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Date:   Tue, 7 Apr 2020 13:00:13 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     kvm@vger.kernel.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, Vasily Gorbik <gor@linux.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        stable@vger.kernel.org
Subject: Re: [PATCH v2 2/5] KVM: s390: vsie: Fix delivery of addressing
 exceptions
In-Reply-To: <20200403153050.20569-3-david@redhat.com>
References: <20200403153050.20569-1-david@redhat.com>
        <20200403153050.20569-3-david@redhat.com>
Organization: IBM
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On Fri,  3 Apr 2020 17:30:47 +0200
David Hildenbrand <david@redhat.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@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;
>  	}
>  	return rc;
>  }

so, the reason why we never noticed this issue before is simply that
nobody tried running a misbehaving nested guest?

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