CVE-2026-64239: mm/damon/sysfs-schemes: delete tried region in regions_rmdirs()

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-64239 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.2 with commit 9277d0367ba18ef4bb98bafb1209e715844cdf7e and fixed in 6.6.143 with commit c0e37017a452addec873865c94cf7a665663a9b2
	Issue introduced in 6.2 with commit 9277d0367ba18ef4bb98bafb1209e715844cdf7e and fixed in 6.12.93 with commit a5fa42214de55e43d165144727ce9facb9fc6b08
	Issue introduced in 6.2 with commit 9277d0367ba18ef4bb98bafb1209e715844cdf7e and fixed in 6.18.35 with commit 0ba6c05156d9ff9fc6ca22b7690e2eec9eca66f7
	Issue introduced in 6.2 with commit 9277d0367ba18ef4bb98bafb1209e715844cdf7e and fixed in 7.0.12 with commit 2c33177023c92e76806c535ddbffaa3d3fc37777
	Issue introduced in 6.2 with commit 9277d0367ba18ef4bb98bafb1209e715844cdf7e and fixed in 7.1 with commit 441f92f7d386b85bad16de49db95a307cba048a2

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64239
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	mm/damon/sysfs-schemes.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/c0e37017a452addec873865c94cf7a665663a9b2
	https://git.kernel.org/stable/c/a5fa42214de55e43d165144727ce9facb9fc6b08
	https://git.kernel.org/stable/c/0ba6c05156d9ff9fc6ca22b7690e2eec9eca66f7
	https://git.kernel.org/stable/c/2c33177023c92e76806c535ddbffaa3d3fc37777
	https://git.kernel.org/stable/c/441f92f7d386b85bad16de49db95a307cba048a2