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: afdad61615cc3 ("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>

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 19a81024fe16b0cedf8d78a4d7d6410cedff5ba7..5dcf9ff1282868bb9bb945a4d0f99117ad2c7fdb 100644 (file)
--- 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);