Re: [PATCH] KVM: arm64: Allow PSCI SYSTEM_OFF/RESET to return

[Marc Zyngier]

Hi David,

On 2020-12-29 16:00, David Brazdil wrote:
> The KVM/arm64 PSCI relay assumes that SYSTEM_OFF and SYSTEM_RESET 
> should
> not return, as dictated by the PSCI spec. However, there is firmware 
> out
> there which breaks this assumption, leading to a hyp panic. Make KVM
> more robust to broken firmware by allowing these to return.
>
> Signed-off-by: David Brazdil <dbrazdil@xxxxxxxxxx>
> ---
>  arch/arm64/kvm/hyp/nvhe/psci-relay.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/psci-relay.c
> b/arch/arm64/kvm/hyp/nvhe/psci-relay.c
> index e3947846ffcb..8e7128cb7667 100644
> --- a/arch/arm64/kvm/hyp/nvhe/psci-relay.c
> +++ b/arch/arm64/kvm/hyp/nvhe/psci-relay.c
> @@ -77,12 +77,6 @@ static unsigned long psci_forward(struct
> kvm_cpu_context *host_ctxt)
>  			 cpu_reg(host_ctxt, 2), cpu_reg(host_ctxt, 3));
>  }
>
> -static __noreturn unsigned long psci_forward_noreturn(struct
> kvm_cpu_context *host_ctxt)
> -{
> -	psci_forward(host_ctxt);
> -	hyp_panic(); /* unreachable */
> -}
> -
>  static unsigned int find_cpu_id(u64 mpidr)
>  {
>  	unsigned int i;
> @@ -251,10 +245,13 @@ static unsigned long psci_0_2_handler(u64
> func_id, struct kvm_cpu_context *host_
>  	case PSCI_0_2_FN_MIGRATE_INFO_TYPE:
>  	case PSCI_0_2_FN64_MIGRATE_INFO_UP_CPU:
>  		return psci_forward(host_ctxt);
> +	/*
> +	 * SYSTEM_OFF/RESET should not return according to the spec.
> +	 * Allow it so as to stay robust to broken firmware.
> +	 */
>  	case PSCI_0_2_FN_SYSTEM_OFF:
>  	case PSCI_0_2_FN_SYSTEM_RESET:
> -		psci_forward_noreturn(host_ctxt);
> -		unreachable();
> +		return psci_forward(host_ctxt);
>  	case PSCI_0_2_FN64_CPU_SUSPEND:
>  		return psci_cpu_suspend(func_id, host_ctxt);
>  	case PSCI_0_2_FN64_CPU_ON:

Thanks for having tracked this.

I wonder whether we should also taint the kernel in this case,
because this is completely unexpected, and a major spec violation.

Ideally, we'd be able to detect this case and prevent pKVM from
getting initialised at all, but I guess there is no way to detect
the sucker without ... calling SYSTEM_RESET?

        M.
--
Jazz is not dead. It just smells funny...