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Subject: Re: [PATCH 2/2] KVM: s390: Return last valid slot if approx index is
 out-of-bounds
To:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        syzbot+d889b59b2bb87d4047a2@syzkaller.appspotmail.com,
        Claudio Imbrenda <imbrenda@linux.ibm.com>
References: <20200408064059.8957-1-sean.j.christopherson@intel.com>
 <20200408064059.8957-3-sean.j.christopherson@intel.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
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Date:   Wed, 8 Apr 2020 09:28:48 +0200
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Precedence: bulk

On 08.04.20 08:40, Sean Christopherson wrote:
> Return the index of the last valid slot from gfn_to_memslot_approx() if
> its binary search loop yielded an out-of-bounds index.  The index can
> be out-of-bounds if the specified gfn is less than the base of the
> lowest memslot (which is also the last valid memslot).
> 
> Note, the sole caller, kvm_s390_get_cmma(), ensures used_slots is
> non-zero.
> 
> Fixes: afdad61615cc3 ("KVM: s390: Fix storage attributes migration with memory slots")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
>  arch/s390/kvm/kvm-s390.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 19a81024fe16..5dcf9ff12828 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -1939,6 +1939,9 @@ static int gfn_to_memslot_approx(struct kvm_memslots *slots, gfn_t gfn)
>  			start = slot + 1;
>  	}
>  
> +	if (start >= slots->used_slots)
> +		return slots->used_slots - 1;
> +
>  	if (gfn >= memslots[start].base_gfn &&
>  	    gfn < memslots[start].base_gfn + memslots[start].npages) {
>  		atomic_set(&slots->lru_slot, start);
> 

Claudio, can you have a look?