Re: [PATCH v1 2/5] KVM: s390: vsie: Fix delivery of addressing exceptions

Christian Borntraeger
2020-04-06T13:24:53Z

On 06.04.20 15:22, David Hildenbrand wrote:
> On 06.04.20 15:17, Christian Borntraeger wrote:
>>
>>
>> On 02.04.20 20:48, David Hildenbrand wrote:
>>> Whenever we get an -EFAULT, we failed to read in guest 2 physical
>>> address space. Such addressing exceptions are reported via a program
>>> intercept to the nested hypervisor.
>>>
>>> We faked the intercept, we have to return to guest 2. Instead, right
>>> now we would be returning -EFAULT from the intercept handler, eventually
>>> crashing the VM.
>>>
>>> Addressing exceptions can only happen if the g2->g3 page tables
>>> reference invalid g2 addresses (say, either a table or the final page is
>>> not accessible - so something that basically never happens in sane
>>> environments.
>>>
>>> Identified by manual code inspection.
>>>
>>> Fixes: a3508fbe9dc6 ("KVM: s390: vsie: initial support for nested virtualization")
>>> Cc: <stable@vger.kernel.org> # v4.8+
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>> arch/s390/kvm/vsie.c | 1 +
>>> 1 file changed, 1 insertion(+)
>>>
>>> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
>>> index 076090f9e666..4f6c22d72072 100644
>>> --- a/arch/s390/kvm/vsie.c
>>> +++ b/arch/s390/kvm/vsie.c
>>> @@ -1202,6 +1202,7 @@ static int vsie_run(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>>> scb_s->iprcc = PGM_ADDRESSING;
>>> scb_s->pgmilc = 4;
>>> scb_s->gpsw.addr = __rewind_psw(scb_s->gpsw, 4);
>>> + rc = 1;
>>
>>
>> kvm_s390_handle_vsie has
>>
>> return rc < 0 ? rc : 0;
>>
>>
>> so rc = 0 would result in the same behaviour, correct?
>
> yes
>
>> Since we DO handle everything as we should, why rc = 1 ?
>
> rc == 1 is the internal representation of "we have to go back into g2".
> rc == 0, in contrast, means "we can go back into g2 (via a NULL
> intercept) or continue executing g3". Returning rc == 1 instead of rc ==
> 0 at this point is just consistency.

Ok, I will add something to the patch description.
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>