KVM: MMU: handle n_free_mmu_pages > n_alloc_mmu_pages in kvm_mmu_change_mmu_pages

kvm_mmu_change_mmu_pages mishandles the case where n_alloc_mmu_pages is
smaller then n_free_mmu_pages, by not checking if the result of
the subtraction is negative.

Its a valid condition which can happen if a large number of pages has
been recently freed.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 8 insertions, 7 deletions

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 7030b5f911bf..49a10d008300 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1407,24 +1407,25 @@ static int kvm_mmu_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp)
  */
 void kvm_mmu_change_mmu_pages(struct kvm *kvm, unsigned int kvm_nr_mmu_pages)
 {
+        int used_pages;
+
+        used_pages = kvm->arch.n_alloc_mmu_pages - kvm->arch.n_free_mmu_pages;
+        used_pages = max(0, used_pages);
+
         /*
          * If we set the number of mmu pages to be smaller be than the
          * number of actived pages , we must to free some mmu pages before we
          * change the value
          */
 
-        if ((kvm->arch.n_alloc_mmu_pages - kvm->arch.n_free_mmu_pages) >
-            kvm_nr_mmu_pages) {
-                int n_used_mmu_pages = kvm->arch.n_alloc_mmu_pages
-                                       - kvm->arch.n_free_mmu_pages;
-
-                while (n_used_mmu_pages > kvm_nr_mmu_pages) {
+        if (used_pages > kvm_nr_mmu_pages) {
+                while (used_pages > kvm_nr_mmu_pages) {
                         struct kvm_mmu_page *page;
 
                         page = container_of(kvm->arch.active_mmu_pages.prev,
                                             struct kvm_mmu_page, link);
                         kvm_mmu_zap_page(kvm, page);
-                        n_used_mmu_pages--;
+                        used_pages--;
                 }
                 kvm->arch.n_free_mmu_pages = 0;
         }