ovl: copy-up: don't unlock between lookup and link

Nothing prevents mischief on upper layer while we are busy copying up the
data.

Move the lookup right before the looked up dentry is actually used.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 01ad3eb8 ("ovl: concurrent copy up of regular files")
Cc: <stable@vger.kernel.org> # v4.11

fs/overlayfs/copy_up.c

@@ -330,15 +330,9 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
 		.link = link
 	};
 
-	upper = lookup_one_len(dentry->d_name.name, upperdir,
-			       dentry->d_name.len);
-	err = PTR_ERR(upper);
-	if (IS_ERR(upper))
-		goto out;
-
 	err = security_inode_copy_up(dentry, &new_creds);
 	if (err < 0)
-		goto out1;
+		goto out;
 
 	if (new_creds)
 		old_creds = override_creds(new_creds);
@@ -362,7 +356,7 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
 	}
 
 	if (err)
-		goto out2;
+		goto out;
 
 	if (S_ISREG(stat->mode)) {
 		struct path upperpath;
@@ -403,6 +397,14 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
 	if (err)
 		goto out_cleanup;
 
+	upper = lookup_one_len(dentry->d_name.name, upperdir,
+			       dentry->d_name.len);
+	if (IS_ERR(upper)) {
+		err = PTR_ERR(upper);
+		upper = NULL;
+		goto out_cleanup;
+	}
+
 	if (tmpfile)
 		err = ovl_do_link(temp, udir, upper, true);
 	else
@@ -416,17 +418,15 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
 
 	/* Restore timestamps on parent (best effort) */
 	ovl_set_timestamps(upperdir, pstat);
-out2:
+out:
 	dput(temp);
-out1:
 	dput(upper);
-out:
 	return err;
 
 out_cleanup:
 	if (!tmpfile)
 		ovl_cleanup(wdir, temp);
-	goto out2;
+	goto out;
 }
 
 /*