linux-kernel - Re: [PATCH 2/2] KVM: s390: Return last valid slot if approx index is out-of-bounds

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Message-Id: <875ba09c-eb1b-1cdd-f198-0bfbdb607d4b@de.ibm.com>
Date:   Wed, 8 Apr 2020 13:40:09 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot+d889b59b2bb87d4047a2@...kaller.appspotmail.com
Subject: Re: [PATCH 2/2] KVM: s390: Return last valid slot if approx index is
 out-of-bounds


On 08.04.20 13:33, Paolo Bonzini wrote:
> On 08/04/20 12:21, Claudio Imbrenda wrote:
>> on s390 memory always starts at 0; you can't even boot a system missing
>> the first pages of physical memory, so this means this situation would
>> never happen in practice. 
>>
>> of course, a malicious userspace program could create an (unbootable) VM
>> and trigger this bug, so the patch itself makes sense.
>>
>> Reviewed-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> 
> What about using KVM just for isolation and not just to run a full-blown
> OS (that is, you might even only have the guest run in problem state)?
> Would that be feasible on s390?

You always need 2 prefix pages. Otherwise the SIE instruction refuses to start.
By default this starts as address 0. You might be also to set the prefix register
to something else adn then this could maybe work.