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Quick-Start Guide for uVisor on mbed OS

This guide will help you get started with uVisor on mbed OS by showing you how to create a sample application for the NXP FRDM-K64F board.

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.

Overview

To get a basic blinky application running on mbed OS with uVisor enabled, you will need the following:

A platform and a toolchain supported by uVisor on mbed OS. You can verify this on the official list. 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.
git. It will be used to download the mbed codebase.
mbed-cli. You can run pip install mbed-cli to install it.

For the remainder of this guide we will assume the following:

You are developing on a *nix machine, in the ~/code folder.
You are building the app for the NXP FRDM-K64F target, with the GNU ARM Embedded Toolchain toolchain.

You can use these instructions as guidelines in the case of other targets on other host OSs.

Start with the `blinky` app

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To create a new mbed application called uvisor-example, just run the following commands:

$ cd ~/code
$ mbed new uvisor-example
$ cd uvisor-example

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:

$ mkdir ~/code/uvisor-example/source

and place a new file main.cpp in it:

/* ~/code/uvisor-example/source/main.cpp */

#include "mbed.h"

DigitalOut led(LED1);

int main(void)
{
    while (true) {
        led = !led;
        wait(0.5);
    }
}

This application blinks an LED from the main thread, which the OS creates by default.

Checkpoint

Compile the application:

$ mbed compile -m K64F -t GCC_ARM

The resulting binary will be located at:

~/code/uvisor-example/BUILD/K64F/GCC_ARM/uvisor-example.bin

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.

In the next sections you will see:

How to enable uVisor on the uvisor-example app.
How to 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.

Enable uVisor

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To enable the uVisor on the app, add the following lines at the beginning of the main.cpp file:

/* ~/code/uvisor-example/source/main.cpp */

#include "mbed.h"
#include "uvisor-lib/uvisor-lib.h"

/* Public box Access Control Lists (ACLs). */
/* Note: These are specific to the NXP FRDM-K64F board. See the section below
 *       for more information. */
static const UvisorBoxAclItem g_public_box_acls[] = {
    /* For the LED */
    {SIM,   sizeof(*SIM),   UVISOR_TACLDEF_PERIPH},
    {PORTB, sizeof(*PORTB), UVISOR_TACLDEF_PERIPH},

    /* For messages printed on the serial port */
    {OSC,   sizeof(*OSC),   UVISOR_TACLDEF_PERIPH},
    {MCG,   sizeof(*MCG),   UVISOR_TACLDEF_PERIPH},
    {UART0, sizeof(*UART0), UVISOR_TACLDEF_PERIPH},
    {PIT,   sizeof(*PIT),   UVISOR_TACLDEF_PERIPH},
};

/* Enable uVisor, using the ACLs we just created. */
UVISOR_SET_MODE_ACL(UVISOR_ENABLED, g_public_box_acls);

/* Rest of the existing code */
...

In the code above we specified 2 elements:

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 below.
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:

{
    "target_overrides": {
        "*": {
            "target.features_add": ["UVISOR"],
            "target.extra_labels_add": ["UVISOR_SUPPORTED"]
        }
    },
    "macros": [
        "FEATURE_UVISOR=1",
        "TARGET_UVISOR_SUPPORTED=1"
    ]
}

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.

Checkpoint

Compile the application again. This time the K64F target will include the new features and labels we provided in mbed_app.json;

$ mbed compile -m K64F -t GCC_ARM

The binary will be located at:

~/code/uvisor-example/BUILD/K64F/GCC_ARM/uvisor-example.bin

Reflash the device and press the reset button. The device LED should be blinking as in the previous case.

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.

Add a secure box

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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.

Configure the secure box

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.

We want the box to have exclusive access to the following resources:

The push-button peripheral (as specified by a peripheral ACL). Nobody else should be able to access the push-button port.
The push-button interrupt (as specified by an IRQ ACL). We want the button IRQ to be rerouted to our box-specific ISR.
The private dynamically allocated buffer (as specified by a dynamic memory ACL).
The private variables (as specified by a static memory ACL).

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.

