[PATCH 1/4] KVM: s390: pv: Use VM_SPARSE area for guest variable storage area

[Heiko Carstens]

The guest variable storage area is allocated with vmalloc and then
donated to the ultravisor. Any kernel access to that area will result
in a secure storage access exception (aka fault).

This is a problem if such a memory area is read via /proc/kcore. This
causes an exception via vread_iter() and results in an unexpected short
read. Avoid this by allocating a custom VM_SPARSE area. If such an area
is read, vread_iter() returns zeroes for the entire area.

Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
---
 arch/s390/include/asm/uv.h |  2 ++
 arch/s390/kernel/uv.c      | 59 ++++++++++++++++++++++++++++++++++++++
 arch/s390/kvm/pv.c         |  6 ++--
 3 files changed, 64 insertions(+), 3 deletions(-)

diff --git a/arch/s390/include/asm/uv.h b/arch/s390/include/asm/uv.h
index d919e69662f5..153fed63adda 100644
--- a/arch/s390/include/asm/uv.h
+++ b/arch/s390/include/asm/uv.h
@@ -635,6 +635,8 @@ int s390_wiggle_split_folio(struct mm_struct *mm, struct folio *folio);
 int __make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb);
 int uv_convert_from_secure(unsigned long paddr);
 int uv_convert_from_secure_folio(struct folio *folio);
+void *uv_alloc_stor_var(unsigned long size);
+void uv_free_stor_var(void *stor_var);
 
 void setup_uv(void);
 
diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index a284f98d9716..98a7f988fc69 100644
--- a/arch/s390/kernel/uv.c
+++ b/arch/s390/kernel/uv.c
@@ -16,6 +16,7 @@
 #include <linux/swap.h>
 #include <linux/pagewalk.h>
 #include <linux/backing-dev.h>
+#include <linux/vmalloc.h>
 #include <asm/facility.h>
 #include <asm/sections.h>
 #include <asm/uv.h>
@@ -209,6 +210,64 @@ int uv_convert_from_secure_pte(pte_t pte)
 	return uv_convert_from_secure_folio(pfn_folio(pte_pfn(pte)));
 }
 
+static int uv_free_range_cb(pte_t *ptep, unsigned long addr, void *data)
+{
+	pte_t pte = ptep_get(ptep);
+
+	if (!pte_present(pte))
+		return 0;
+	__free_page(pte_page(pte));
+	return 0;
+}
+
+void uv_free_stor_var(void *stor_var)
+{
+	unsigned long addr, size;
+	struct vm_struct *area;
+
+	if (!stor_var)
+		return;
+	area = find_vm_area(stor_var);
+	if (WARN_ON_ONCE(!area || !(area->flags & VM_SPARSE)))
+		return;
+	size = get_vm_area_size(area);
+	addr = (unsigned long)area->addr;
+	apply_to_existing_page_range(&init_mm, addr, size, uv_free_range_cb, NULL);
+	free_vm_area(area);
+}
+EXPORT_SYMBOL_FOR_MODULES(uv_free_stor_var, "kvm");
+
+static int uv_alloc_range_cb(pte_t *ptep, unsigned long addr, void *data)
+{
+	struct page *page;
+	pte_t pte;
+
+	page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
+	if (!page)
+		return -ENOMEM;
+	pte = __pte(page_to_phys(page) | pgprot_val(PAGE_KERNEL));
+	set_pte(ptep, pte);
+	return 0;
+}
+
+void *uv_alloc_stor_var(unsigned long size)
+{
+	struct vm_struct *area;
+	unsigned long addr;
+
+	area = get_vm_area(size, VM_SPARSE);
+	if (!area)
+		return NULL;
+	addr = (unsigned long)area->addr;
+	if (apply_to_page_range(&init_mm, addr, size, uv_alloc_range_cb, NULL))
+		goto out;
+	return area->addr;
+out:
+	uv_free_stor_var(area->addr);
+	return NULL;
+}
+EXPORT_SYMBOL_FOR_MODULES(uv_alloc_stor_var, "kvm");
+
 /*
  * Calculate the expected ref_count for a folio that would otherwise have no
  * further pins. This was cribbed from similar functions in other places in
diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
index 1beacc841ca8..dc5ac29b4c31 100644
--- a/arch/s390/kvm/pv.c
+++ b/arch/s390/kvm/pv.c
@@ -337,7 +337,7 @@ int kvm_s390_pv_create_cpu(struct kvm_vcpu *vcpu, u16 *rc, u16 *rrc)
 /* only free resources when the destroy was successful */
 static void kvm_s390_pv_dealloc_vm(struct kvm *kvm)
 {
-	vfree(kvm->arch.pv.stor_var);
+	uv_free_stor_var(kvm->arch.pv.stor_var);
 	free_pages(kvm->arch.pv.stor_base,
 		   get_order(uv_info.guest_base_stor_len));
 	kvm_s390_clear_pv_state(kvm);
@@ -369,7 +369,7 @@ static int kvm_s390_pv_alloc_vm(struct kvm *kvm)
 	/* Allocate variable storage */
 	vlen = ALIGN(virt * ((npages * PAGE_SIZE) / HPAGE_SIZE), PAGE_SIZE);
 	vlen += uv_info.guest_virt_base_stor_len;
-	kvm->arch.pv.stor_var = vzalloc(vlen);
+	kvm->arch.pv.stor_var = uv_alloc_stor_var(vlen);
 	if (!kvm->arch.pv.stor_var)
 		goto out_err;
 	return 0;
@@ -414,7 +414,7 @@ static int kvm_s390_pv_dispose_one_leftover(struct kvm *kvm,
 	 */
 	free_pages(leftover->stor_base, get_order(uv_info.guest_base_stor_len));
 	free_pages(leftover->old_gmap_table, CRST_ALLOC_ORDER);
-	vfree(leftover->stor_var);
+	uv_free_stor_var(leftover->stor_var);
 done_fast:
 	atomic_dec(&kvm->mm->context.protected_count);
 	return 0;
-- 
2.53.0