[PATCH 1/2] configfs: pin the symlink target's dirent instead of chasing ->ci_dentry

[Vasileios Almpanis <vasilisalmpanis@gmail.com>]

create_link() reads the target's configfs_dirent from
item->ci_dentry->d_fsdata, relying on the item reference taken by
get_target().  That reference pins the item, not its dentry: the dentry is
pinned by DCACHE_PERSISTENT, which configfs_remove_dir() releases via
simple_rmdir() while the item is still alive.  A symlink racing with rmdir
of its target can therefore find ->ci_dentry freed and its dirent
released, triggering WARN_ON(!atomic_read(&sd->s_count)) in configfs_get().

Take the dirent in get_target() as well, under ->d_lock and atomically
with the item reference, and pass it down to create_link().  A hashed
dentry has not been killed yet, so its ->d_fsdata reference keeps the
dirent alive there.

Cc: stable@vger.kernel.org
Fixes: 7063fbf22611 ("[PATCH] configfs: User-driven configuration filesystem")
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
---
 fs/configfs/symlink.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/fs/configfs/symlink.c b/fs/configfs/symlink.c
index 31eb28b27309..3b31c714400f 100644
--- a/fs/configfs/symlink.c
+++ b/fs/configfs/symlink.c
@@ -76,9 +76,9 @@ static int configfs_get_target_path(struct config_item *item,
 
 static int create_link(struct config_item *parent_item,
 		       struct config_item *item,
+		       struct configfs_dirent *target_sd,
 		       struct dentry *dentry)
 {
-	struct configfs_dirent *target_sd = item->ci_dentry->d_fsdata;
 	char *body;
 	int ret;
 
@@ -115,6 +115,7 @@ static int create_link(struct config_item *parent_item,
 
 
 static int get_target(const char *symname, struct config_item **target,
+		      struct configfs_dirent **target_sd,
 		      struct super_block *sb)
 {
 	struct path path __free(path_put) = {};
@@ -125,7 +126,20 @@ static int get_target(const char *symname, struct config_item **target,
 		return ret;
 	if (path.dentry->d_sb != sb)
 		return -EPERM;
-	*target = configfs_get_config_item(path.dentry);
+	/*
+	 * A hashed dentry guarantees that neither the item nor the dirent
+	 * have been released yet, as removals unhash before dropping.
+	 * Grab both references here. An item reference alone would not keep
+	 * ->ci_dentry alive.
+	 */
+	spin_lock(&path.dentry->d_lock);
+	if (!d_unhashed(path.dentry)) {
+		struct configfs_dirent *sd = path.dentry->d_fsdata;
+
+		*target = config_item_get(sd->s_element);
+		*target_sd = configfs_get(sd);
+	}
+	spin_unlock(&path.dentry->d_lock);
 	if (!*target)
 		return -ENOENT;
 	return 0;
@@ -139,6 +153,7 @@ int configfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
 	struct configfs_dirent *sd;
 	struct config_item *parent_item;
 	struct config_item *target_item = NULL;
+	struct configfs_dirent *target_sd = NULL;
 	const struct config_item_type *type;
 
 	sd = dentry->d_parent->d_fsdata;
@@ -182,7 +197,7 @@ int configfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
 	 *  AV, a thoroughly annoyed bastard.
 	 */
 	inode_unlock(dir);
-	ret = get_target(symname, &target_item, dentry->d_sb);
+	ret = get_target(symname, &target_item, &target_sd, dentry->d_sb);
 	inode_lock(dir);
 	if (ret)
 		goto out_put;
@@ -196,13 +211,14 @@ int configfs_symlink(struct mnt_idmap *idmap, struct inode *dir,
 		ret = type->ct_item_ops->allow_link(parent_item, target_item);
 	if (!ret) {
 		mutex_lock(&configfs_symlink_mutex);
-		ret = create_link(parent_item, target_item, dentry);
+		ret = create_link(parent_item, target_item, target_sd, dentry);
 		mutex_unlock(&configfs_symlink_mutex);
 		if (ret && type->ct_item_ops->drop_link)
 			type->ct_item_ops->drop_link(parent_item,
 						     target_item);
 	}
 
+	configfs_put(target_sd);
 	config_item_put(target_item);
 
 out_put:
-- 
2.47.3