linux-kernel - Re: [PATCH v2 2/5] KVM: s390: vsie: Fix delivery of addressing exceptions

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Date:   Tue, 7 Apr 2020 13:35:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, 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>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 2/5] KVM: s390: vsie: Fix delivery of addressing
 exceptions

On 07.04.20 13:00, Claudio Imbrenda wrote:
> On Fri,  3 Apr 2020 17:30:47 +0200
> David Hildenbrand <david@...hat.com> 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;
>>  	}
>>  	return rc;
>>  }
> 
> so, the reason why we never noticed this issue before is simply that
> nobody tried running a misbehaving nested guest?

Yes, actually, a misbehaving nested hypervisor.

> 
> Reviewed-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> 

Thanks!

-- 
Thanks,

David / dhildenb