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: 3694725 ("KVM: Dynamically size memslot array based on number of used slots") Reported-by: Qian Cai <[email protected]> Cc: Peter Xu <[email protected]> Signed-off-by: Sean Christopherson <[email protected]> Message-Id: <[email protected]> Acked-by: Christian Borntraeger <[email protected]> Signed-off-by: Paolo Bonzini <[email protected]>
kawasaki · Mar 26, 2020 · 0774a96 · 0774a96
1 parent d536167
commit 0774a96
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diff --git 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
@@ -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
@@ -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;