mm/shmem: take care of UFFDIO_COPY_MODE_WP

Pass wp_copy into shmem_mfill_atomic_pte() through the stack, then apply
the UFFD_WP bit properly when the UFFDIO_COPY on shmem is with
UFFDIO_COPY_MODE_WP.  wp_copy lands mfill_atomic_install_pte() finally.

Note: we must do pte_wrprotect() if !writable in
mfill_atomic_install_pte(), as mk_pte() could return a writable pte (e.g.,
when VM_SHARED on a shmem file).

Link: https://lkml.kernel.org/r/20220405014841.14185-1-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 18 insertions, 5 deletions

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 9cdaed939203..2b6ec3352147 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -78,10 +78,19 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	 * Always mark a PTE as write-protected when needed, regardless of
 	 * VM_WRITE, which the user might change.
 	 */
-	if (wp_copy)
+	if (wp_copy) {
 		_dst_pte = pte_mkuffd_wp(_dst_pte);
-	else if (writable)
+		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);
 
 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
 
@@ -96,7 +105,12 @@ int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	}
 
 	ret = -EEXIST;
-	if (!pte_none(*dst_pte))
+	/*
+	 * We allow to overwrite a pte marker: consider when both MISSING|WP
+	 * registered, we firstly wr-protect a none pte which has no page cache
+	 * page backing it, then access the page.
+	 */
+	if (!pte_none_mostly(*dst_pte))
 		goto out_unlock;
 
 	if (page_in_cache) {
@@ -480,11 +494,10 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 			err = mfill_zeropage_pte(dst_mm, dst_pmd,
 						 dst_vma, dst_addr);
 	} else {
-		VM_WARN_ON_ONCE(wp_copy);
 		err = shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
 					     dst_addr, src_addr,
 					     mode != MCOPY_ATOMIC_NORMAL,
-					     page);
+					     wp_copy, page);
 	}
 
 	return err;