Trusted Applications¶
This document tells how to implement a Trusted Application for OP-TEE, using OP-TEE’s so called TA-devkit to both build and sign the Trusted Application binary. In this document, a Trusted Application running in the OP-TEE os is referred to as a TA. Note that in the default setup a private test key is distributed along with the optee_os source is used for signing Trusted Applications. See TASign for more details, including offline signing of TAs.
TA Mandatory files¶
The Makefile for a Trusted Application must be written to rely on OP-TEE TA-devkit resources in order to successfully build the target application. TA-devkit is built when one builds optee_os.
To build a TA, one must provide:
- Makefile, a make file that should set some configuration variables and include the TA-devkit make file.
- sub.mk, a make file that lists the sources to build (local source files, subdirectories to parse, source file specific build directives).
- user_ta_header_defines.h, a specific ANSI-C header file to define most of the TA properties.
- An implementation of at least the TA entry points, as extern functions:
TA_CreateEntryPoint()
,TA_DestroyEntryPoint()
,TA_OpenSessionEntryPoint()
,TA_CloseSessionEntryPoint()
,TA_InvokeCommandEntryPoint()
TA file layout example¶
As an example, hello_world looks like this:
hello_world/
├── ...
└── ta
├── Makefile BINARY=<uuid>
├── Android.mk Android way to invoke the Makefile
├── sub.mk srcs-y += hello_world_ta.c
├── include
│ └── hello_world_ta.h Header exported to non-secure: TA commands API
├── hello_world_ta.c Implementation of TA entry points
└── user_ta_header_defines.h TA_UUID, TA_FLAGS, TA_DATA/STACK_SIZE, ...
TA Makefile Basics¶
Required variables¶
The main TA-devkit make file is located in optee_os at
ta/mk/ta_dev_kit.mk
. The make file supports make targets such as all
and
clean
to build a TA or a library and clean the built objects.
The make file expects a couple of configuration variables:
- TA_DEV_KIT_DIR
- Base directory of the TA-devkit. Used by the TA-devkit itself to locate its tools.
- BINARY and LIBNAME
- These are exclusive, meaning that you cannot use both at the same time. If
building a TA,
BINARY
shall provide the TA filename used to load the TA. The built and signed TA binary file will be named${BINARY}.ta
. In native OP-TEE, it is the TA UUID, used by tee-supplicant to identify TAs. If one is building a static library (that will be later linked by a TA), thenLIBNAME
shall provide the name of the library. The generated library binary file will be namedlib${LIBNAME}.a
- CROSS_COMPILE and CROSS_COMPILE32
- Cross compiler for the TA or the library source files.
CROSS_COMPILE32
is optional. It allows to target AArch32 builds on AArch64 capable systems. On AArch32 systems,CROSS_COMPILE32
defaults toCROSS_COMPILE
.
Optional variables¶
Some optional configuration variables can be supported, for example:
- O
- Base directory for build objects filetree. If not set, TA-devkit defaults to ./out from the TA source tree base directory.
Example Makefile¶
A typical Makefile for a TA looks something like this
# Append specific configuration to the C source build (here log=info)
# The UUID for the Trusted Application
BINARY=8aaaf200-2450-11e4-abe2-0002a5d5c51b
# Source the TA-devkit make file
include $(TA_DEV_KIT_DIR)/mk/ta_dev_kit.mk
sub.mk directives¶
The make file expects that current directory contains a file sub.mk
that is
the entry point for listing the source files to build and other specific build
directives. Here are a couple of examples of directives one can implement in a
sub.mk make file:
# Adds /hello_world_ta.c from current directory to the list of the source
# file to build and link.
srcs-y += hello_world_ta.c
# Includes path **./include/** from the current directory to the include
# path.
global-incdirs-y += include/
# Adds directive -Wno-strict-prototypes only to the file hello_world_ta.c
cflags-hello_world_ta.c-y += -Wno-strict-prototypes
# Removes directive -Wno-strict-prototypes from the build directives for
# hello_world_ta.c only.
cflags-remove-hello_world_ta.c-y += -Wno-strict-prototypes
# Adds the static library foo to the list of the linker directive -lfoo.
libnames += foo
# Adds the directory path to the libraries pathes list. Archive file
# libfoo.a is expected in this directory.
libdirs += path/to/libfoo/install/directory
# Adds the static library binary to the TA build dependencies.
libdeps += path/to/greatlib/libgreatlib.a
Android Build Environment¶
OP-TEE’s TA-devkit supports building in an Android build environment. One can
write an Android.mk
file for the TA (stored side by side with the Makefile).
Android’s build system will parse the Android.mk
file for the TA which in
turn will parse a TA-devkit Android make file to locate TA build resources. Then
the Android build will execute a make
command to built the TA through its
generic Makefile file.
A typical Android.mk
file for a TA looks like this (Android.mk
for
hello_world is used as an example here).
