[PATCH] arm64: KVM: Fix SMCCC handling of unimplemented SMC/HVC calls

[Christoffer Dall]

On Tue, Jan 16, 2018 at 10:23:47AM +0000, Marc Zyngier wrote:
> KVM doesn't follow the SMCCC when it comes to unimplemented calls,
> and inject an UNDEF instead of returning an error. Since firmware
> calls are now used for security mitigation, they are becoming more
> common, and the undef is counter productive.
> 
> Instead, let's follow the SMCCC which states that -1 must be returned
> to the caller when getting an unknown function number.
> 
> Cc: stable at vger.kernel.org
> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>

Reviewed-by: Christoffer Dall <christoffer.dall at linaro.org>

And applied to kvmarm/master.  I'll attempt to herd this upstream asap.
-Christoffer

> ---
> This has been identified as an issue when working on the Spectre issues,
> as guest have to perform HVC/SMC calls to invalidate their branch predictors,
> and need to perform some form of discovery. 32bit is affected too, but the
> mitigation doesn't involve firmware, so we can fix it separately.
> 
>  arch/arm64/kvm/handle_exit.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 304203fa9e33..e60494f1eef9 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -45,7 +45,7 @@ static int handle_hvc(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  
>  	ret = kvm_psci_call(vcpu);
>  	if (ret < 0) {
> -		kvm_inject_undefined(vcpu);
> +		vcpu_set_reg(vcpu, 0, ~0UL);
>  		return 1;
>  	}
>  
> @@ -54,7 +54,7 @@ static int handle_hvc(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  
>  static int handle_smc(struct kvm_vcpu *vcpu, struct kvm_run *run)
>  {
> -	kvm_inject_undefined(vcpu);
> +	vcpu_set_reg(vcpu, 0, ~0UL);
>  	return 1;
>  }
>  
> -- 
> 2.14.2
>