Abstract:
In a general aspect, a system can include a processor having a secure mode and a non-secure mode, and a secure module configured to respond to tokens posted by the processor in the secure mode. Each token can identify a secure asset, and source and destination addresses within secure and public address spaces. The secure module can include a memory storing secure assets identifiable by the tokens and a memory access circuit to read data from source addresses and write processed data to destination addresses. The system can further include a cryptography engine configured to process the read data using identified secure assets. The secure module can respond to tokens posted in the non-secure mode. The memory can store, with each secure asset, a respective rule defining the address spaces where the memory access circuit may read and write data. The secure module can ignore tokens that do not satisfy respective rules.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to a data processing system including a Trusted Execution Environment (TEE), and more specifically to a secure module that stores secure assets for use in the TEE. 
       BACKGROUND 
       [0002]      FIG. 1  is a block diagram of an exemplary data processing system including a TEE implemented around a specific secure module  10  offering a secure asset store. The secure module  10 , a host processor or CPU  12 , and a memory  14  are interconnected through a system bus B. 
         [0003]    The memory  14  is partitioned into a secure area  14   a  and a public area  14   b . The secure area  14   a  contains code and data that are part of the TEE. It is only accessible by trusted elements, such as the secure module  10 , and the CPU only when it operates in secure mode, which is identified by a secure-mode signal S issued by the CPU on the bus. In secure mode, the CPU thus executes trusted code stored in the secure memory area and has access to secure data stored in that same memory area. The public memory area  14   b  is accessible both in secure mode and non-secure mode. 
         [0004]    The secure module  10  may be selected from the “VaultIP” product line offered by Inside Secure, for instance the VaultIP- 130  that is schematically depicted here. 
         [0005]    The secure module or vault  10  includes an internal volatile memory  16  storing several assets that should not be exposed in plaintext format to an untrusted environment. The assets typically include temporary symmetric keys or session keys. These assets or keys are usable for encryption or decryption processes accelerated internally by a cipher core  18 . The cipher core  18  may actually include multiple cipher engines, such as AES, 3DES, SHA-256, etc. A sequencer  20  controls the cipher core  18  and other elements of the vault based on firmware stored in a non-volatile memory  22 . 
         [0006]    The vault includes a secure mailbox  24  connected to the bus. The mailbox is configured to receive commands called tokens from the CPU in secure mode. Typically, a token may be used to install an asset in the internal memory  16 , read an asset, request encryption or decryption of data using an identified asset and an identified cipher, etc. 
         [0007]    The data processed by the cipher core is exchanged with the system memory  14  through a direct memory access (DMA) controller  26  connected to the bus B in bus-master mode. The secure or public memory area accessed by the DMA controller may be selected, like for the CPU, by a secure-mode signal S issued on the bus. 
         [0008]    A system such as disclosed above, where a TEE coexists with an untrusted or rich execution environment, introduces significant overhead when operations require switching back and forth between security domains. Switching a CPU to secure mode typically requires flushing the cache memory, saving the contexts of multiple parallel tasks, flushing the contexts, loading the trusted code from the secure memory area, and executing the trusted code in single-task mode with the cache memory and interruptions disabled. 
       SUMMARY 
       [0009]    In a general aspect a data processing system with a trusted execution environment can include a host processor having a secure mode for operating in the trusted execution environment and a non-secure mode; a system bus connected to the host processor; resources connected to the system bus, accessible through addresses within a secure address space used by the trusted execution environment and a public address space; and a secure module connected to the system bus, configured to respond to tokens posted by the host processor in secure mode, wherein each token identifies a secure asset, and source and destination addresses within the secure and public address spaces. The secure module includes an internal memory storing secure assets identifiable by the tokens; a memory access circuit connected to read data from the source addresses and write processed data to the destination addresses; and a cryptography engine configured to process the read data using the identified secure assets. The secure module is configured to also respond to tokens posted by the host processor in non-secure mode. The internal memory of the secure module stores a rule together with each secure asset, defining permissions as to the address spaces where the memory access circuit may read and write the data. The secure module ignores tokens that do not satisfy the permissions defined for the corresponding assets. 
         [0010]    The data processing system may include cross-domain rules for tokens posted by the host processor in non-secure mode, that allow reading data from one of the public and secure address spaces and writing resulting data to the other of the public and secure address spaces. 
         [0011]    A cross-domain rule may allow reading data from the public address space and writing resulting data to the secure address space in response to a decryption token. 
         [0012]    A cross-domain rule may allow reading data from the secure address space and writing resulting data to the public address space in response to an encryption token. 
         [0013]    All rules for assets installed in the internal memory by the host processor in non-secure mode may constrain access to the public address space. 
         [0014]    The rules may include a flag identifying the secure or public address space where source data is located, and each rule may constrain read access to the address space identified by the corresponding flag. 
         [0015]    The resources accessible through the secure address space may include a system memory area and a secure peripheral. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]    Other advantages and features will become more clearly apparent from the following description of particular embodiments, which are provided for exemplary purposes only and represented in the appended drawings, in which: 
