Code sharing between independently linked XIP binaries

Author:: Tamas Ban
Organization:: Arm Limited
Contact:: tamas.ban@arm.com

Motivation

Cortex-M devices are usually constrained in terms of flash and RAM. Therefore, it is often challenging to fit bigger projects in the available memory. The PSA specifications require a device to both have a secure boot process in place at device boot-up time, and to have a partition in the SPE which provides cryptographic services at runtime. These two entities have some overlapping functionality. Some cryptographic primitives (e.g. hash calculation and digital signature verification) are required both in the bootloader and the runtime environment. In the current TF-M code base, both firmware components use the mbed-crypto library to implement these requirements. During the build process, the mbed-crpyto library is built twice, with different configurations (the bootloader requires less functionality) and then linked to the corresponding firmware component. As a result of this workflow, the same code is placed in the flash twice. For example, the code for the SHA-256 algorithm is included in MCUboot, but the exact same code is duplicated in the SPE cryptography partition. In most cases, there is no memory isolation between the bootloader and the SPE, because both are part of the PRoT code and run in the secure domain. So, in theory, the code of the common cryptographic algorithms could be reused among these firmware components. This could result in a big reduction in code footprint, because the cryptographic algorithms are usually flash hungry. Code size reduction can be a good opportunity for very constrained devices, which might need to use TF-M Profile Small anyway.

Technical challenge

Code sharing in a regular OS environment is easily achievable with dynamically linked libraries. However, this is not the case in Cortex-M systems where applications might run bare-metal, or on top of an RTOS, which usually lacks dynamic loading functionality. One major challenge to be solved in the Cortex-M space is how to share code between independently linked XIP applications that are tied to a certain memory address range to be executable and have absolute function and global data memory addresses. In this case, the code is not relocatable, and in most cases, there is no loader functionality in the system that can perform code relocation. Also, the lack of an MMU makes the address space flat, constant and not reconfigurable at runtime by privileged code.

One other difficulty is that the bootloader and the runtime use the same RAM area during execution. The runtime firmware is executed strictly after the bootloader, so normally, it can reuse the whole secure RAM area, as it would be the exclusive user. No attention needs to be paid as to where global data is placed by the linker. The bootloader does not need to retain its state. The low level startup of the runtime firmware can freely overwrite the RAM with its data without corrupting bootloader functionality. However, with code sharing between bootloader and runtime firmware, these statements are no longer true. Global variables used by the shared code must either retain their value or must be reinitialised during low level startup of the runtime firmware. The startup code is not allowed to overwrite the shared global variables with arbitrary data. The following design proposal provides a solution to these challenges.

Design concept

The bootloader is sometimes implemented as ROM code (BL1) or stored in a region of the flash which is lockable, to prevent tampering. In a secure system, the bootloader is immutable code and thus implements a part of the Root of Trust anchor in the device, which is trusted implicitly. The shared code is primarily part of the bootloader, and is reused by the runtime SPE firmware at a later stage. Not all of the bootloader code is reused by the runtime SPE, only some cryptographic functions.

Simplified steps of building with code sharing enabled:

Complete the bootloader build process to have a final image that contains the absolute addresses of the shared functions, and the global variables used by these functions.

Extract the addresses of the functions and related global variables that are intended to be shared from the bootloader executable.

When building runtime firmware, provide the absolute addresses of the shared symbols to the linker, so that it can pick them up, instead of instantiating them again.

The execution flow looks like this:

SPE        MCUboot func1()    MCUboot func2()     MCUboot func3()
 |
 |     Hash()
 |------------->|
                |----------------->|
                                   |
                      Return       |
      Return    |<-----------------|
 |<-------------|
 |
 |
 |----------------------------------------------------->|
                                                        |
         Function pointer in shared global data()       |
 |<-----------------------------------------------------|
 |
 |                       Return
 |----------------------------------------------------->|
                                                        |
                         Return                         |
 |<-----------------------------------------------------|
 |
 |

The execution flow usually returns from a shared function back to the SPE with an ordinary function return. So usually, once a shared function is called in the call path, all further functions in the call chain will be shared as well. However, this is not always the case, as it is possible for a shared function to call a non-shared function in SPE code through a global function pointer.

For shared global variables, a dedicated data section must be allocated in the linker configuration file. This area must have the same memory address in both MCUboot’s and the SPE’s linker files, to ensure the integrity of the variables. For simplicity’s sake, this section is placed at the very beginning of the RAM area. Also, the RAM wiping functionality at the end of the secure boot flow (that is intended to remove any possible secrets from the RAM) must not clear this area. Furthermore, it must be ensured that the linker places shared globals into this data section. There are two way to achieve this:

Put a filter pattern in the section body that matches the shared global variables.

