KVM: s390: Return last valid slot if approx index is out-of-bounds

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: afdad61615 ("KVM: s390: Fix storage attributes migration with memory slots")
Cc: stable@vger.kernel.org # 4.19.x: 0774a964ef56: KVM: Fix out of range accesses to memslots
Cc: stable@vger.kernel.org # 4.19.x
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200408064059.8957-3-sean.j.christopherson@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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);