linux-kernel - [PATCH v5 05/14] KVM: s390: pv: leak the topmost page table when destroy fails

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Date:   Mon, 20 Sep 2021 15:24:53 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     kvm@...r.kernel.org
Cc:     cohuck@...hat.com, borntraeger@...ibm.com, frankja@...ux.ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, david@...hat.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ulrich.Weigand@...ibm.com
Subject: [PATCH v5 05/14] KVM: s390: pv: leak the topmost page table when destroy fails

Each secure guest must have a unique address space control element and
we must avoid that new guests use the same ASCE, to avoid errors.
Since the ASCE mostly consists of the topmost page table address (and
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 should not happen unless the hardware is broken or KVM has
some unknown serious bug.

Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
---
 arch/s390/include/asm/gmap.h |  2 ++
 arch/s390/kvm/pv.c           |  4 ++-
 arch/s390/mm/gmap.c          | 55 ++++++++++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+), 1 deletion(-)

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..76b0d64ce8fa 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -168,9 +168,11 @@ 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
+		s390_replace_asce(kvm->arch.gmap);
 	return cc ? -EIO : 0;
 }
 
diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 9bb2c7512cd5..5a138f6220c4 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2706,3 +2706,58 @@ void s390_reset_acc(struct mm_struct *mm)
 	mmput(mm);
 }
 EXPORT_SYMBOL_GPL(s390_reset_acc);
+
+/*
+ * 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.
+ */
+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);
+
+/*
+ * 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.
+ */
+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));
+
+	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