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configfs: pin the symlink target's dirent instead of chasing ->ci_dentry

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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: 7063fbf226 ("[PATCH] configfs: User-driven configuration filesystem")
Signed-off-by: Vasileios Almpanis <vasilisalmpanis@gmail.com>
Tested-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260730093435.195441-2-vasilisalmpanis@gmail.com
Signed-off-by: Breno Leitao <leitao@debian.org>
This commit is contained in:
Vasileios Almpanis 2026-07-30 11:30:24 +02:00 committed by Breno Leitao
parent df2908090c
commit a7c1290eef
1 changed files with 20 additions and 4 deletions
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24
fs/configfs/symlink.c
View File

@ -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: