Re: [PATCH] ArmPkg/ArmMmuLib AARCH64: avoid EL0 accessible mappings

["Alexander Graf" <agraf@csgraf.de>]

On 22.09.21 18:19, Ard Biesheuvel wrote:
> We never run any code at EL0, and so it would seem that any access
> permissions set for EL0 (via the AP[1] attribute in the page tables) are
> irrelevant. We currently set EL0 and EL1 permissions to the same value
> arbitrarily.
>
> However, this causes problems on hardware like the Apple M1 running the
> hypervisor framework, which enters EL1 with SCTLR_EL1.SPAN enabled,
> which causes the Privileged Access Never (PAN) feature to be enabled on
> any exception taken to EL1, including the IRQ exceptions that handle our
> timer interrupt. When PAN is enabled, EL1 has no access to any mappings
> that are also accessible to EL0, causing the firmware to crash if it
> attempts to access such a mapping.
>
> Even though it is debatable whether or not SCTLR_EL1.SPAN should be
> disabled at entry or whether the firmware should put all UNKNOWN bits in
> all system registers in a consistent state (which it should), using EL0
> permissions serves no purpose whatsoever so let's fix that regardless.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>


I can confirm that this unbreaks HVF guests running on M1 with
SCTLR_EL1.SPAN=0 as reset state.


Tested-by: Alexander Graf <agraf@csgraf.de>

Alex