Re: SetMemorySpaceAttributes with EFI_MEMORY_XP

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 20 March 2017 at 10:32, Michael Zimmermann <sigmaepsilon92@gmail.com> wrote:
> Hi,
>
> I didn't test ArmVirtQemuKernel but I'm trying to use some of the code
> for another platform.
> So does this call ever succeed with PcdDxeNxMemoryProtectionPolicy
> being enabled?
> https://github.com/tianocore/edk2/blob/76874be3d411bf8daac051718e20932e0bf97d70/ArmVirtPkg/HighMemDxe/HighMemDxe.c#L95
> Status = gDS->SetMemorySpaceAttributes (CurBase, CurSize, Attributes);
>
> Neither the memory that was added by this Dxe nor the one added
> automatically by GCD has the EFI_MEMORY_XP capability which causes
> SetMemorySpaceAttributes to return EFI_UNSUPPORTED.
>

That is a very good point. I have been caught by this more than once
already (and I did test this, but not as thoroughly as I should have,
apparently)

This is caused by the unfortunate situation in EDK2 that GCD
permission attributes are ambiguous: it does not distinguish between
'the memory controller allows this range to be configured as
non-executable' and 'the nature of the contents of this memory region
allows it to be mapped without executable attributes', and therefore,
RO/XP are never used in the GCD memory space map.

The solution is to use the CPU_ARCH_PROTOCOL interface explicitly to
set the XP attribute on the memory itself (but not on the descriptors
in the GCD or UEFI memory maps). I will spin a patch to fix this.

Thanks,
Ard.