NAME

TPMLIB_RegisterCallbacks - Register callbacks for implementing customized behavior of certain functions

LIBRARY

TPM library (libtpms, -ltpms)

SYNOPSIS

#include <libtpms/tpm_types.h>

#include <libtpms/tpm_library.h>

#include <libtpms/tpm_error.h>

TPM_RESULT TPMLIB_RegisterCallbacks(struct tpmlibrary_callbacks *);

DESCRIPTION

The TPMLIB_RegisterCallbacks() functions allows to register several callback functions with libtpms that enable a user to implement customized behavior of several library-internal functions. This feature will typically be used if the behavior of the provided internal functions is not as needed. An example would be that libtpms writes all data into files with certain names. If, however, the data needs to be written into a special type of storage the user will register callbacks with the library that are invoked when the TPM needs to write, read or delete data from storage and the user may then implement custom behavior in these functions.

The following shows the data structure used for registering the callbacks.

    struct libtpms_callbacks {  
            int sizeOfStruct;
            TPM_RESULT (*tpm_nvram_init)(void);
            TPM_RESULT (*tpm_nvram_loaddata)(unsigned char **data,
                                             uint32_t *length,
                                             uint32_t tpm_number,
                                             const char *name);
            TPM_RESULT (*tpm_nvram_storedata)(const unsigned char *data,
                                              uint32_t length,
                                              uint32_t tpm_number,
                                              const char *name);
            TPM_RESULT (*tpm_nvram_deletename)(uint32_t tpm_number,
                                               const char *name,
                                               TPM_BOOL mustExist);
            TPM_RESULT (*tpm_io_init)(void);
            TPM_RESULT (*tpm_io_getlocality)(TPM_MODIFIER_INDICATOR *localityModifer,
                                             uint32_t tpm_number);
            TPM_RESULT (*tpm_io_getphysicalpresence)(TPM_BOOL *physicalPresence,
                                                     uint32_t tpm_number);
    };

Currently 7 callbacks are supported. If a callback pointer in the above structure is set to NULL the default library-internal implementation of that function will be used.

If one of the callbacks in either the tpm_nvram or tpm_io group is set, then all of the callbacks in the respective group should be implemented.

tpm_nvram_init

This function is called before any access to persistent storage is done. It allows the user to perform initialization of access to persistent storage.

Upon success this function should return TPM_SUCCESS, a failure code otherwise.

The default implementation requires that the environment variable TPM_PATH is set and points to a directory where the TPM's state can be written to. If the variable is not set, it will return TPM_FAIL and the initialization of the TPM in TPMLIB_MainInit() will fail.

tpm_nvram_loaddata

This function is called when the TPM wants to load state from persistent storage. The implementing function must allocate a buffer (data) and return it to the TPM along with the length of the buffer (length). The tpm_number is always 0 and can be ignored. The name parameter is either one of TPM_SAVESTATE_NAME, TPM_VOLATILESTATE_NAME, or TPM_PERMANENT_ALL_NAME and indicates which one of the 3 types of state is supposed to be loaded.

Upon success this function should return TPM_SUCCESS, a failure code otherwise.

The default implementation writes the TPM's state into files in a directory where the TPM_PATH environment variable pointed to when TPMLIB_MainInit() was executed. Failure to write the TPM's state into files will put the TPM into failure mode.

If this function is not set (NULL), then the original NVChip file will be read when using a TPM 2. This file contains the memory dump of internal data structures and is neither portable between endianesses or architectures of different sizes (32 bit, 64 bit), nor will it allow handling extensions of those internal data structures it carries through additions in the TPM 2 code. In the worst case this may result in memory access errors by internal functions and result in crashes. Therefore, it is recommended to set this function and handle the writing of the TPM state.

tpm_nvram_storedata

This function is called when the TPM wants to store state to persistent storage. The data and length parameters provide the data to be stored and the number of bytes. The implementing function must not free the data buffer. The tpm_number is always 0 and can be ignored. The name parameter is either one of TPM_SAVESTATE_NAME, TPM_VOLATILESTATE_NAME, or TPM_PERMANENT_ALL_NAME and indicates which one of the 3 types of state is supposed to be stored.

Upon success this function should return TPM_SUCCESS, a failure code otherwise.

