[PATCH 1/7] KVM: Fix out of range accesses to memslots

[Sean Christopherson <sean.j.christopherson@intel.com>]

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: 36947254e5f9 ("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>
---
 arch/s390/kvm/kvm-s390.c | 3 +++
 include/linux/kvm_host.h | 3 +++
 virt/kvm/kvm_main.c      | 3 +++
 3 files changed, 9 insertions(+)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 807ed6d722dd..cb15fdda1fee 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/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;
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 35bc52e187a2..b19dee4ed7d9 100644
--- a/include/linux/kvm_host.h
+++ b/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];
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 28eae681859f..f744bc603c53 100644
--- a/virt/kvm/kvm_main.c
+++ b/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;
-- 
2.24.1