           [0017]      FIG. 1  is a block diagram of an exemplary data processing system including a Trusted Execution Environment implemented around a secure module having an asset store; 
           [0018]      FIG. 2  is a time diagram of a decryption process as it may be carried out by the system of  FIG. 1 ; 
           [0019]      FIG. 3  is a block diagram of a data processing system including a Trusted Execution Environment implemented around an embodiment of a secure module storing assets with rules; and 
           [0020]      FIG. 4  is a time diagram of a decryption process as it may be carried out by the system of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 2  is a time diagram that illustrates a possible use of the trusted execution environment in the data processing system of  FIG. 1 . A decryption process is taken as an exemplary use case. Operations carried out in the host CPU  12 , in the vault  10  and in the system memory (RAM)  14  are illustrated on respective vertical axes. Horizontal arrows show communications between these elements. 
         [0022]    At a time t 0 , the CPU writes a ciphertext data block CTXT in the public area  14   b  of the memory. The ciphertext data is, for example, a DRM-protected media segment. The key for decrypting the data is an asset stored in the internal memory  16  (Asset store) of the vault. This asset has been installed beforehand by the CPU in secure mode. 
         [0023]    When the data CTXT has been written, the CPU switches to secure mode. As mentioned above, such an action has a significant overhead and takes several clock cycles. 
         [0024]    At a time t 1 , the CPU, now in secure mode, writes a token in the vault&#39;s mailbox. The token is a decrypt request specifying the source address of the ciphertext data block (in public memory), the destination address in secure memory, the asset to use, and eventually the cipher to use and the size of the data block. 
         [0025]    The CPU remains in secure mode and idles until the result of the operation is returned at a time t 5 , when the vault has completed the request. When the CPU executes an operation in secure mode, it may not switch to non-secure mode until the operation is completed. 
         [0026]    In response to the token, the vault processes the asset, i.e. fetches the asset in the internal memory, checks if it may be used, and programs the corresponding cipher engine. 
         [0027]    At a time t 2 , the vault issues a request through its DMA controller  26  to fetch a data sequence at the specified source address, in public memory. 
         [0028]    At a time t 3 , the requested data, i.e. the ciphertext data CTXT, is received from the memory by the DMA controller and decrypted by the cipher core. 
         [0029]    At a time t 4 , the processed data, plaintext data PTXT, is written in secure memory by the DMA controller. 
         [0030]    At a time t 5 , the vault returns a status indicating that the request was successfully processed. The CPU wakes up and may switch back to non-secure mode. 
         [0031]    In practice, the data block to process identified in the token may be larger than the data sequence that can be accepted by the cipher core  18  and internal buffering in the vault. In such a case, steps t 2  to t 4  may be repeated in a loop to process the data block piecewise in multiple cycles. 
         [0032]    The time diagram of  FIG. 2  thus shows that the CPU switches to secure mode for posting each token, and stays idle until the operation requested in the token finishes. This represents a significant overhead during which the CPU cannot attend to other tasks. The overhead of switching to secure mode may in practice be longer than the actual time taken by the vault to process the token (t 5 -t 1 ). 
         [0033]    The vault in the system of  FIG. 1  is typically designed to ignore or reject tokens posted to it by untrusted code, i.e. posted by the CPU in non-secure mode. Indeed, untrusted code could post tokens that direct decrypted, plaintext data to the public memory, where it would be exposed and compromised. Switching the CPU to secure mode is generally required for posting tokens that use secure assets or require access to the secure memory domain. 
         [0034]    It is proposed herein to modify the vault structure so that tokens involving assets can be posted to it in non-secure mode while preserving data security requirements. Since tokens can be posted in non-secure mode, the overhead of switching to secure mode is avoided. Moreover, the CPU no longer needs to idle while the vault processes the posted token. 
         [0035]      FIG. 3  is a block diagram of a data processing system similar to that of  FIG. 1 , with a modified secure module  10 ′. Similar elements to those of  FIG. 1  bear same reference labels. A difference is that the internal memory  16 ′ stores each asset Ax with a rule Px defining permissions for using the asset. The sequencer  20 ′ is designed to interpret the rules assigned to the assets based on the asset installer domain (asset installed in secure or non-secure mode), the token poster domain (token posted in secure or non-secure mode), the source data domain (public or secure memory), and the operation direction (same domain or cross-domain). Moreover, the mailbox  24 ′ is no longer “secure” in that the CPU may post tokens to it both in secure and non-secure modes. 
         [0036]    The source data domain and the operation direction may be encoded in individual bits forming a 2-bit rule Px stored with the corresponding asset Ax. Since the installer domain is specific to each asset, it may be encoded in a third bit of the rule Px. The token domain may vary between different uses of a same asset, so it may be evaluated by the sequencer upon processing each token. The permissions may be expressed as read and write accesses to each of the secure and public areas of the RAM memory  14 . Table 1 below is an example summarizing the RAM read and write permissions based on the four parameters mentioned above. The column X-Domain is a flag indicating whether the operation should cross domains (Yes) or not (No). The Read RAM and Write RAM columns are flags that set the actual permissions resulting from the information in the previous columns—these permission flags indicate the memory domains (Secure, Public) to which read or write access is allowed. Some combinations do not allow access at all to the memory. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Asset 
                   
