mm/userfaultfd: fail uffd-wp registration if not supported

We should fail uffd-wp registration immediately if the arch does not even
have CONFIG_HAVE_ARCH_USERFAULTFD_WP defined.  That'll block also relevant
ioctls on e.g.  UFFDIO_WRITEPROTECT because that'll check against
VM_UFFD_WP, which can only be applied with a success registration.

Remove the WP feature bit too for those archs when handling UFFDIO_API
ioctl.

Link: https://lkml.kernel.org/r/20210428225030.9708-5-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Joe Perches <joe@perches.com>
Cc: Kirill A. Shutemov <kirill@shutemov.name>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Shaohua Li <shli@fb.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Wang Qing <wangqing@vivo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -1304,8 +1304,12 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	vm_flags = 0;
 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
 		vm_flags |= VM_UFFD_MISSING;
-	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP)
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
+#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
+		goto out;
+#endif
 		vm_flags |= VM_UFFD_WP;
+	}
 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MINOR) {
 #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
 		goto out;
@@ -1942,6 +1946,9 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 	uffdio_api.features = UFFD_API_FEATURES;
 #ifndef CONFIG_HAVE_ARCH_USERFAULTFD_MINOR
 	uffdio_api.features &= ~UFFD_FEATURE_MINOR_HUGETLBFS;
+#endif
+#ifndef CONFIG_HAVE_ARCH_USERFAULTFD_WP
+	uffdio_api.features &= ~UFFD_FEATURE_PAGEFAULT_FLAG_WP;
 #endif
 	uffdio_api.ioctls = UFFD_API_IOCTLS;
 	ret = -EFAULT;