The default implementation reads the TPM's state from files in a directory where the TPM_PATH environment variable pointed to when TPMLIB_MainInit() was executed. Failure to read the TPM's state from files may put the TPM into failure mode.

If this function is not set (NULL), the memory dump will be written to the NVChip file (TPM 2) and the same comments apply as when the tpm_nvram_loaddata interface function is not set.

tpm_nvram_deletename

This function is called when the TPM wants to delete state on persistent storage. The tpm_number is always 0 and can be ignored. The name parameter is either one of TPM_SAVESTATE_NAME, TPM_VOLATILESTATE_NAME, or TPM_PERMANENT_ALL_NAME and indicates which one of the 3 types of state is supposed to be deleted. The mustExist parameter indicates whether the given data must exist and the implementing function should return TPM_FAIL if the data did not exist.

Upon success this function should return TPM_SUCCESS, a failure code otherwise.

The default implementation deletes the TPM's state files in a directory where the TPM_PATH environment variable pointed to when TPMLIB_MainInit() was executed. Failure to delete the TPM's state files may put the TPM into failure mode.

tpm_io_init

This function is called to initialize the IO subsystem of the TPM.

Upon success this function should return TPM_SUCCESS, a failure code otherwise.

The default implementation simply returns TPM_SUCCESS.

tpm_io_getlocality

This function is called when the TPM needs to determine the locality under which a command is supposed to be executed. The implementing function should return the number of the locality by writing it into the localityModifier pointer.

This function is expected to return TPM_SUCCESS along with a valid locality number. The program implementing the called function is trusted to manage locality numbers correctly.

The default implementation returns 0 as the locality.

tpm_io_getphysicalpresence

This function is called when the TPM needs to determine whether physical presence has been asserted. The implementing function should write either TRUE or FALSE into the physicalPresence pointer.

Upon success this function should return TPM_SUCCESS, a failure code otherwise.

The default implementation returns FALSE for physical presence.

RETURN VALUE

Upon successful completion, TPMLIB_MainInit() returns TPM_SUCCESS, an error value otherwise.

ERRORS

TPM_SUCCESS: The function completed successfully.
TPM_FAIL: General failure.

For a complete list of TPM error codes please consult the include file libtpms/tpm_error.h

EXAMPLE

 #include <libtpms/tpm_types.h>
 #include <libtpms/tpm_library.h>
 #include <libtpms/tpm_error.h>
 static TPM_MODIFIER_INDICATOR locality;
 static TPM_RESULT mytpm_io_init(void)
 {
        return TPM_SUCCESS;
 }
 static TPM_RESULT mytpm_io_getlocality(TPM_MODIFIER_INDICATOR *locModif,
                                        uint32_t tpm_number)
 {
        *locModif = locality;
        return TPM_SUCCESS:
 }
 static TPM_RESULT mytpm_io_getphysicalpresence(TPM_BOOL *physicalPresence,
                                                uint32_t tpm_number)
 {
        *physicalPresence = FALSE;
        return TPM_SUCCESS;
 }
 int main(void) {
     TPM_RESULT res;
     unsigned char *respbuffer;
     uint32_t resp_size;
     uint32_t respbufsize;
     unsigned char *command;
     uint32_t command_size;
     struct libtpms_callbacks cbs = {
         .sizeOfStruct               = sizeof(struct libtpms_callbacks),
         .tpm_nvram_init             = NULL,
         .tpm_nvram_loaddata         = NULL,
         .tpm_nvram_storedata        = NULL,
         .tpm_nvram_deletename       = NULL,
         .tpm_io_init                = mytpm_io_init,
         .tpm_io_getlocality         = mytpm_io_getlocality,
         .tpm_io_getphysicalpresence = mytpm_io_getphysicalpresence,
     };
     [...]
     if (TPMLIB_RegisterCallbacks(&cbs) != TPM_SUCCESS) {
         fprintf(stderr, "Could not register the callbacks.\n");
         return 1;
     }
     if (TPMLIB_MainInit()) != TPM_SUCCESS) {
         fprintf(stderr, "Could not start the TPM.\n");
         return 1;
     }
     [...]
     /* build TPM command */
     [...]
     res = TPMLIB_Process(&respbuffer, &resp_size,
                          &respbufsize,
                          command, command_size);
     [...]
     TPMLIB_Terminate();
     return 0;
 }