Re: [edk2-devel] [PATCH v1] MdeModulePkg: Fixup MAT Attributes After Splitting EFI Memory Map

["Oliver Smith-Denny" <osde@linux.microsoft.com>]

On 4/17/2024 9:52 AM, Ard Biesheuvel wrote:
> So the purpose of the MAT is to describe RT code (and to a lesser
> extent, RT data) regions where we cannot apply either RO or XP to the
> whole thing. IIRC there was never an intent to exhaustively describe
> all memory runtime regions. Also note that RO was introduced at this
> point, because WP was already being used in the ordinary memory map in
> a deviating manner. RO is defined both for the memory map and the MAT,
> and so it can occur in either.
> 

At the principle level, I think we can say that we want all runtime code
regions to RO and all runtime data regions to be XP. Regardless of
whatever situation we have today, I think this is a reasonable
principle to maintain. If you don't want those attributes, a different
memory type should be allocated. If we agree on this principle, I think
we should put it into practice. Again, the UEFI spec calls out that
EfiRuntimeServicesCode is for image code. From a security and safety
standpoint, we know we want image code to be RO.

To help with any existing (mis)use of EfiRuntimeServicesCode, I do
think we should put a big old assert in the MAT generation logic that
says I found a EfiRuntimeServicesCode section that is not described
in an image record, something is wrong with your configuration, you
are not using EfiRuntimeServicesCode correctly. If I am missing a
legitimate use of EfiRuntimeServicesCode, please help educate me.

Also, I know that modern Windows security features rely on the MAT
describing all EfiRuntimeServicesCode and EfiRuntimeServicesData
regions. Here is an MSDN link that makes a statement towards
that:

https://learn.microsoft.com/en-us/windows-hardware/drivers/bringup/unified-extensible-firmware-interface

In a more actionable way, the Windows testing infrastructure will
test to ensure that there are no EfiRuntimeServices[Code|Data]
sections in the EFI memory map that are not described in the MAT.
Again, there are various security features that rely on this.

Thanks,
Oliver


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