ovl: fix remove/copy-up race

ovl_remove_and_whiteout() needs to check if upper dentry exists or not
after having locked upper parent directory.

Previously we used a "type" value computed before locking the upper parent
directory, which is susceptible to racing with copy-up.

There's a similar check in ovl_check_empty_and_clear().  This one is not
actually racy, since copy-up doesn't change the "emptyness" property of a
directory.  Add a comment to this effect, and check the existence of upper
dentry locally to make the code cleaner.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/overlayfs/dir.c

@@ -284,8 +284,7 @@ static struct dentry *ovl_clear_empty(struct dentry *dentry,
 	return ERR_PTR(err);
 }
 
-static struct dentry *ovl_check_empty_and_clear(struct dentry *dentry,
-						enum ovl_path_type type)
+static struct dentry *ovl_check_empty_and_clear(struct dentry *dentry)
 {
 	int err;
 	struct dentry *ret = NULL;
@@ -294,8 +293,17 @@ static struct dentry *ovl_check_empty_and_clear(struct dentry *dentry,
 	err = ovl_check_empty_dir(dentry, &list);
 	if (err)
 		ret = ERR_PTR(err);
-	else if (type == OVL_PATH_MERGE)
+	else {
+		/*
+		 * If no upperdentry then skip clearing whiteouts.
+		 *
+		 * Can race with copy-up, since we don't hold the upperdir
+		 * mutex.  Doesn't matter, since copy-up can't create a
+		 * non-empty directory from an empty one.
+		 */
+		if (ovl_dentry_upper(dentry))
 			ret = ovl_clear_empty(dentry, &list);
+	}
 
 	ovl_cache_free(&list);
 
@@ -487,8 +495,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
 	return err;
 }
 
-static int ovl_remove_and_whiteout(struct dentry *dentry,
-				   enum ovl_path_type type, bool is_dir)
+static int ovl_remove_and_whiteout(struct dentry *dentry, bool is_dir)
 {
 	struct dentry *workdir = ovl_workdir(dentry);
 	struct inode *wdir = workdir->d_inode;
@@ -500,7 +507,7 @@ static int ovl_remove_and_whiteout(struct dentry *dentry,
 	int err;
 
 	if (is_dir) {
-		opaquedir = ovl_check_empty_and_clear(dentry, type);
+		opaquedir = ovl_check_empty_and_clear(dentry);
 		err = PTR_ERR(opaquedir);
 		if (IS_ERR(opaquedir))
 			goto out;
@@ -515,7 +522,8 @@ static int ovl_remove_and_whiteout(struct dentry *dentry,
 	if (IS_ERR(whiteout))
 		goto out_unlock;
 
-	if (type == OVL_PATH_LOWER) {
+	upper = ovl_dentry_upper(dentry);
+	if (!upper) {
 		upper = lookup_one_len(dentry->d_name.name, upperdir,
 				       dentry->d_name.len);
 		err = PTR_ERR(upper);
@@ -529,7 +537,6 @@ static int ovl_remove_and_whiteout(struct dentry *dentry,
 	} else {
 		int flags = 0;
 
-		upper = ovl_dentry_upper(dentry);
 		if (opaquedir)
 			upper = opaquedir;
 		err = -ESTALE;
@@ -648,7 +655,7 @@ static int ovl_do_remove(struct dentry *dentry, bool is_dir)
 		cap_raise(override_cred->cap_effective, CAP_CHOWN);
 		old_cred = override_creds(override_cred);
 
-		err = ovl_remove_and_whiteout(dentry, type, is_dir);
+		err = ovl_remove_and_whiteout(dentry, is_dir);
 
 		revert_creds(old_cred);
 		put_cred(override_cred);
@@ -781,7 +788,7 @@ static int ovl_rename2(struct inode *olddir, struct dentry *old,
 	}
 
 	if (overwrite && (new_type == OVL_PATH_LOWER || new_type == OVL_PATH_MERGE) && new_is_dir) {
-		opaquedir = ovl_check_empty_and_clear(new, new_type);
+		opaquedir = ovl_check_empty_and_clear(new);
 		err = PTR_ERR(opaquedir);
 		if (IS_ERR(opaquedir)) {
 			opaquedir = NULL;