                 Source 
                   
                   
                   
               
               
                 Installer 
                 Token 
                 Data 
                   
                   
                 Write 
               
               
                 Domain 
                 Domain 
                 Domain 
                 X-Domain 
                 Read RAM 
                 RAM 
               
               
                   
               
             
             
               
                 Secure 
                 Secure 
                 Secure 
                 No 
                 Secure 
                 Secure 
               
               
                 Secure 
                 Secure 
                 Secure 
                 Yes 
                 Secure 
                 Public 
               
             
          
           
               
                 Secure 
                 Secure 
                 Public 
                 No 
                 Not allowed 
               
             
          
           
               
                 Secure 
                 Secure 
                 Public 
                 Yes 
                 Public 
                 Secure 
               
             
          
           
               
                 Secure 
                 Non-secure 
                 Secure 
                 No 
                 Not allowed 
               
             
          
           
               
                 Secure 
                 Non-secure 
                 Secure 
                 Yes 
                 Secure 
                 Public 
               
               
                 Secure 
                 Non-secure 
                 Public 
                 No 
                 Public 
                 Public 
               
               
                 Secure 
                 Non-secure 
                 Public 
                 Yes 
                 Public 
                 Secure 
               
             
          
           
               
                 Non-secure 
                 Secure 
                 Secure 
                 Not allowed 
               
             
          
           
               