/* ~/code/uvisor-example/source/secure_box.cpp */

#include "mbed.h"
#include "uvisor-lib/uvisor-lib.h"

/* Private static memory for the secure box */
typedef struct {
    uint32_t * buffer;          /* Static private memory, pointing to dynamically allocated private memory */
    uint32_t counter;           /* Static private memory */
    int index;                  /* Static private memory */
    RawSerial * pc;             /* Static private memory, pointing to dynamically allocated private memory */
} PrivateButtonStaticMemory;

/* ACLs list for the secure box: Timer (PIT). */
static const UvisorBoxAclItem g_private_button_acls[] = {
    {PORTC, sizeof(*PORTC), UVISOR_TACLDEF_PERIPH},     /* Private peripheral */
};

static void private_button_main_thread(const void *);

/* Secure box configuration */
UVISOR_BOX_NAMESPACE(NULL);                   /* We won't specify a box namespace for this example. */
UVISOR_BOX_HEAPSIZE(4096);                    /* Heap size for the secure box */
UVISOR_BOX_MAIN(private_button_main_thread,   /* Main thread for the secure box */
                osPriorityNormal,             /* Priority of the secure box's main thread */
                1024);                        /* Stack size for the secure box's main thread */
UVISOR_BOX_CONFIG(private_button,             /* Name of the secure box */
                  g_private_button_acls,      /* ACLs list for the secure box */
                  1024,                       /* Stack size for the secure box */
                  PrivateButtonStaticMemory); /* Private static memory for the secure box. */

Create the secure box's main thread function

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.

/* ~/code/uvisor-example/source/secure_box.cpp */

/* The previous code goes here. */
...

#define uvisor_ctx ((PrivateButtonStaticMemory *) __uvisor_ctx)

#define PRIVATE_BUTTON_BUFFER_COUNT 8

static void private_button_on_press(void)
{
    /* Store the thread count into the buffer and reset it. */
    uvisor_ctx->buffer[uvisor_ctx->index] = uvisor_ctx->counter;
    uvisor_ctx->counter = 0;

    /* Update the index. Behave as a circular buffer. */
    if (uvisor_ctx->index < PRIVATE_BUTTON_BUFFER_COUNT - 1) {
        uvisor_ctx->index++;
    } else {
        uvisor_ctx->index = 0;

        /* For debug purposes: Print the content of the buffer. */
        uvisor_ctx->pc->printf("Thread count between button presses: ");
        for (int i = 0; i < PRIVATE_BUTTON_BUFFER_COUNT; ++i) {
            uvisor_ctx->pc->printf("%lu ", uvisor_ctx->buffer[i]);
        }
        uvisor_ctx->pc->printf("\r\n");
    }

}

/* Main thread for the secure box */
static void private_button_main_thread(const void *)
{
    /* Allocate serial port to ensure that code in this secure box
     * won't touch handle in the default security context when printing */
    if (!(uvisor_ctx->pc = new RawSerial(USBTX, USBRX)))
        return;

    /* Create the buffer and cache its pointer to the private static memory. */
    uvisor_ctx->buffer = (uint32_t *) malloc(PRIVATE_BUTTON_BUFFER_COUNT * sizeof(uint32_t));
    if (uvisor_ctx->buffer == NULL) {
        uvisor_ctx->pc->printf("ERROR: Failed to allocate memory for the button buffer\r\n");
        mbed_die();
    }
    uvisor_ctx->index = 0;
    uvisor_ctx->counter = 0;

    /* Setup the push-button callback. */
    InterruptIn button(SW2);                        /* Private IRQ */
    button.mode(PullUp);
    button.fall(&private_button_on_press);

    /* Increment the private counter every second. */
    while (1) {
        uvisor_ctx->counter++;
        wait(1.0);
    }
}

A few things to note in the code above:

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 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).

Checkpoint

Compile the application again:

$ mbed compile -m K64F -t GCC_ARM

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 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.

