[PATCH] KVM: x86/mmu: Don't put invalid SPs back on the list of active pages

[Sean Christopherson]

Delete a shadow page from the invalidation list instead of throwing it
back on the list of active pages when it's a root shadow page with
active users.  Invalid active root pages will be explicitly freed by
mmu_free_root_page() when the root_count hits zero, i.e. they don't need
to be put on the active list to avoid leakage.

Use sp->role.invalid to detect that a shadow page has already been
zapped, i.e. is not on a list.

WARN if an invalid page is encountered when zapping pages, as it should
now be impossible.

Signed-off-by: Sean Christopherson <sean.j.christopher...@intel.com>
---
 arch/x86/kvm/mmu/mmu.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index fdd05c233308..fa5bd3f987dd 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -2757,10 +2757,13 @@ static bool __kvm_mmu_prepare_zap_page(struct kvm *kvm,
        if (!sp->root_count) {
                /* Count self */
                (*nr_zapped)++;
-               list_move(&sp->link, invalid_list);
+               if (sp->role.invalid)
+                       list_add(&sp->link, invalid_list);
+               else
+                       list_move(&sp->link, invalid_list);
                kvm_mod_used_mmu_pages(kvm, -1);
        } else {
-               list_move(&sp->link, &kvm->arch.active_mmu_pages);
+               list_del(&sp->link);
 
                /*
                 * Obsolete pages cannot be used on any vCPUs, see the comment
@@ -5732,12 +5735,11 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
                        break;
 
                /*
-                * Skip invalid pages with a non-zero root count, zapping pages
-                * with a non-zero root count will never succeed, i.e. the page
-                * will get thrown back on active_mmu_pages and we'll get stuck
-                * in an infinite loop.
+                * Invalid pages should never land back on the list of active
+                * pages.  Skip the bogus page, otherwise we'll get stuck in an
+                * infinite loop if the page gets put back on the list (again).
                 */
-               if (sp->role.invalid && sp->root_count)
+               if (WARN_ON(sp->role.invalid))
                        continue;
 
                /*
@@ -6015,7 +6017,7 @@ void kvm_mmu_zap_all(struct kvm *kvm)
        spin_lock(&kvm->mmu_lock);
 restart:
        list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
-               if (sp->role.invalid && sp->root_count)
+               if (WARN_ON(sp->role.invalid))
                        continue;
                if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))
                        goto restart;
-- 
2.26.0