[PATCH 04/11] KVM: arm64: Move PC rollback on SError to HYP

[James Morse]

Hi Marc,

On 26/10/2020 13:34, Marc Zyngier wrote:
> Instead of handling the "PC rollback on SError during HVC" at EL1 (which
> requires disclosing PC to a potentially untrusted kernel), let's move
> this fixup to ... fixup_guest_exit(), which is where we do all fixups.

> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index d687e574cde5..668f02c7b0b3 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -411,6 +411,21 @@ static inline bool fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
>  	if (ARM_EXCEPTION_CODE(*exit_code) != ARM_EXCEPTION_IRQ)
>  		vcpu->arch.fault.esr_el2 = read_sysreg_el2(SYS_ESR);
>  
> +	if (ARM_SERROR_PENDING(*exit_code)) {
> +		u8 esr_ec = kvm_vcpu_trap_get_class(vcpu);
> +
> +		/*
> +		 * HVC already have an adjusted PC, which we need to
> +		 * correct in order to return to after having injected
> +		 * the SError.
> +		 *
> +		 * SMC, on the other hand, is *trapped*, meaning its
> +		 * preferred return address is the SMC itself.
> +		 */
> +		if (esr_ec == ESR_ELx_EC_HVC32 || esr_ec == ESR_ELx_EC_HVC64)
> +			*vcpu_pc(vcpu) -= 4;

Isn't *vcpu_pc(vcpu) the PC of the previous entry for this vcpu?.... its not the PC of the
exit until __sysreg_save_el2_return_state() saves it, which happens just after
fixup_guest_exit().

Mess with ELR_EL2 directly?


Thanks,

James

> +	}
> +
>  	/*
>  	 * We're using the raw exception code in order to only process
>  	 * the trap if no SError is pending. We will come back to the
>