                 Non-secure 
                 Secure 
                 Public 
                 Yes 
                 Not allowed 
               
             
          
           
               
                 Non-secure 
                 Secure 
                 Public 
                 No 
                 Public 
                 Public 
               
             
          
           
               
                 Non-secure 
                 Non-secure 
                 Secure 
                 Not allowed 
               
             
          
           
               
                 Non-secure 
                 Non-secure 
                 Public 
                 No 
                 Public 
                 Public 
               
             
          
           
               
                 Non-secure 
                 Non-secure 
                 Public 
                 Yes 
                 Not allowed 
               
               
                   
               
             
          
         
       
     
         [0037]    The permissions are crafted in particular so that, in lower security contexts, such as when the token domain is non-secure, ciphertext data stored in the public memory can only be decrypted to secure memory, and plaintext data stored in the secure memory is always encrypted when it is transferred outside the secure domain. 
         [0038]    A few generic rules result from this table. Read access to the memory, when allowed, is constrained to the same domain as the source data. Read and write accesses, when allowed, are constrained to the public memory if the asset originates from the non-secure domain, whereby cross-domain operations are not allowed. 
         [0039]    Note in particular that rules allow cross-domain operations in both directions for non-secure tokens, i.e. tokens posted by the CPU in non-secure mode. 
         [0040]    The permissions may be checked by the sequencer  20 ′ as a token is processed. The sequencer reads the asset identified by the token, together with its rule, for instance three bits corresponding to the asset installer domain, the source data domain and the cross-domain flag. The token domain information is known when the token is posted. From these four pieces of information, the sequencer establishes the permissions according to the two last columns of the table. If the token conveys source and destination addresses that do not match the permissions, the token is ignored or rejected. 
         [0041]      FIG. 4  is a time diagram of a decryption process as it may be carried out by the system of  FIG. 3 . It is similar to the diagram of  FIG. 2 , except that the CPU may post the decrypt token at time t 1  while remaining in non-secure mode. The CPU may then idle awaiting the return status of the operation or switch to another task. 
         [0042]    The asset processing phase includes permission checking as mentioned above, based on the rule stored with the asset. If the permission check succeeds, and the asset is intended for the operation, the requested operation is performed. Otherwise the operation is discarded and a failure status may be returned to the CPU. 
         [0043]    As shown in  FIG. 3 , the system may include a trusted peripheral connected to the bus, such as an HDMI interface  30  implementing HDCP protection. Typically, if the system receives DRM protected content, the system may be configured to decrypt this content into the secure memory  14   a  through the vault. From there, the CPU, in secure mode, may transfer the decrypted data to the HDMI interface for display. The CPU, still in secure mode, would have previously configured the HDCP protocol with a key asset stored in the vault. 
         [0044]    The system may typically be configured to map heterogeneous resources, such as the memory and the HDMI interface, to a contiguous virtual memory space. A secure virtual memory space may be allocated to both the secure memory area  14   a  and the HDMI interface. In this context, reading and writing in public or secure memory areas may be referred to more generally as reading and writing in public or secure address spaces. Then, instead of first writing the decrypted data into the secure memory area  14   a  at time t 4 , the DMA controller may be provided with a virtual destination address that corresponds to the HDMI interface, whereby the decrypted data may be directly transferred to a media renderer. 
         [0045]    In addition, a VaultIP- 130  circuit is designed with a secure interface for directly transferring HDCP keys from the asset store to an external HDMI interface. 
         [0046]    With such a structure, the CPU may remain in non-secure mode throughout the decryption and rendering of DRM-protected content. The only times the CPU would need to switch into secure mode is when installing an initial key asset for the DRM decryption in the vault, and eventually when renewing the keys as they expire. 
         [0047]    A DRM decryption process has been described as a particular application example using assets stored with rules. Similar benefits may be obtained in other applications, such as DRM encryption or any other application using cryptography and a trusted execution environment.