CVE-2026-64239

In the Linux kernel, the following vulnerability has been resolved:

mm/damon/sysfs-schemes: delete tried region in regions_rmdirs()

DAMON sysfs maintains the DAMOS tried region directory objects via a

linked list. When the user requests refresh of the directories, DAMON

sysfs removes all the region directories first, and then generate updated

regions directory on the empty space. The removal function

(damon_sysfs_scheme_regions_rm_dirs()) only puts the kobj objects.

Deletion of the container region object from the linked list is done

inside the kobj release callback function.

If somehow the callback invocation is delayed, the list will contain

regions list that gonna be freed. If the updated region directories

creation is started in this situation, the list can be corrupted and

use-after-free can happen.

Because the kobj objects are managed by only DAMON sysfs, the issue cannot

happen in normal situation. But, such delays can be made on kernels that

built with CONFIG_DEBUG_KOBJECT_RELEASE. On the kernel, the issue can

indeed be reproduced like below.

# damo start --damos_action stat

# cd /sys/kernel/mm/damon/admin/kdamonds/0/

# for i in {1..10}; do echo update_schemes_tried_regions > state; done

# dmesg | grep underflow

[ 89.296152] refcount_t: underflow; use-after-free.

Fix the issue by removing the region object from the list when

decrementing the reference count.

Also update damos_sysfs_populate_region_dir() to add the region object to

the list only after the kobject_init_and_add() is success, so that fail of

kobject_init_and_add() is not leaving the deallocated object on the list.

The issue was discovered [1] by Sashiko.