Hi Brian,

MM-IOVEC is permitted in isolation 2-3.

It just may have some conflicts with the isolation rules. What you’ve quoted is just one of the conflicts.

However, “Access to an input or output vector’s buffer for the duration of the call is expected by the client, so this

does not itself present a new attack surface”.

So MM-IOVEC could be implemented for isolation 2 and 3.

But you have to be careful as the “implementation uses to map input and output vectors can be imprecise” because of, for example, the HW limitation of MPU.

The MPU has the alignment constraints, for example, 32-byte.

And then if you’d like to map a 16-byte vector, you might have to provide 16-byte more data for the RoT Service to access.

Whether this imprecise map is acceptable depends on the security requirements.

But it is true that TF-M has not implemented that.

-an attacker may tamper NS input data while the RoT service is processing those data

Usually, input data should be accessed by RoT Services with READ-ONLY permission.

-Kevin

From: Quach, Brian via TF-M <tf-m@lists.trustedfirmware.org>
Sent: Tuesday, November 14, 2023 7:37 AM
To: tf-m@lists.trustedfirmware.org; Shreve, Erik <e-shreve@ti.com>
Subject: [TF-M] PSA_FRAMEWORK_HAS_MM_IOVEC

Hi,

I saw that PSA_FRAMEWORK_HAS_MM_IOVEC is not permitted for Isolation level 2-3 and transient copies of the input and output data into a 5KB scratch buffer are always made when making a PSA call. 

This makes sense as FF-Mv1.1 states:

“In a system using isolation level 3, a Secure Partitions is not permitted to access another Secure

Partition’s Private data. MM-IOVEC can provide a mechanism for one Secure Partition to access the

other’s Private data.”

But I think the requirements for isolation level 3 could be fulfilled by:

If SP detects a Secure caller, it could make a transient copy of I/O data.

If SP detects a Non-Secure caller, it could use MMIOVECs or a similar method to access NS memory directly to avoid overhead and limitations of copying the I/O data. 

Is this logical/ correct?

With this approach, an attacker may tamper NS input data while the RoT service is processing those data but rules of isolation level 3 are maintained.

Regards,

Brian Quach

SimpleLink MCU

Texas Instruments Inc.