kvm: fix sorting of memslots with base_gfn == 0

Before commit 0e60b0799fed (kvm: change memslot sorting rule from size
to GFN, 2014-12-01), the memslots' sorting key was npages, meaning
that a valid memslot couldn't have its sorting key equal to zero.
On the other hand, a valid memslot can have base_gfn == 0, and invalid
memslots are identified by base_gfn == npages == 0.

Because of this, commit 0e60b0799fed broke the invariant that invalid
memslots are at the end of the mslots array.  When a memslot with
base_gfn == 0 was created, any invalid memslot before it were left
in place.

This can be fixed by changing the insertion to use a ">=" comparison
instead of "<=", but some care is needed to avoid breaking the case
of deleting a memslot; see the comment in update_memslots.

Thanks to Tiejun Chen for posting an initial patch for this bug.

Reported-by: Jamie Heilman <jamie@audible.transient.net>
Reported-by: Andy Lutomirski <luto@amacapital.net>
Tested-by: Jamie Heilman <jamie@audible.transient.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 5 deletions

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f5283438ee05..050974c051b5 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -687,11 +687,23 @@ static void update_memslots(struct kvm_memslots *slots,
                 slots->id_to_index[mslots[i].id] = i;
                 i++;
         }
-        while (i > 0 &&
-               new->base_gfn > mslots[i - 1].base_gfn) {
-                mslots[i] = mslots[i - 1];
-                slots->id_to_index[mslots[i].id] = i;
-                i--;
+
+        /*
+         * The ">=" is needed when creating a slot with base_gfn == 0,
+         * so that it moves before all those with base_gfn == npages == 0.
+         *
+         * On the other hand, if new->npages is zero, the above loop has
+         * already left i pointing to the beginning of the empty part of
+         * mslots, and the ">=" would move the hole backwards in this
+         * case---which is wrong.  So skip the loop when deleting a slot.
+         */
+        if (new->npages) {
+                while (i > 0 &&
+                       new->base_gfn >= mslots[i - 1].base_gfn) {
+                        mslots[i] = mslots[i - 1];
+                        slots->id_to_index[mslots[i].id] = i;
+                        i--;
+                }
         }
 
         mslots[i] = *new;