mm/uffd: always wr-protect pte in pte|pmd_mkuffd_wp()

This patch is a cleanup to always wr-protect pte/pmd in mkuffd_wp paths.

The reasons I still think this patch is worthwhile, are:

  (1) It is a cleanup already; diffstat tells.

  (2) It just feels natural after I thought about this, if the pte is uffd
      protected, let's remove the write bit no matter what it was.

  (2) Since x86 is the only arch that supports uffd-wp, it also redefines
      pte|pmd_mkuffd_wp() in that it should always contain removals of
      write bits.  It means any future arch that want to implement uffd-wp
      should naturally follow this rule too.  It's good to make it a
      default, even if with vm_page_prot changes on VM_UFFD_WP.

  (3) It covers more than vm_page_prot.  So no chance of any potential
      future "accident" (like pte_mkdirty() sparc64 or loongarch, even
      though it just got its pte_mkdirty fixed <1 month ago).  It'll be
      fairly clear when reading the code too that we don't worry anything
      before a pte_mkuffd_wp() on uncertainty of the write bit.

We may call pte_wrprotect() one more time in some paths (e.g.  thp split),
but that should be fully local bitop instruction so the overhead should be
negligible.

Although this patch should logically also fix all the known issues on
uffd-wp too recently on page migration (not for numa hint recovery - that
may need another explcit pte_wrprotect), but this is not the plan for that
fix.  So no fixes, and stable doesn't need this.

Link: https://lkml.kernel.org/r/20221214201533.1774616-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ives van Hoorne <ives@codesandbox.io>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 16 deletions

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 0499907b6f1a..f8d31b82aceb 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -74,24 +74,10 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	_dst_pte = pte_mkdirty(_dst_pte);
 	if (page_in_cache && !vm_shared)
 		writable = false;
-
-	/*
-	 * Always mark a PTE as write-protected when needed, regardless of
-	 * VM_WRITE, which the user might change.
-	 */
-	if (wp_copy) {
-		_dst_pte = pte_mkuffd_wp(_dst_pte);
-		writable = false;
-	}
-
 	if (writable)
 		_dst_pte = pte_mkwrite(_dst_pte);
-	else
-		/*
-		 * We need this to make sure write bit removed; as mk_pte()
-		 * could return a pte with write bit set.
-		 */
-		_dst_pte = pte_wrprotect(_dst_pte);
+	if (wp_copy)
+		_dst_pte = pte_mkuffd_wp(_dst_pte);
 
 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);