ovl: make sure that ovl_create_real() returns a hashed dentry

e8bd877 ("ovl: fix possible double unlink") added a sanity
check of !d_unhashed(child) to try to verify that child dentry was not
unlinked while parent dir was unlocked.

This "was not unlink" check has a false positive result in the case of
casefolded parent dir, because in that case, ovl_create_temp() returns
an unhashed dentry after ovl_create_real() gets an unhashed dentry from
ovl_lookup_upper() and makes it positive.

To avoid returning unhashed dentry from ovl_create_temp(), let
ovl_create_real() lookup again after making the newdentry positive,
so it always returns a hashed positive dentry (or an error).

This fixes the error in ovl_parent_lock() in ovl_check_rename_whiteout()
after ovl_create_temp() and allows mount of overlayfs with casefolding
enabled layers.

Reported-by: André Almeida <andrealmeid@igalia.com>
Closes: https://lore.kernel.org/r/18704e8c-c734-43f3-bc7c-b8be345e1bf5@igalia.com/
Suggested-by: Neil Brown <neil@brown.name>
Reviewed-by: Neil Brown <neil@brown.name>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

Author: amir73il

fs/overlayfs/dir.c

@@ -212,12 +212,32 @@ struct dentry *ovl_create_real(struct ovl_fs *ofs, struct dentry *parent,
 			err = -EPERM;
 		}
 	}
-	if (!err && WARN_ON(!newdentry->d_inode)) {
+	if (err)
+		goto out;
+
+	if (WARN_ON(!newdentry->d_inode)) {
 		/*
 		 * Not quite sure if non-instantiated dentry is legal or not.
 		 * VFS doesn't seem to care so check and warn here.
 		 */
 		err = -EIO;
+	} else if (d_unhashed(newdentry)) {
+		struct dentry *d;
+		/*
+		 * Some filesystems (i.e. casefolded) may return an unhashed
+		 * negative dentry from the ovl_lookup_upper() call before
+		 * ovl_create_real().
+		 * In that case, lookup again after making the newdentry
+		 * positive, so ovl_create_upper() always returns a hashed
+		 * positive dentry.
+		 */
+		d = ovl_lookup_upper(ofs, newdentry->d_name.name, parent,
+				     newdentry->d_name.len);
+		dput(newdentry);
+		if (IS_ERR_OR_NULL(d))
+			err = d ? PTR_ERR(d) : -ENOENT;
+		else
+			return d;
 	}
 out:
 	if (err) {