linux-kernel - Re: [PATCH v10 02/19] KVM: s390: pv: handle secure storage violations for protected guests

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Date:   Thu, 5 May 2022 19:10:39 +0200
From:   Thomas Huth <thuth@...hat.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, frankja@...ux.ibm.com, pasic@...ux.ibm.com,
        david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com,
        mimu@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v10 02/19] KVM: s390: pv: handle secure storage violations
 for protected guests

On 14/04/2022 10.02, Claudio Imbrenda wrote:
> With upcoming patches, protected guests will be able to trigger secure
> storage violations in normal operation.
> 
> A secure storage violation is triggered when a protected guest tries to
> access secure memory that has been mapped erroneously, or that belongs
> to a different protected guest or to the ultravisor.
> 
> With upcoming patches, protected guests will be able to trigger secure
> storage violations in normal operation.

You've already used this sentence as 1st sentence of the patch description. 
Looks weird to read it again. Maybe scratch the 1st sentence?

> This happens for example if a
> protected guest is rebooted with lazy destroy enabled and the new guest
> is also protected.
> 
> When the new protected guest touches pages that have not yet been
> destroyed, and thus are accounted to the previous protected guest, a
> secure storage violation is raised.
> 
> This patch adds handling of secure storage violations for protected
> guests.
> 
> This exception is handled by first trying to destroy the page, because
> it is expected to belong to a defunct protected guest where a destroy
> should be possible. If that fails, a normal export of the page is
> attempted.
 >
> Therefore, pages that trigger the exception will be made non-secure
> before attempting to use them again for a different secure guest.

I'm an complete ignorant here, but isn't this somewhat dangerous? Could it 
happen that a VM could destroy/export the pages of another secure guest that 
way?

  Thomas