[PATCH v4 05/23] KVM: WARN and error out if memslot update encounters existing slot with no pages

[Sean Christopherson <seanjc@google.com>]

WARN and return -EIO if an existing memslot with npages=0 is encountered
when updating memslots, as KVM returns NULL if there is no memslot since
commit 0577d1abe704 ("KVM: Terminate memslot walks via used_slots".  Before
that, KVM pre-allocated all possible memslots and relied on npages==0 to
denote "no valid memslot".  The check on npages was deliberately left
behind to aid debug, but that was over six years ago.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/kvm_main.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 218456372b11..dcd28b45ef30 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2086,9 +2086,11 @@ static int kvm_set_memory_region(struct kvm *kvm,
 	 * and/or destroyed by kvm_set_memslot().
 	 */
 	old = id_to_memslot(slots, id);
+	if (WARN_ON_ONCE(old && !old->npages))
+		return -EIO;
 
 	if (!mem->memory_size) {
-		if (!old || !old->npages)
+		if (!old)
 			return -EINVAL;
 
 		if (WARN_ON_ONCE(kvm->nr_memslot_pages < old->npages))
@@ -2100,7 +2102,7 @@ static int kvm_set_memory_region(struct kvm *kvm,
 	base_gfn = (mem->guest_phys_addr >> PAGE_SHIFT);
 	npages = (mem->memory_size >> PAGE_SHIFT);
 
-	if (!old || !old->npages) {
+	if (!old) {
 		change = KVM_MR_CREATE;
 
 		/*
-- 
2.56.0.385.gac671c819a-goog