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

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Date:   Sun, 20 Dec 2020 10:44:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        linux-kernel@...r.kernel.org
Cc:     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 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?

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).

@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)
> 


-- 
Thanks,

David / dhildenb