userfaultfd: mark uffd_wp regardless of VM_WRITE flag

When a PTE is set by UFFD operations such as UFFDIO_COPY, the PTE is
currently only marked as write-protected if the VMA has VM_WRITE flag
set.  This seems incorrect or at least would be unexpected by the users.

Consider the following sequence of operations that are being performed
on a certain page:

	mprotect(PROT_READ)
	UFFDIO_COPY(UFFDIO_COPY_MODE_WP)
	mprotect(PROT_READ|PROT_WRITE)

At this point the user would expect to still get UFFD notification when
the page is accessed for write, but the user would not get one, since
the PTE was not marked as UFFD_WP during UFFDIO_COPY.

Fix it by always marking PTEs as UFFD_WP regardless on the
write-permission in the VMA flags.

Link: https://lkml.kernel.org/r/20220217211602.2769-1-namit@vmware.com
Fixes: 292924b2 ("userfaultfd: wp: apply _PAGE_UFFD_WP bit")
Signed-off-by: Nadav Amit <namit@vmware.com>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/userfaultfd.c

@@ -72,12 +72,15 @@ 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;
-	if (writable) {
+
+	/*
+	 * 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);
-		else
+	else if (writable)
 		_dst_pte = pte_mkwrite(_dst_pte);
-	}
 
 	dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);