security/intel/txt: Improve MTRR setup for GETSEC[ENTERACCS]

The BIOS ACM will check that enabled variable MTRRs do not cover more
than the ACM's size, rounded up to 4 KiB. If that is not the case,
launching the ACM will result in a lovely TXT reset. How boring.

The new algorithm simply performs a reverse bit scan in a loop, and
allocates one MTRR for each set bit in the rounded-up size to cache.
Before allocating anything, it checks if there are enough variable
MTRRs; if not, it will refuse to cache anything. This will result in
another TXT reset, initiated by the processor, with error type 5:

   Load memory type error in Authenticated Code Execution Area.

This can only happen if the ACM has specific caching requirements that
the current code does not know about, or something has been compromised.
Therefore, causing a TXT reset should be a reasonable enough approach.

Also, disable all MTRRs before clearing the variable MTRRs and only
enable them again once they have been set up with the new values.

Tested on Asrock B85M Pro4 with a BIOS ACM whose size is 101504 bytes.
Without this patch, launching the ACM would result in a TXT reset. This
no longer happens when this patch is applied.

Change-Id: I8d411f6450928357544be20250262c2005d1e75d
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44880
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions