configfs: fix lockless traversals of ->s_children

[ Upstream commit 9b9e8bb ]

Having the parent directory locked protects entries from removal
by another thread, but it does *not* protect cursors from being
moved around by lseek() - or freed, for that matter.

Fixes: 6f61076 ("configfs: Introduce configfs_dirent_lock")
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Al Viro

fs/configfs/dir.c

@@ -225,15 +225,16 @@ static int configfs_dirent_exists(struct configfs_dirent *parent_sd,
 {
 	struct configfs_dirent * sd;
 
+	spin_lock(&configfs_dirent_lock);
 	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
 		if (sd->s_element) {
-			const unsigned char *existing = configfs_get_name(sd);
-			if (strcmp(existing, new))
-				continue;
-			else
+			if (strcmp(configfs_get_name(sd), new) == 0) {
+				spin_unlock(&configfs_dirent_lock);
 				return -EEXIST;
+			}
 		}
 	}
+	spin_unlock(&configfs_dirent_lock);
 
 	return 0;
 }
@@ -551,11 +552,28 @@ static void configfs_detach_rollback(struct dentry *dentry)
 			configfs_detach_rollback(sd->s_dentry);
 }
 
+/*
+ * Find the next non-cursor.  configfs_dirent_lock held by caller.
+ */
+static struct configfs_dirent *next_dirent(struct configfs_dirent *parent,
+					   struct configfs_dirent *last)
+{
+	struct configfs_dirent *s;
+
+	s = list_prepare_entry(last, &parent->s_children, s_sibling);
+
+	list_for_each_entry_continue(s, &parent->s_children, s_sibling) {
+		if (s->s_element)
+			return s;
+	}
+	return NULL;
+}
+
 static void detach_attrs(struct config_item * item)
 {
 	struct dentry * dentry = dget(item->ci_dentry);
-	struct configfs_dirent * parent_sd;
-	struct configfs_dirent * sd, * tmp;
+	struct configfs_dirent *parent_sd;
+	struct configfs_dirent *sd, *next;
 
 	if (!dentry)
 		return;
@@ -564,15 +582,19 @@ static void detach_attrs(struct config_item * item)
 		 dentry->d_name.name);
 
 	parent_sd = dentry->d_fsdata;
-	list_for_each_entry_safe(sd, tmp, &parent_sd->s_children, s_sibling) {
-		if (!sd->s_element || !(sd->s_type & CONFIGFS_NOT_PINNED))
+
+	spin_lock(&configfs_dirent_lock);
+	for (sd = next_dirent(parent_sd, NULL); sd; sd = next) {
+		next = next_dirent(parent_sd, sd);
+		if (!(sd->s_type & CONFIGFS_NOT_PINNED))
 			continue;
-		spin_lock(&configfs_dirent_lock);
 		list_del_init(&sd->s_sibling);
 		spin_unlock(&configfs_dirent_lock);
 		configfs_drop_dentry(sd, dentry);
 		configfs_put(sd);
+		spin_lock(&configfs_dirent_lock);
 	}
+	spin_unlock(&configfs_dirent_lock);
 
 	/**
 	 * Drop reference from dget() on entrance.
@@ -621,18 +643,20 @@ static void detach_groups(struct config_group *group)
 	struct dentry * dentry = dget(group->cg_item.ci_dentry);
 	struct dentry *child;
 	struct configfs_dirent *parent_sd;
-	struct configfs_dirent *sd, *tmp;
+	struct configfs_dirent *sd, *next;
 
 	if (!dentry)
 		return;
 
 	parent_sd = dentry->d_fsdata;
-	list_for_each_entry_safe(sd, tmp, &parent_sd->s_children, s_sibling) {
-		if (!sd->s_element ||
-		    !(sd->s_type & CONFIGFS_USET_DEFAULT))
+	spin_lock(&configfs_dirent_lock);
+	for (sd = next_dirent(parent_sd, NULL); sd; sd = next) {
+		next = next_dirent(parent_sd, sd);
+		if (!(sd->s_type & CONFIGFS_USET_DEFAULT))
 			continue;
 
 		child = sd->s_dentry;
+		spin_unlock(&configfs_dirent_lock);
 
 		inode_lock(d_inode(child));
 
@@ -644,7 +668,9 @@ static void detach_groups(struct config_group *group)
 
 		d_delete(child);
 		dput(child);
+		spin_lock(&configfs_dirent_lock);
 	}
+	spin_unlock(&configfs_dirent_lock);
 
 	/**
 	 * Drop reference from dget() on entrance.
@@ -1096,13 +1122,15 @@ configfs_find_subsys_dentry(struct configfs_dirent *root_sd,
 	struct configfs_dirent *p;
 	struct configfs_dirent *ret = NULL;
 
+	spin_lock(&configfs_dirent_lock);
 	list_for_each_entry(p, &root_sd->s_children, s_sibling) {
 		if (p->s_type & CONFIGFS_DIR &&
 		    p->s_element == subsys_item) {
 			ret = p;
 			break;
 		}
 	}
+	spin_unlock(&configfs_dirent_lock);
 
 	return ret;
 }