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

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Date:   Wed, 8 Apr 2020 12:21:38 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.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 Tue,  7 Apr 2020 23:40:59 -0700
Sean Christopherson <sean.j.christopherson@...el.com> 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@...el.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);

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>