# Define base path for the TA sources filetree
LOCAL_PATH := $(call my-dir)
# Define the module name as the signed TA binary filename.
local_module := 8aaaf200-2450-11e4-abe2-0002a5d5c51b.ta
# Include the devkit Android make script
include $(OPTEE_OS_DIR)/mk/aosp_optee.mk
TA Mandatory Entry Points¶
A TA must implement a couple of mandatory entry points, these are:
TEE_Result TA_CreateEntryPoint(void)
{
/* Allocate some resources, init something, ... */
...
/* Return with a status */
return TEE_SUCCESS;
}
void TA_DestroyEntryPoint(void)
{
/* Release resources if required before TA destruction */
...
}
TEE_Result TA_OpenSessionEntryPoint(uint32_t ptype,
TEE_Param param[4],
void **session_id_ptr)
{
/* Check client identity, and alloc/init some session resources if any */
...
/* Return with a status */
return TEE_SUCCESS;
}
void TA_CloseSessionEntryPoint(void *sess_ptr)
{
/* check client and handle session resource release, if any */
...
}
TEE_Result TA_InvokeCommandEntryPoint(void *session_id,
uint32_t command_id,
uint32_t parameters_type,
TEE_Param parameters[4])
{
/* Decode the command and process execution of the target service */
...
/* Return with a status */
return TEE_SUCCESS;
}
TA Properties¶
Trusted Application properties shall be defined in a header file named
user_ta_header_defines.h
, which should contain:
TA_UUID
defines the TA uuid valueTA_FLAGS
define some of the TA propertiesTA_STACK_SIZE
defines the RAM size to be reserved for TA stackTA_DATA_SIZE
defines the RAM size to be reserved for TA heap (TEE_Malloc() pool)
Refer to TA Properties to understand how to configure these macros.
Hint
UUIDs can be generated using python
python -c 'import uuid; print(uuid.uuid4())'
or in most Linux systems using either
cat /proc/sys/kernel/random/uuid # Linux only
uuidgen # available from the util-linux package in most distributions
Example of a property header file¶
#ifndef USER_TA_HEADER_DEFINES_H
#define USER_TA_HEADER_DEFINES_H
#define TA_UUID
{ 0x8aaaf200, 0x2450, 0x11e4, \
{ 0xab, 0xe2, 0x00, 0x02, 0xa5, 0xd5, 0xc5, 0x1b} }
#define TA_FLAGS (TA_FLAG_EXEC_DDR | \
TA_FLAG_SINGLE_INSTANCE | \
TA_FLAG_MULTI_SESSION)
#define TA_STACK_SIZE (2 * 1024)
#define TA_DATA_SIZE (32 * 1024)
#define TA_CURRENT_TA_EXT_PROPERTIES \
{ "gp.ta.description", USER_TA_PROP_TYPE_STRING, "Foo TA for some purpose." }, \
{ "gp.ta.version", USER_TA_PROP_TYPE_U32, &(const uint32_t){ 0x0100 } }
#endif /* USER_TA_HEADER_DEFINES_H */
Note
It is recommended to use the TA_CURRENT_TA_EXT_PROPERTIES
as above to
define extra properties of the TA.
Note
Generating a fresh UUID with suitable formatting for the header file can be done using:
python -c "import uuid; u=uuid.uuid4(); print(u); \
n = [', 0x'] * 11; \
n[::2] = ['{:12x}'.format(u.node)[i:i + 2] for i in range(0, 12, 2)]; \
print('\n' + '#define TA_UUID\n\t{ ' + \
'0x{:08x}'.format(u.time_low) + ', ' + \
'0x{:04x}'.format(u.time_mid) + ', ' + \
'0x{:04x}'.format(u.time_hi_version) + ', \\ \n\n\t\t{ ' + \
'0x{:02x}'.format(u.clock_seq_hi_variant) + ', ' + \
'0x{:02x}'.format(u.clock_seq_low) + ', ' + \
'0x' + ''.join(n) + '} }')"
Checking TA parameters¶
GlobalPlatforms TEE Client APIs TEEC_InvokeCommand()
and
TEE_OpenSession()
allow clients to invoke a TA with some invocation
parameters: values or references to memory buffers. It is mandatory that TA’s
verify the parameters types before using the parameters themselves. For this a
TA can rely on the macro TEE_PARAM_TYPE_GET(param_type, param_index)
to get
the type of a parameter and check its value according to the expected parameter.
