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

[Marc Zyngier]

On 2020-10-27 14:56, James Morse wrote:
> 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().

Hmmm. Good point. The move was obviously done in haste, thank you for 
pointing
this blatant bug.

> Mess with ELR_EL2 directly?

Yes, that's the best course of action. We never run this code anyway.

Thanks,

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