Re: [PATCH 2/2] configfs: unhash the dentry before dropping the item in rmdir

[Breno Leitao]

On Thu, Jul 30, 2026 at 11:30:25AM +0200, Vasileios Almpanis wrote:
> configfs_get_config_item() treats a hashed dentry as proof that
> sd->s_element is a live config_item.  configfs_rmdir() breaks that:
> simple_rmdir() leaves the dentry hashed, the last reference to the item is
> dropped right after, and the dentry is only unhashed by d_delete() once
> ->rmdir() has returned.  configfs_symlink() resolves its target holding no
> lock on it, so get_target() can land in that window:
> 
>   BUG: KASAN: slab-use-after-free in config_item_get+0x26/0x90
>    get_target fs/configfs/symlink.c:128 [inline]
>    configfs_symlink+0x4ab/0x1030 fs/configfs/symlink.c:185
> 
> Unhash in configfs_remove_dir(), while the item is still guaranteed to be
> there.  A reference obtained just before that stays harmless, as
> create_link() rechecks CONFIGFS_USET_DROPPING, already set by
> configfs_detach_prep().  Both configfs_unregister_subsystem() paths
> d_drop() after detaching, so this only makes rmdir match them.

Is the claim that "a reference obtained just before that stays harmless,
as create_link() rechecks CONFIGFS_USET_DROPPING" accurate?