KVM: s390: pv: fix race when making a page secure

Holding the pte lock for the page that is being converted to secure is
needed to avoid races. A previous commit removed the locking, which
caused issues. Fix by locking the pte again.

Fixes: 5cbe24350b7d ("KVM: s390: move pv gmap functions into kvm")
Reported-by: David Hildenbrand <david@redhat.com>
Tested-by: David Hildenbrand <david@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
[david@redhat.com: replace use of get_locked_pte() with folio_walk_start()]
Link: https://lore.kernel.org/r/20250312184912.269414-2-imbrenda@linux.ibm.com
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-ID: <20250312184912.269414-2-imbrenda@linux.ibm.com>

2 files changed, 21 insertions, 107 deletions

diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
index 02adf151d4de..6d8944d1b4a0 100644
--- a/arch/s390/kvm/gmap.c
+++ b/arch/s390/kvm/gmap.c
@@ -23,116 +23,25 @@
 #include "gmap.h"
 
 /**
- * should_export_before_import - Determine whether an export is needed
- * before an import-like operation
- * @uvcb: the Ultravisor control block of the UVC to be performed
- * @mm: the mm of the process
- *
- * Returns whether an export is needed before every import-like operation.
- * This is needed for shared pages, which don't trigger a secure storage
- * exception when accessed from a different guest.
- *
- * Although considered as one, the Unpin Page UVC is not an actual import,
- * so it is not affected.
- *
- * No export is needed also when there is only one protected VM, because the
- * page cannot belong to the wrong VM in that case (there is no "other VM"
- * it can belong to).
- *
- * Return: true if an export is needed before every import, otherwise false.
- */
-static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_struct *mm)
-{
-	/*
-	 * The misc feature indicates, among other things, that importing a
-	 * shared page from a different protected VM will automatically also
-	 * transfer its ownership.
-	 */
-	if (uv_has_feature(BIT_UV_FEAT_MISC))
-		return false;
-	if (uvcb->cmd == UVC_CMD_UNPIN_PAGE_SHARED)
-		return false;
-	return atomic_read(&mm->context.protected_count) > 1;
-}
-
-static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
-{
-	struct folio *folio = page_folio(page);
-	int rc;
-
-	/*
-	 * Secure pages cannot be huge and userspace should not combine both.
-	 * In case userspace does it anyway this will result in an -EFAULT for
-	 * the unpack. The guest is thus never reaching secure mode.
-	 * If userspace plays dirty tricks and decides to map huge pages at a
-	 * later point in time, it will receive a segmentation fault or
-	 * KVM_RUN will return -EFAULT.
-	 */
-	if (folio_test_hugetlb(folio))
-		return -EFAULT;
-	if (folio_test_large(folio)) {
-		mmap_read_unlock(gmap->mm);
-		rc = kvm_s390_wiggle_split_folio(gmap->mm, folio, true);
-		mmap_read_lock(gmap->mm);
-		if (rc)
-			return rc;
-		folio = page_folio(page);
-	}
-
-	if (!folio_trylock(folio))
-		return -EAGAIN;
-	if (should_export_before_import(uvcb, gmap->mm))
-		uv_convert_from_secure(folio_to_phys(folio));
-	rc = make_folio_secure(folio, uvcb);
-	folio_unlock(folio);
-
-	/*
-	 * In theory a race is possible and the folio might have become
-	 * large again before the folio_trylock() above. In that case, no
-	 * action is performed and -EAGAIN is returned; the callers will
-	 * have to try again later.
-	 * In most cases this implies running the VM again, getting the same
-	 * exception again, and make another attempt in this function.
-	 * This is expected to happen extremely rarely.
-	 */
-	if (rc == -E2BIG)
-		return -EAGAIN;
-	/* The folio has too many references, try to shake some off */
-	if (rc == -EBUSY) {
-		mmap_read_unlock(gmap->mm);
-		kvm_s390_wiggle_split_folio(gmap->mm, folio, false);
-		mmap_read_lock(gmap->mm);
-		return -EAGAIN;
-	}
-
-	return rc;
-}
-
-/**
  * gmap_make_secure() - make one guest page secure
  * @gmap: the guest gmap
  * @gaddr: the guest address that needs to be made secure
  * @uvcb: the UVCB specifying which operation needs to be performed
  *
  * Context: needs to be called with kvm->srcu held.
- * Return: 0 on success, < 0 in case of error (see __gmap_make_secure()).
+ * Return: 0 on success, < 0 in case of error.
  */
 int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
 {
 	struct kvm *kvm = gmap->private;
-	struct page *page;
-	int rc = 0;
+	unsigned long vmaddr;
 
 	lockdep_assert_held(&kvm->srcu);
 
-	page = gfn_to_page(kvm, gpa_to_gfn(gaddr));
-	mmap_read_lock(gmap->mm);
-	if (page)
-		rc = __gmap_make_secure(gmap, page, uvcb);
-	kvm_release_page_clean(page);
-	mmap_read_unlock(gmap->mm);
-
-	return rc;
+	vmaddr = gfn_to_hva(kvm, gpa_to_gfn(gaddr));
+	if (kvm_is_error_hva(vmaddr))
+		return -EFAULT;
+	return make_hva_secure(gmap->mm, vmaddr, uvcb);
 }
 
 int gmap_convert_to_secure(struct gmap *gmap, unsigned long gaddr)
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index ebecb96bacce..020502af7dc9 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4952,6 +4952,7 @@ static int vcpu_post_run_handle_fault(struct kvm_vcpu *vcpu)
 {
 	unsigned int flags = 0;
 	unsigned long gaddr;
+	int rc;
 
 	gaddr = current->thread.gmap_teid.addr * PAGE_SIZE;
 	if (kvm_s390_cur_gmap_fault_is_write())
@@ -4961,16 +4962,6 @@ static int vcpu_post_run_handle_fault(struct kvm_vcpu *vcpu)
 	case 0:
 		vcpu->stat.exit_null++;
 		break;
-	case PGM_NON_SECURE_STORAGE_ACCESS:
-		kvm_s390_assert_primary_as(vcpu);
-		/*
-		 * This is normal operation; a page belonging to a protected
-		 * guest has not been imported yet. Try to import the page into
-		 * the protected guest.
-		 */
-		if (gmap_convert_to_secure(vcpu->arch.gmap, gaddr) == -EINVAL)
-			send_sig(SIGSEGV, current, 0);
-		break;
 	case PGM_SECURE_STORAGE_ACCESS:
 	case PGM_SECURE_STORAGE_VIOLATION:
 		kvm_s390_assert_primary_as(vcpu);
@@ -4995,6 +4986,20 @@ static int vcpu_post_run_handle_fault(struct kvm_vcpu *vcpu)
 			send_sig(SIGSEGV, current, 0);
 		}
 		break;
+	case PGM_NON_SECURE_STORAGE_ACCESS:
+		kvm_s390_assert_primary_as(vcpu);
+		/*
+		 * This is normal operation; a page belonging to a protected
+		 * guest has not been imported yet. Try to import the page into
+		 * the protected guest.
+		 */
+		rc = gmap_convert_to_secure(vcpu->arch.gmap, gaddr);
+		if (rc == -EINVAL)
+			send_sig(SIGSEGV, current, 0);
+		if (rc != -ENXIO)
+			break;
+		flags = FAULT_FLAG_WRITE;
+		fallthrough;
 	case PGM_PROTECTION:
 	case PGM_SEGMENT_TRANSLATION:
 	case PGM_PAGE_TRANSLATION: