KVM: Fix out of range accesses to memslots

Reset the LRU slot if it becomes invalid when deleting a memslot to fix
an out-of-bounds/use-after-free access when searching through memslots.

Explicitly check for there being no used slots in search_memslots(), and
in the caller of s390's approximation variant.

Fixes: 36947254 ("KVM: Dynamically size memslot array based on number of used slots")
Reported-by: Qian Cai <cai@lca.pw>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200320205546.2396-2-sean.j.christopherson@intel.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/s390/kvm/kvm-s390.c

@@ -2002,6 +2002,9 @@ static int kvm_s390_get_cmma(struct kvm *kvm, struct kvm_s390_cmma_log *args,
 	struct kvm_memslots *slots = kvm_memslots(kvm);
 	struct kvm_memory_slot *ms;
 
+	if (unlikely(!slots->used_slots))
+		return 0;
+
 	cur_gfn = kvm_s390_next_dirty_cmma(slots, args->start_gfn);
 	ms = gfn_to_memslot(kvm, cur_gfn);
 	args->count = 0;

include/linux/kvm_host.h

@@ -1032,6 +1032,9 @@ search_memslots(struct kvm_memslots *slots, gfn_t gfn)
 	int slot = atomic_read(&slots->lru_slot);
 	struct kvm_memory_slot *memslots = slots->memslots;
 
+	if (unlikely(!slots->used_slots))
+		return NULL;
+
 	if (gfn >= memslots[slot].base_gfn &&
 	    gfn < memslots[slot].base_gfn + memslots[slot].npages)
 		return &memslots[slot];

virt/kvm/kvm_main.c

@@ -882,6 +882,9 @@ static inline void kvm_memslot_delete(struct kvm_memslots *slots,
 
 	slots->used_slots--;
 
+	if (atomic_read(&slots->lru_slot) >= slots->used_slots)
+		atomic_set(&slots->lru_slot, 0);
+
 	for (i = slots->id_to_index[memslot->id]; i < slots->used_slots; i++) {
 		mslots[i] = mslots[i + 1];
 		slots->id_to_index[mslots[i].id] = i;