Advisory TFMV-3

Title	`abort()` function may not take effect in TF-M Crypto multi-part MAC/hashing/cipher operations.
CVE ID	CVE-2021-32032
Public Disclosure Date	May 10, 2021
Versions Affected	Affected all versions up to and including TF-M v1.3.0
Configurations	All
Impact	It can cause memory leakage in TF-M Crypto service, eventually making TF-M Crypto service unavailable and impacting other services relied on it.
Fix Version	commit 7e2e52
Credit	Chongqing Lei, Southeast University Zhen Ling, Associate Professor, Southeast University Xinwen Fu, Professor, University of Massachusetts Lowell

Background

PSA multi-part crypto operation sequence

PSA Crypto API specification defines a common sequence for all multi-part crypto operations. The sequence can be simplified to the following steps:

setup() sets up the multi-part operation.
update() adds data/configurations into the multi-part operation.
finish() completes the multi-part operation.

PSA Crypto API specification requests that the corresponding abort() function shall be called when update() or finish() function fails. The abort() function aborts the ongoing multi-part operation and cleans up the operation context.

TF-M multi-part crypto operation functions eventually call the underlying crypto library (Mbed TLS by default) to perform those steps, including abort() step.

PSA multi-part crypto operation objects

PSA Crypto API specification defines an operation object for each type of multi-part crypto operations. For example, psa_mac_operation_t for multi-part MAC operations and psa_hash_operation_t for multi-part hashing operations.

TF-M Crypto service relies on the underlying crypto library (Mbed TLS by default) to implement those objects. The structures of those objects are crypto library specific and hidden to TF-M. The underlying crypto library usually stores and manages the context of ongoing multi-part crypto operations in the corresponding PSA operation object. For example, Mbed TLS stores multi-part hashing operation context in its psa_hash_operation_t implementation.

The context is cleaned up in crypto library abort() function when the client calls abort() to handle a previous error. The clean-up execution can include zeroing the memory area and freeing allocated memory.

TF-M multi-part crypto operation objects

TF-M Crypto service defines a dedicated operation structure tfm_crypto_operation_s to wrap PSA multi-part crypto operation object and maintains its own status, as shown in the code block below.

struct tfm_crypto_operation_s {

    ...

    union {
        psa_cipher_operation_t cipher;    /*!< Cipher operation context */
        psa_mac_operation_t mac;          /*!< MAC operation context */
        psa_hash_operation_t hash;        /*!< Hash operation context */
        psa_key_derivation_operation_t key_deriv; /*!< Key derivation operation context */
    } operation;
};

TF-M Crypto service assigns a tfm_crypto_operation_s object for each multi-part crypto operation sequence during setup() step. The tfm_crypto_operation_s object content will be cleaned after the sequence completes or fails.

Impact

During multi-part hashing/MAC/cipher operations, if the underlying crypto library function returns an error code, TF-M update() and finish() functions will immediately clean up the structure tfm_crypto_operation_s content and exit.

When tfm_crypto_operation_s content is cleaned in TF-M update() and finish() functions, the content in PSA multi-part crypto operation object inside tfm_crypto_operation_s is also cleaned. If the underlying crypto library stores operation context in the PSA operation object, the operation context is lost before clients call abort() to handle the error.

Therefore, the underlying crypto library abort() function can be unable to perform normal abort operation if it cannot fetch the context or its content. In other words, the underlying crypto library abort() may not work normally or take effect.

In theory when the case analyzed above occurs:

If the underlying crypto library dynamically allocates some memory regions during multi-part operation and stores those memory region pointers in the PSA multi-part operation object, the underlying crypto library will be unable to locate and free those allocated memory regions in abort(). It will cause memory leakage in TF-M Crypto service. It may further make TF-M Crypto service unavailable and affect other services relying on TF-M Crypto service.
The underlying crypto library abort() may still consider the field values in the context as valid. abort() may perform unexpected behaviors or access invalid memory regions. It may trigger further faults and block TF-M Crypto service or even the whole system.

Note

The actual consequences depend on the implementation of the multi-part operations in the underlying crypto library.

Impacted PSA Crypto API functions

The following PSA multi-part crypto operation functions are impacted:

Multi-part hashing operations
- psa_hash_update()
- psa_hash_finish()
- psa_hash_verify()
- psa_hash_clone()
Multi-part MAC operations
- psa_mac_update()
- psa_mac_sign_finish()
- psa_mac_verify_finish()
Multi-part cipher operations
- psa_cipher_generate_iv()
- psa_cipher_set_iv()
- psa_cipher_update()
- psa_cipher_finish()

Justifications on unaffected multi-part operations

TF-M multi-part AEAD operations and multi-part key derivation operations are not impacted by this issue.

TF-M Crypto service has not implemented multi-part AEAD operations. TF-M multi-part AEAD functions directly return an error of unsupported operations.

In TF-M key derivation implementation, the psa_key_derivation_operation_t object is only cleaned in the abort() function after the underlying crypto library completes abort.

Mitigation

The clean-up operation shall be removed from error handling routines in the following TF-M Crypto functions:

Multi-part hashing operations
- tfm_crypto_hash_update()
- tfm_crypto_hash_finish()
- tfm_crypto_hash_verify()
- tfm_crypto_hash_clone()
Multi-part MAC operations
- tfm_crypto_mac_update()
- tfm_crypto_mac_sign_finish()
- tfm_crypto_mac_verify_finish()
Multi-part cipher operations
- tfm_crypto_cipher_generate_iv()
- tfm_crypto_cipher_set_iv()
- tfm_crypto_cipher_update()
- tfm_crypto_cipher_finish()

Note

This mitigation assumes that client follows the sequence specified in PSA Crypto API specification to call abort() when an error occurs during multi-part crypto operations.