KVM: s390: prevent buffer overrun on memory hotplug during migration

We must not go beyond the pre-allocated buffer. This can happen when
a new memory slot is added during migration.

Reported-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: stable@vger.kernel.org # 4.13+
Fixes: 190df4a212a7 (KVM: s390: CMMA tracking, ESSA emulation, migration mode)
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>

arch/s390/kvm/priv.c

@@ -1006,7 +1006,7 @@ static inline int do_essa(struct kvm_vcpu *vcpu, const int orc)
 		cbrlo[entries] = gfn << PAGE_SHIFT;
 	}
 
-	if (orc) {
+	if (orc && gfn < ms->bitmap_size) {
 		/* increment only if we are really flipping the bit to 1 */
 		if (!test_and_set_bit(gfn, ms->pgste_bitmap))
 			atomic64_inc(&ms->dirty_pages);