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

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Message-Id: <123476b6-acbd-a5ed-7416-1092c064bc86@de.ibm.com>
Date:   Mon, 6 Apr 2020 15:24:44 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Janosch Frank <frankja@...ux.ibm.com>, stable@...r.kernel.org
Subject: Re: [PATCH v1 2/5] KVM: s390: vsie: Fix delivery of addressing
 exceptions



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@...r.kernel.org> # v4.8+
>>> Signed-off-by: David Hildenbrand <david@...hat.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@...ibm.com>