KVM: Restrict non-existing slot state transitions

The API documentation states:

	When changing an existing slot, it may be moved in the guest
	physical memory space, or its flags may be modified.

An "existing slot" requires a non-zero npages (memory_size).  The only
transition we should therefore allow for a non-existing slot should be
to create the slot, which includes setting a non-zero memory_size.  We
currently allow calls to modify non-existing slots, which is pointless,
confusing, and possibly wrong.

With this we know that the invalidation path of __kvm_set_memory_region
is always for a delete or move and never for adding a zero size slot.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1cd693a76a51..3caf8162eb6b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -758,10 +758,15 @@ int __kvm_set_memory_region(struct kvm *kvm,
         new.npages = npages;
         new.flags = mem->flags;
 
-        /* Disallow changing a memory slot's size. */
+        /*
+         * Disallow changing a memory slot's size or changing anything about
+         * zero sized slots that doesn't involve making them non-zero.
+         */
         r = -EINVAL;
         if (npages && old.npages && npages != old.npages)
                 goto out_free;
+        if (!npages && !old.npages)
+                goto out_free;
 
         /* Check for overlaps */
         r = -EEXIST;
@@ -780,7 +785,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
         r = -ENOMEM;
 
         /* Allocate if a slot is being created */
-        if (npages && !old.npages) {
+        if (!old.npages) {
                 new.user_alloc = user_alloc;
                 new.userspace_addr = mem->userspace_addr;