/* ~/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:

/* ~/code/uvisor-example/source/secure_box.cpp */

/* Function called through RPC */
static int get_index() {
    /* Access to private memory here */
    return uvisor_ctx->index;
}

UVISOR_BOX_RPC_GATEWAY_SYNC (private_button, secure_get_index, get_index, int, void);

#define PRIVATE_BUTTON_BUFFER_COUNT 8

Listening for RPC messages

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:

/* ~/code/uvisor-example/source/secure_box.cpp */

static void listen_for_rpc() {
    /* List of functions to wait for */
    static const TFN_Ptr my_fn_array[] = {
        (TFN_Ptr) get_index
    };

    while (1) {
        int caller_id;
        int status = rpc_fncall_waitfor(my_fn_array, 1, &caller_id, UVISOR_WAIT_FOREVER);

        if (status) {
            uvisor_error(USER_NOT_ALLOWED);
        }
    }
}

/* Main thread for the secure box */
static void private_button_main_thread(const void *)
{
    /* allocate serial port to ensure that code in this secure box
     * won't touch handle in the default security context when printing */
    if (!(uvisor_ctx->pc = new RawSerial(USBTX, USBRX)))
        return;

    /* Start listening for RPC messages in a separate thread */
    Thread rpc_thread(osPriorityNormal, 1024);
    rpc_thread.start(&listen_for_rpc);

    /* ... Rest of the private_button_main_thread function ... */

Calling the public secure entry point

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:

/* ~/code/uvisor-example/source/main.cpp */

#include "secure-box.h"

And then replace the main function with:

/* ~/code/uvisor-example/source/main.cpp */

int main(void)
{
    while (true) {
        led = !led;
        printf("Secure index is %d\r\n", secure_get_index());
        Thread::wait(500);
    }
}

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.

Wrap-up

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In this guide we showed you how to:

Enable uVisor on an existing application.
Add a secure box to your application.
- Protect static and dynamic memories in a secure box.
- Gain exclusive access to a peripheral and an IRQ in a secure box.
- Expose public secure entry points to a secure box.

You can now modify the example or create a new one to protect your resources into a secure box. You may find the following resources useful:

If you found any bug or inconsistency in this guide, please raise an issue.

Appendix

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This section contains additional information that you may find useful when setting up a secure box.

The NVIC APIs

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 uVisor owns the interrupt vector table.
All ISRs are relocated to SRAM.
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.
An IRQ that belongs to a box can only be modified when that box context is active.

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:

#define MY_IRQ 42

/* Set the ISR for MY_IRQ at runtime.
 * Without uVisor: Relocate the interrupt vector table to SRAM and set my_isr as
                   the ISR for MY_IRQ.
 * With    uVisor: Register MY_IRQ for the current box with my_isr as ISR. */
NVIC_SetVector(MY_IRQ, &my_isr);

/* Change the IRQ state. */
NVIC_SetPriority(MY_IRQ, 3);
NVIC_EnableIRQ(MY_IRQ);

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.

For more information on the uVisor APIs, see the uVisor API documentation document.

The public box ACLs

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 section. In this case, though, start with an empty ACLs list:

static const UvisorBoxAclItem g_public_box_acls[] = {
}

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 document. The main idea is that you compile the application in debug mode:

$ mbed compile -m K64F -t GCC_ARM --profile mbed-os/tools/profiles/debug.json

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 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:

***********************************************************
                    BUS FAULT
***********************************************************

...

* MEMORY MAP
  Address:           0x4004800C
  Region/Peripheral: SIM
    Base address:    0x40047000
    End address:     0x40048060

...

Now that you know which peripheral is causing the fault (the SIM peripheral, in this example), you can add its entry to the ACLs list:

static const UvisorBoxAclItem g_public_box_acls[] = {
    {SIM, sizeof(*SIM), UVISOR_TACLDEF_PERIPH},
};

Note: If the fault debug screen does not show the name of the peripheral, you need to look it up in the target device reference manual.

For readability, do not use the hard-coded addresses of your peripherals, but rather use the symbols that the target CMSIS module provides.

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.

24 KiB Raw Blame History

Quick-Start Guide for uVisor on mbed OS

Overview

Start with the blinky app

Enable uVisor

Add a secure box

Configure the secure box

Create the secure box's main thread function

Expose public secure entry points to the secure box

Defining a secure entry point

Implementing a secure entry point

Listening for RPC messages

Calling the public secure entry point

Wrap-up

Appendix

The NVIC APIs

The public box ACLs

24 KiB

Raw Blame History

Start with the `blinky` app