[PATCH 4/4] efi/arm64: unmap the kernel while executing UEFI services

[Ard Biesheuvel]

On 26 January 2018 at 17:05, Will Deacon <will.deacon at arm.com> wrote:
> On Thu, Jan 25, 2018 at 10:31:31AM +0000, Ard Biesheuvel wrote:
>> Now that all UEFI runtime service wrappers ensure that byref
>> arguments are moved into the UEFI marshalling buffer (which
>> is not part of the kernel mapping), we can proceed and unmap
>> the kernel while UEFI runtime service calls are in progress.
>>
>> This is done by setting the EPD1 bit and flushing the TLB of
>> the local CPU. This makes it independent of KPTI or whether
>> non-global mappings are being used.
>
> One snag with this is that it will break SPE, so I'd prefer this behaviour
> to be predicated on kpti so that the arm64_kernel_unmapped_at_el0() check
> in drivers/perf/arm_spe_pmu.c remains valid.
>

The problem with that is that they serve two different purposes: kpti
protects against meltdown, this protects against Spectre variant 1.