[PATCH v6 01/17] KVM: s390: pv: leak the topmost page table when destroy fails

Claudio Imbrenda
2021-12-03T16:58:31Z

Each secure guest must have a unique ASCE (address space control
element); we must avoid that new guests use the same page for their
ASCE, to avoid errors.

Since the ASCE mostly consists of the address of the topmost page table
(plus some flags), we must not return that memory to the pool unless
the ASCE is no longer in use.

Only a successful Destroy Secure Configuration UVC will make the ASCE
reusable again.

If the Destroy Configuration UVC fails, the ASCE cannot be reused for a
secure guest (either for the ASCE or for other memory areas). To avoid
a collision, it must not be used again. This is a permanent error and
the page becomes in practice unusable, so we set it aside and leak it.
On failure we already leak other memory that belongs to the ultravisor
(i.e. the variable and base storage for a guest) and not leaking the
topmost page table was an oversight.

This error (and thus the leakage) should not happen unless the hardware
is broken or KVM has some unknown serious bug.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Fixes: 29b40f105ec8d55 ("KVM: s390: protvirt: Add initial vm and cpu lifecycle handling")
---
arch/s390/include/asm/gmap.h | 2 ++
arch/s390/kvm/pv.c | 9 +++--
arch/s390/mm/gmap.c | 65 ++++++++++++++++++++++++++++++++++++
3 files changed, 73 insertions(+), 3 deletions(-)

diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h
index 40264f60b0da..746e18bf8984 100644
--- a/arch/s390/include/asm/gmap.h
+++ b/arch/s390/include/asm/gmap.h
@@ -148,4 +148,6 @@ void gmap_sync_dirty_log_pmd(struct gmap *gmap, unsigned long dirty_bitmap[4],
unsigned long gaddr, unsigned long vmaddr);
int gmap_mark_unmergeable(void);
void s390_reset_acc(struct mm_struct *mm);
+void s390_remove_old_asce(struct gmap *gmap);
+int s390_replace_asce(struct gmap *gmap);
#endif /* _ASM_S390_GMAP_H */
diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
index 00d272d134c2..b906658ffc2e 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -168,10 +168,13 @@ 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 */
- if (!cc)
+ /* Intended memory leak on "impossible" error */
+ if (!cc) {
kvm_s390_pv_dealloc_vm(kvm);
- return cc ? -EIO : 0;
+ return 0;
+ }
+ s390_replace_asce(kvm->arch.gmap);
+ return -EIO;
}

int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index dfee0ebb2fac..5d6618016263 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2714,3 +2714,68 @@ void s390_reset_acc(struct mm_struct *mm)
mmput(mm);
}
EXPORT_SYMBOL_GPL(s390_reset_acc);
+
+/**
+ * s390_remove_old_asce - Remove the topmost level of page tables from the
+ * list of page tables of the gmap.
+ * @gmap the gmap whose table is to be removed
+ *
+ * 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.
+ */
+void s390_remove_old_asce(struct gmap *gmap)
+{
+ struct page *old;
+
+ old = virt_to_page(gmap->table);
+ spin_lock(&gmap->guest_table_lock);
+ list_del(&old->lru);
+ spin_unlock(&gmap->guest_table_lock);
+ /* in case the ASCE needs to be "removed" multiple times */
+ INIT_LIST_HEAD(&old->lru);
+}
+EXPORT_SYMBOL_GPL(s390_remove_old_asce);
+
+/**
+ * s390_replace_asce - Try to replace the current ASCE of a gmap with
+ * another equivalent one.
+ * @gmap the gmap
+ *
+ * If the allocation of the new top level page table fails, the ASCE is not
+ * replaced.
+ * In any case, the old ASCE is always removed from the list. Therefore the
+ * caller has to make sure to save a pointer to it beforehands, unless an
+ * intentional leak is intended.
+ */
+int s390_replace_asce(struct gmap *gmap)
+{
+ unsigned long asce;
+ struct page *page;
+ void *table;
+
+ s390_remove_old_asce(gmap);
+
+ page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER);
+ if (!page)
+ return -ENOMEM;
+ table = page_to_virt(page);
+ memcpy(table, gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT));
+
+ /*
+ * The caller has to deal with the old ASCE, but here we make sure
+ * the new one is properly added to the list of page tables, so that
+ * it will be freed when the VM is torn down.
+ */
+ spin_lock(&gmap->guest_table_lock);
+ list_add(&page->lru, &gmap->crst_list);
+ spin_unlock(&gmap->guest_table_lock);
+
+ asce = (gmap->asce & ~PAGE_MASK) | __pa(table);
+ WRITE_ONCE(gmap->asce, asce);
+ WRITE_ONCE(gmap->mm->context.gmap_asce, asce);
+ WRITE_ONCE(gmap->table, table);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(s390_replace_asce);
--
2.31.1