For example, if a TA expects that command ID 0 comes with params[0]
being a
input value, params[1]
being a output value, and params[2]
being a
in/out memory reference (buffer), then the TA should implemented the following
sequence:
TEE_Result handle_command_0(void *session, uint32_t cmd_id,
uint32_t param_types, TEE_Param params[4])
{
if ((TEE_PARAM_TYPE_GET(param_types, 0) != TEE_PARAM_TYPE_VALUE_IN) ||
(TEE_PARAM_TYPE_GET(param_types, 1) != TEE_PARAM_TYPE_VALUE_OUT) ||
(TEE_PARAM_TYPE_GET(param_types, 2) != TEE_PARAM_TYPE_MEMREF_INOUT) ||
(TEE_PARAM_TYPE_GET(param_types, 3) != TEE_PARAM_TYPE_NONE)) {
return TEE_ERROR_BAD_PARAMETERS
}
/* process command */
...
}
TEE_Result TA_InvokeCommandEntryPoint(void *session, uint32_t command_id,
uint32_t param_types, TEE_Param params[4])
{
switch (command_id) {
case 0:
return handle_command_0(session, param_types, params);
default:
return TEE_ERROR_NOT_SUPPORTED;
}
}
Signing of TAs¶
All REE Filesystem Trusted Applications need to be signed. The
signature is verified by optee_os upon loading of the TA. Within the
optee_os source is a directory keys
. The public part of
keys/default_ta.pem
will be compiled into the optee_os binary and the
signature of each TA will be verified against this key upon loading. Currently
keys/default_ta.pem
must contain an RSA key.
Warning
optee_os comes with a default private key in its source to facilitate easy development, testing, debugging and QA. Never deploy an optee_os binary with this key in production. Instead replace this key as soon as possible with a public key and keep the private part of the key offline, preferably on an HSM.
Note
Currently only a single key for signing TAs is supported by optee_os.
TAs are signed using the sign_encrypt.py
script referenced from
ta/mk/ta_dev_kit.mk
in optee_os. Its default behaviour is to sign a
compiled TA binary and attach the signature to form a complete TA for
deployment. For offline signing, a three-step process is required: In a
first step a digest of the compiled binary has to be generated, in the second
step this digest is signed offline using the private key and finally in the
third step the binary and its signature are stitched together into the full TA.
Offline Signing of TAs¶
There are two types of TAs that can be signed offline. The in-tree TAs, which come with the OP-TEE
OS (for example the pkcs11
TA) and are generated during the compilation of the TA DEV KIT. The
second type are any external TAs coming from the user. In both cases however, the signing process
is the same.
Offline signing is done with the following sequence of steps:
0. (Preparation) Generate a 2048 or 4096 bit RSA key for signing in a secure environment and extract the public key. For example
openssl genrsa -out rsa2048.pem 2048
openssl rsa -in rsa2048.pem -pubout -out rsa2048_pub.pem
1. Build the OP-TEE OS with the variable TA_PUBLIC_KEY
set to the public
key generated above
TA_PUBLIC_KEY=/path/to/public_key.pem make all
The build script will do two things:
- It will embed the
TA_PUBLIC_KEY
key into the OP-TEE core image, which will be used toauthenticate the TAs. - It will generate .stripped.elf files of the in-tree TAs and sign them with the dummy keypointed to by
TA_SIGN_KEY
, thus creating .ta files. Note that the generated .ta files arenot to be used as they are not compatible with the public key embedded into the OP-TEE core image.
2. Build any external TA. Same as with the in-tree TAs, the building procedure can use the dummy key
pointed to by TA_SIGN_KEY
, however they are not to be used due to the incompatibility reasons
mentioned in the paragraph above.
There are now two ways to generate the final .ta files. Either re-sign the .ta files with a customized sign_encrypt.py script (left to the user to implement) or stitch the .stripped.elf files and their signatures together (explained in steps 3-5). In both cases however, note that the private key used must be the one generated in step 0.
- Manually generate a digest of the generated .stripped.elf files using
sign_encrypt.py digest --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid)
- Sign this digest offline, for example with OpenSSL
base64 --decode digestfile | \
openssl pkeyutl -sign -inkey $TA_SIGN_KEY \
-pkeyopt digest:sha256 -pkeyopt rsa_padding_mode:pss \
-pkeyopt rsa_pss_saltlen:digest -pkeyopt rsa_mgf1_md:sha256 | \
base64 > sigfile
or with pkcs11-tool using a Nitrokey HSM
echo "0000: 3031300D 06096086 48016503 04020105 000420" | \
xxd -c 19 -r > /tmp/sighdr
cat /tmp/sighdr $(base64 --decode digestfile) > /tmp/hashtosign
pkcs11-tool --id $key_id -s --login -m RSA-PKCS-PSS --hash-algorithm SHA256 --mgf MGF1-SHA256 \
--input-file /tmp/hashtosign | \
base64 > sigfile
- Manually stitch the TA and signature together
sign_encrypt.py stitch --key $(TA_SIGN_KEY) --uuid $(user-ta-uuid)
By default, the UUID is taken as the base file name for all files. Different file
names and paths can be set through additional options to sign_encrypt.py
. Consult
sign_encrypt.py --help
for a full list of options and parameters.