is because all ACLs declared in the public box are by default shared with all the other secure boxes. This also means that the messages we are printing on the serial port are not secure, because other boxes have access to that peripheral.
> **Warning**: Instantiating an object in the `secure_box.cpp` global scope will automatically map it to the public box context, not the `private_button` one. If you want an object to be private to a box, you need to instantiate it inside the code that runs in the context of that box (such as the `InterruptIn` object), or alternatively statically initialize it in the box private static memory (such as the `buffer`, `index` and `counter` variables in `PrivateButtonStaticMemory`).
Reflash the device, and press the reset button. The device LED should be blinking as in the previous case.
If you don't see the LED blinking, it means that the application halted somewhere, probably because uVisor captured a fault. You can set up the uVisor debug messages to see if there is any problem. Follow the [Debugging uVisor on mbed OS](DEBUGGING.md) document for a step-by-step guide.
If the LED is blinking, it means that the app is running fine. If you now press the `SW2` button on the NXP FRDM-K64F board, the `private_button_on_press` function will be executed, printing the values in the timer buffer after `PRIVATE_BUTTON_BUFFER_COUNT` presses. You can observe these values by opening a serial port connection to the device, with a baud rate of 9600.
## Expose public secure entry points to the secure box
So far the code in the secure box cannot communicate to other boxes. To let other boxes call functions in our secure box you can define public secure entry points. These entry points can map to private functions within the context of a secure box, and the arguments and return values are automatically serialized using an RPC protocol to ensure no private memory can be leaked to external boxes.
You can define a public secure entry point to retrieve the index value from the secure box. This index value is increased every time the `SW2` button is pressed.
### Defining a secure entry point
Create a new source file, `~/code/uvisor-example/source/secure_box.h`. In here we will define the functions that can be called through RPC.
```cpp
/* ~/code/uvisor-example/source/secure_box.h */
#ifndef SECURE_BOX_H_
#define SECURE_BOX_H_
#include "uvisor-lib/uvisor-lib.h"
UVISOR_EXTERN int (*secure_get_index)(void);
#endif
```
### Implementing a secure entry point
Now that you have defined the secure entry point, you can map the entry point to a function running in the secure box. This is done through the `UVISOR_BOX_RPC_GATEWAY_SYNC` macro. Open `~/code/uvisor-example/source/secure_box.cpp`, and replace the line with `#define PRIVATE_BUTTON_BUFFER_COUNT 8` by:
To receive RPC messages you will need to spin up a new thread, running in the secure box context. You can do this in the main thread of the secure box. In `~/code/uvisor-example/source/secure_box.cpp`, replace the first five lines of `private_button_main_thread` with:
To call the public secure entry point from any other box, you can use the `secure_get_index` function. It will automatically do an RPC call into the secure box and serialize the return value. You can try this out from the main box. In `~/code/uvisor-example/source/main.cpp`, first include the header file for the secure box:
```cpp
/* ~/code/uvisor-example/source/main.cpp */
#include "secure-box.h"
```
And then replace the `main` function with:
```cpp
/* ~/code/uvisor-example/source/main.cpp */
int main(void)
{
while (true) {
led = !led;
printf("Secure index is %d\r\n", secure_get_index());
You can observe the secure index by opening a serial port connection to the device, with a baud rate of 9600. When you press the `SW2` button the index will be increased.
The ARM CMSIS header files provide APIs to configure, enable and disable IRQs in the NVIC module. These APIs are all prefixed with `NVIC_` and can be found in the `core_cm*.h` files in your CMSIS module.
In addition, the CMSIS header files also provide APIs to set and get an interrupt vector at runtime. This requires the interrupt vector table, which is usually located in flash, to be relocated to SRAM.
When the uVisor is enabled, all NVIC APIs are rerouted to the corresponding uVisor vIRQ APIs, which virtualize the interrupt module. The uVisor interrupt model has the following features:
* The first call to any NVIC API will register the IRQ for exclusive use with the active box. IRQs cannot be unregistered.
* Code in a box can only change the state of an IRQ (enable it, change its priority, etc.) if the box registered that IRQ with uVisor at runtime first.
Although this behaviour is different from the original NVIC one, it is backward compatible. This means that legacy code (like a device HAL) will still work after uVisor is enabled. The general use case is the following:
> **Note**: When enabling the IRQ before setting the vector, uVisor uses the handler specified in the original vector table. This mirrors the NVIC hardware behavior.
The code samples that we provide in this guide give you a ready-made list of ACLs for the public box. The list includes peripherals that we already know will be necessary to make the example app work, and it is specific to the NXP FRDM-K64F target.
This section shows how to discover the needed ACLs for the public box. You might need to follow these instructions in case you want to generate the ACLs list for a different target or a different app.
At the moment the uVisor does not provide a way to detect and list all the faulting ACLs for a given platform automatically. This is a planned feature that will be released in the future.
In order to generate the list of ACLs, use the code provided in the [Enable uVisor](#enable-uvisor) section. In this case, though, start with an empty ACLs list:
You now need to compile your application using uVisor in debug mode. This operation requires some more advanced steps, which are described in detail in the [Debugging uVisor on mbed OS](DEBUGGING.md) document. The main idea is that you compile the application in debug mode:
and then use a GDB-compatible interface to flash the device, enable semihosting and access the uVisor debug messages. Please read the [Debugging uVisor on mbed OS](DEBUGGING.md) document for the detailed instructions.
Once the uVisor debug messages are enabled, you will see your application fail. The failure is due to the first missing ACL being hit by the public box code. The message will look like:
Repeat the process multiple times until all ACLs have been added to the list. When no other ACL is needed any more, the system will run without hitting a uVisor fault.
