[PATCH v2 02/13] KVM: s390: pv: leak the ASCE page when destroy fails

Claudio Imbrenda
2021-07-28T14:26:54Z

When a protected VM is created, the topmost level of page tables of its
ASCE is marked by the Ultravisor; any attempt to use that memory for
protected virtualization will result in failure.

Only a successful Destroy Configuration UVC will remove the marking.

When the Destroy Configuration UVC fails, the topmost level of page
tables of the VM does not get its marking cleared; to avoid issues it
must not be used again.

Since the page becomes in practice unusable, we set it aside and leak it.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
---
arch/s390/kvm/pv.c | 53 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 52 insertions(+), 1 deletion(-)

diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
index e007df11a2fe..1ecdc1769ed9 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -155,6 +155,55 @@ static int kvm_s390_pv_alloc_vm(struct kvm *kvm)
return -ENOMEM;
}

+/*
+ * Remove the topmost level of page tables from the list of page tables of
+ * the gmap.
+ * This means that it will not be freed when the VM is torn down, and needs
+ * to be handled separately by the caller, unless an intentional leak is
+ * intended.
+ */
+static void kvm_s390_pv_remove_old_asce(struct kvm *kvm)
+{
+ struct page *old;
+
+ old = virt_to_page(kvm->arch.gmap->table);
+ list_del(&old->lru);
+ /* in case the ASCE needs to be "removed" multiple times */
+ INIT_LIST_HEAD(&old->lru);
+}
+
+/*
+ * Try to replace the current ASCE with another equivalent one.
+ * If the allocation of the new top level page table fails, the ASCE is not
+ * replaced.
+ * In any case, the old ASCE is removed from the list, therefore the caller
+ * has to make sure to save a pointer to it beforehands, unless an
+ * intentional leak is intended.
+ */
+static int kvm_s390_pv_replace_asce(struct kvm *kvm)
+{
+ unsigned long asce;
+ struct page *page;
+ void *table;
+
+ kvm_s390_pv_remove_old_asce(kvm);
+
+ page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER);
+ if (!page)
+ return -ENOMEM;
+ list_add(&page->lru, &kvm->arch.gmap->crst_list);
+
+ table = page_to_virt(page);
+ memcpy(table, kvm->arch.gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT));
+
+ asce = (kvm->arch.gmap->asce & ~PAGE_MASK) | __pa(table);
+ WRITE_ONCE(kvm->arch.gmap->asce, asce);
+ WRITE_ONCE(kvm->mm->context.gmap_asce, asce);
+ WRITE_ONCE(kvm->arch.gmap->table, table);
+
+ return 0;
+}
+
/* this should not fail, but if it does, we must not free the donated memory */
int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
{
@@ -169,9 +218,11 @@ int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
atomic_set(&kvm->mm->context.is_protected, 0);
KVM_UV_EVENT(kvm, 3, "PROTVIRT DESTROY VM: rc %x rrc %x", *rc, *rrc);
WARN_ONCE(cc, "protvirt destroy vm failed rc %x rrc %x", *rc, *rrc);
- /* Inteded memory leak on "impossible" error */
+ /* Intended memory leak on "impossible" error */
if (!cc)
kvm_s390_pv_dealloc_vm(kvm);
+ else
+ kvm_s390_pv_replace_asce(kvm);
return cc ? -EIO : 0;
}

--
2.31.1