Mark the global variables in the source code with special attribute __attribute__((section(<NAME_OF_SHARED_SYMBOL_SECTION>)))

RAM memory layout in MCUboot with code sharing enabled:

+------------------+
|  Shared symbols  |
+------------------+
| Shared boot data |
+------------------+
|      Data        |
+------------------+
|    Stack (MSP)   |
+------------------+
|      Heap        |
+------------------+

RAM memory layout in SPE with code sharing enabled:

+-------------------+
|  Shared symbols   |
+-------------------+
| Shared boot data  |
+-------------------+
|    Stack (MSP)    |
+-------------------+
|    Stack (PSP)    |
+-------------------+
| Partition X Data  |
+-------------------+
| Partition X Stack |
+-------------------+
          .
          .
          .
+-------------------+
| Partition Z Data  |
+-------------------+
| Partition Z Stack |
+-------------------+
|     PRoT Data     |
+-------------------+
|       Heap        |
+-------------------+

Patching mbedTLS

In order to share some global function pointers from mbed-crypto that are related to dynamic memory allocation, their scope must be extended from private to global. This is needed because some compiler toolchain only extract the addresses of public functions and global variables, and extraction of addresses is a requirement to share them among binaries. Therefore, a short patch was created for the mbed-crypto library, which “globalises” these function pointers:

lib/ext/mbedcrypto/0002-Enable-crypto-code-sharing-between-independent-binar.patch

The patch need to manually applied in the mbedtls repo, if code sharing is enabled. The patch has no effect on the functional behaviour of the cryptographic library, it only extends the scope of some variables.

Tools support

All the currently supported compilers provide a way to achieve the above objectives. However, there is no standard way, which means that the code sharing functionality must be implemented on a per compiler basis. The following steps are needed:

Extraction of the addresses of all global symbols.

The filtering out of the addresses of symbols that aren’t shared. The goal is to not need to list all the shared symbols by name. Only a simple pattern has to be provided, which matches the beginning of the symbol’s name. Matching symbols will be shared. Examples are in : bl2/shared_symbol_template.txt

Provision of the addresses of shared symbols to the linker during the SPE build process.

The resolution of symbol collisions during SPE linking. Because mbed-crypto is linked to both firmware components as a static library, the external shared symbols will conflict with the same symbols found within it. In order to prioritize the external symbol, the symbol with the same name in mbed-crypto must be marked as weak in the symbol table.

The above functionalities are implemented in the toolchain specific CMake files:

toolchain_ARMCLANG.cmake

toolchain_GNUARM.cmake

By the following two functions:

target_share_symbols(): Extract and filter shared symbol addresses from MCUboot.

target_link_shared_code(): Link shared code to the SPE and resolve symbol conflict issues.

ARMCLANG

The toolchain specific steps are:

Extract all symbols from MCUboot: add -symdefs to the compiler command line

Filter shared symbols: call CMake script FilterSharedSymbols.cmake

Weaken duplicated (shared) symbols in the mbed-crypto static library that are linked to the SPE: arm-none-eabi-objcopy

Link shared code to SPE: Add the filtered output of -symdefs to the SPE source file list.

GNUARM

The toolchain specific steps are:

Extract all symbols from MCUboot: arm-none-eabi-nm

Filter shared symbols: call CMake script: FilterSharedSymbols.cmake

Strip unshared code from MCUboot: arm-none-eabi-strip

Weaken duplicated (shared) symbols in the mbed-crypto static library that are linked to the SPE: arm-none-eabi-objcopy

Link shared code to SPE: Add -Wl -R <SHARED_STRIPPED_CODE.axf> to the compiler command line

IAR

Functionality currently not implemented, but the toolchain supports doing it.

Memory footprint reduction

Build type: MinSizeRel Platform: mps2/an521 Version: TF-Mv1.2.0 + code sharing patches MCUboot image encryption support is disabled.

	ConfigDefault		ConfigProfile-M		ConfigProfile-S
	ARMCLANG	GNUARM	ARMCLANG	GNUARM	ARMCLANG	GNUARM
CODE_SHARING=OFF	122268	124572	75936	75996	50336	50224
CODE_SHARING=ON	113264	115500	70400	70336	48840	48988
Difference	9004	9072	5536	5660	1496	1236

If MCUboot image encryption support is enabled then saving could be up to ~13-15KB.

Note

Code sharing on Musca-B1 was tested only with SW only crypto, so crypto hardware acceleration must be turned off: -DCRYPTO_HW_ACCELERATOR=OFF

Useability considerations

Functions that only use local variables can be shared easily. However, functions that rely on global variables are a bit tricky. They can still be shared, but all global variables must be placed in the shared symbol section, to prevent overwriting and to enable the retention of their values.

Some global variables might need to be reinitialised to their original values by runtime firmware, if they have been used by the bootloader, but need to have their original value when runtime firmware starts to use them. If so, the reinitialising functionality must be implemented explicitly, because the low level startup code in the SPE does not initialise the shared variables, which means they retain their value after MCUboot stops running.

If a bug is discovered in the shared code, it cannot be fixed with a firmware upgrade, if the bootloader code is immutable. If this is the case, disabling code sharing might be a solution, as the new runtime firmware could contain the fixed code instead of relying on the unfixed shared code. However, this would increase code footprint.

API backward compatibility also can be an issue. If the API has changed in newer version of the shared code. Then new code cannot rely on the shared version. The changed code and all the other shared code where it is referenced from must be ignored and the updated version of the functions must be compiled in the SPE binary. The mbedTLS library is API compatible with its current version (v2.24.0) since the mbedtls-2.7.0 release (2018-02-03).

To minimise the risk of incompatibility, use the same compiler flags to build both firmware components.

The artifacts of the shared code extraction steps must be preserved so as to remain available if new SPE firmware (that relies on shared code) is built and released. Those files are necessary to know the address of shared symbols when linking the SPE.

Further improvements

This design focuses only on sharing the cryptography code. However, other code could be shared as well. Some possibilities:

Flash driver
Serial driver
Image metadata parsing code
etc.