linux-kernel - Re: [PATCH v1 1/4] s390/kvm: VSIE: stop leaking host addresses

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Date:   Mon, 4 Jan 2021 14:58:02 +0100
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1 1/4] s390/kvm: VSIE: stop leaking host addresses

On Sun, 20 Dec 2020 10:44:56 +0100
David Hildenbrand <david@...hat.com> wrote:

> On 18.12.20 15:18, Claudio Imbrenda wrote:
> > The addresses in the SIE control block of the host should not be
> > forwarded to the guest. They are only meaningful to the host, and
> > moreover it would be a clear security issue.  
> 
> It's really almost impossible for someone without access to
> documentation to understand what we leak. I assume we're leaking the
> g1 address of a page table (entry), used for translation of g2->g3 to
> g1. Can you try making that clearer?

this is correct.

I guess I can improve the text of the commit
 
> In that case, it's pretty much a random number (of a random page used
> as a leave page table) and does not let g1 identify locations of
> symbols etc. If so, I don't think this is a "clear security issue"
> and suggest squashing this into the actual fix (#p4 I assume).

yeah __maybe__ I overstated the importance ;)

But I would still like to keep it as a separate patch, looks more
straightforward to me
 
> @Christian, @Janosch? Am I missing something?
> 
> > 
> > Subsequent patches will actually put the right values in the guest
> > SIE control block.
> > 
> > Fixes: a3508fbe9dc6d ("KVM: s390: vsie: initial support for nested
> > virtualization") Cc: stable@...r.kernel.org
> > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > ---
> >  arch/s390/kvm/vsie.c | 5 -----
> >  1 file changed, 5 deletions(-)
> > 
> > diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> > index 4f3cbf6003a9..ada49583e530 100644
> > --- a/arch/s390/kvm/vsie.c
> > +++ b/arch/s390/kvm/vsie.c
> > @@ -416,11 +416,6 @@ static void unshadow_scb(struct kvm_vcpu
> > *vcpu, struct vsie_page *vsie_page) memcpy((void *)((u64)scb_o +
> > 0xc0), (void *)((u64)scb_s + 0xc0), 0xf0 - 0xc0);
> >  		break;
> > -	case ICPT_PARTEXEC:
> > -		/* MVPG only */
> > -		memcpy((void *)((u64)scb_o + 0xc0),
> > -		       (void *)((u64)scb_s + 0xc0), 0xd0 - 0xc0);
> > -		break;
> >  	}
> >  
> >  	if (scb_s->ihcpu != 0xffffU)
> >   
> 
>