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Context Management

In general, we don't prevent things unless there is a good reason for that. Put another way, we try to allow anything that doesn't cause a security problem.

David Wooten, During an e-mail exchange about context management

TPMs, for all their tremendous capability, are very limited in their memory, largely to reduce cost. This means objects, sessions, and sequences must be swapped in and out of the TPM as needed, much like a virtual memory manager swaps memory pages to and from disk drives. In both cases, the calling application thinks it has access to many more objects and sessions (in the TPM case) or much more memory (in the virtual memory case) than can actually be present at any given time.

For a system where only a single application sends commands to the TPM, these swapping operations can be performed by the application itself. [1] However, when multiple applications and/or processes are accessing the TPM, two components of the TSS stack are required: the TPM Access Broker (TAB) and the resource manager (RM).

This chapter describes the high-level architecture of the TAB and RM. Then it describes the features and commands of the TPM that support swapping objects, sessions, and sequences in and out of TPM memory, the details of how different swappable entities are handled by the related commands, and some special cases.

TAB and the Resource Manager: A High-Level Description

The TAB and RM were first described in Chapter 7 as layers in the TSS. This section provides some insights into the internals of the TAB and RM.

The TAB and RM transparently isolate TPM applications and processes from the messiness of arbitrating multiprocess access to the TPM and swapping objects, sessions, and sequences in and out of TPM memory as needed. The TAB and RM are closely related and typically integrated into the same software module. Depending on system design, they may reside in the top layer of a TPM device-driver stack, or they may be integrated as a daemon process sandwiched between the TSS system API layer above and the TPM device driver below.

TAB

The TAB's responsibility is fairly simple: arbitrate multiple processes accessing the TPM. At a minimum, all processes must be guaranteed that, between the time their command byte stream begins to be transmitted to the TPM and the time the response byte stream is fully received from the TPM, no other processes communicate with the TPM. Some examples of multiprocess collisions that could occur in the absence of a TAB are as follows:

• Process A's command byte stream begins to be transmitted to the TPM, and before it's done, process B's command byte stream starts being transmitted. The byte stream sent to the TPM will be a mix of command bytes from both processes, and the TPM will probably return an error code.

• Process A sends a command, and Process B reads the response.

• Process A's command byte stream is transmitted, and then, while its response is being read, Process B's byte stream starts being transmitted.

• Process A creates and loads a key exclusively for its own use. Process B context saves (using the TPM2_ContextSave command) the key and then, sometime later, context loads it (using the TPM2_ContextLoad command) and uses it for its own purposes.

A TAB can be implemented in a couple of different ways: either with a TPM lock or without one.

In the lock architecture, a software method of sending a “lock” down to the TAB could be designed. This would signal the TAB that no other process would be allowed access to the TPM until the process that locked it is completes. This would have the advantage of allowing a TAB to complete multiple commands to the TPM without interruption. And interestingly enough, this architecture would eliminate the need for an RM, assuming you could make it work. An application could claim a lock on the TPM and send commands to the TPM while managing all TPM contexts itself (this management includes cleaning up after itself by evicting all objects, sequences, and sessions before releasing the lock). The Achilles heel of this architecture is that the application might fail to release the lock, starving all other applications. There could be some sort of timeout mechanism, but that would place artificial limits on the time that an application could exclusively use the TPM. This in turn would force applications to do some fairly complex management of corner cases related to TPM contexts since they couldn't depend on the lock being active long enough to complete their TPM operations. This approach was initially considered by the TCG TSS working group and rejected after some very wise advice from Paul England of Microsoft.

The simpler lock-less architecture allows the transmission of a TPM command and reception of its response atomically without interruption from any other processes' interactions with the TPM. This reduces the time that the process exclusively accesses the TPM to the time it takes to send a command and receive a response. This architecture requires an underlying RM because competing applications cannot be in the business of managing each other's objects, sessions, and sequences. For example:

1. Process A happens to be the only application accessing the TPM for a while, and during this time it starts three sessions and loads three keys.

2. Then process B decides it's ready to talk to the TPM and wants to create a session and a key. If the TPM has only three session slots and three object slots, process B must first unload at least one of process A's sessions and one of its objects.

3. This forces process B to manage process A's TPM contexts— an untenable position in terms of inter-process isolation (security) and software complexity. Without a central RM, applications must manage all TPM contexts themselves. It's almost impossible to make this work, and it guarantees that processes can mess with each other's TPM contexts.

Hence the need for an RM.

  • [1] Even in this case, the use of an RM is advantageous because it relieves the application of the burden of performing the swapping.
 
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