Re: [PATCH] KVM: arm64: Do not allow ID_AA64MMFR0_EL1.ASIDbits to be overridden

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Dec 03, 2024 at 07:02:36PM +0000, Marc Zyngier wrote:
> Catalin reports that a hypervisor lying to a guest about the size
> of the ASID field may result in unexpected issues:
> 
> - if the underlying HW does only supports 8 bit ASIDs, the ASID
>   field in a TLBI VAE1* operation is only 8 bits, and the HW will
>   ignore the other 8 bits
> 
> - if on the contrary the HW is 16 bit capable, the ASID field
>   in the same TLBI operation is always 16 bits, irrespective of
>   the value of TCR_ELx.AS.
> 
> This could lead to missed invalidations if the guest was lead to
> assume that the HW had 8 bit ASIDs while they really are 16 bit wide.
> 
> In order to avoid any potential disaster that would be hard to debug,
> prenent the migration between a host with 8 bit ASIDs to one with
> wider ASIDs (the converse was obviously always forbidden). This is
> also consistent with what we already do for VMIDs.
> 
> If it becomes absolutely mandatory to support such a migration path
> in the future, we will have to trap and emulate all TLBIs, something
> that nobody should look forward to.
> 
> Fixes: d5a32b60dc18 ("KVM: arm64: Allow userspace to change ID_AA64MMFR{0-2}_EL1")
> Reported-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: James Morse <james.morse@arm.com>
> Cc: Oliver Upton <oliver.upton@linux.dev>

Acked-by: Catalin Marinas <catalin.marinas@arm.com>