Security vulnerabilities in the context switch implementation of the security monitor

[neeluk7]

Hello,

We have been analyzing several CC systems, especially the context switch implementations, to check if any security-sensitive architectural state is not swapped appropriately:

Security vulnerability:
The existing context switch implementation swaps the F-extension and V-extension architectural state only if the status of these states is Dirty in the sstatus CSR. (Link to code)

This can lead to timing channel attacks from the untrusted host due to the variation in context switch time.

This can be fixed by swapping these architectural state regardless of whether the state is Dirty or not, to achieve a constant time context switch.

Best Regards.