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

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Message-ID: <527f7bdf-d8f9-59b4-e70a-54e358ee9e26@redhat.com>
Date:   Mon, 6 Apr 2020 15:22:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.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 15:17, 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.

-- 
Thanks,

David / dhildenb