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

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Date:   Mon, 6 Apr 2020 15:17:57 +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 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?
Since we DO handle everything as we should, why rc = 1 ?