[RFC PATCH] mm/damon/sysfs-schemes: delete tried region in regions_rmdirs()

[RFC PATCH] mm/damon/sysfs-schemes: delete tried region in regions_rmdirs()

[SeongJae Park]

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.

The issue was discovered [1] by Sashiko.

[1] https://lore.kernel.org/20260513011920.119183-1-sj@kernel.org

Fixes: 9277d0367ba1 ("mm/damon/sysfs-schemes: implement scheme region directory")
Cc: <stable@vger.kernel.org> # 6.2.x
Signed-off-by: SeongJae Park <sj@kernel.org>
---
 mm/damon/sysfs-schemes.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
index 5d966ac864193..52627ce3a1a62 100644
--- a/mm/damon/sysfs-schemes.c
+++ b/mm/damon/sysfs-schemes.c
@@ -88,7 +88,6 @@ static void damon_sysfs_scheme_region_release(struct kobject *kobj)
 	struct damon_sysfs_scheme_region *region = container_of(kobj,
 			struct damon_sysfs_scheme_region, kobj);
 
-	list_del(&region->list);
 	kfree(region);
 }
 
@@ -164,7 +163,7 @@ static void damon_sysfs_scheme_regions_rm_dirs(
 	struct damon_sysfs_scheme_region *r, *next;
 
 	list_for_each_entry_safe(r, next, &regions->regions_list, list) {
-		/* release function deletes it from the list */
+		list_del(&r->list);
 		kobject_put(&r->kobj);
 		regions->nr_regions--;
 	}

base-commit: 950de73f0f8bb17fc322e8f9ec09a5fbb4a72ed8
-- 
2.47.3

Re: [RFC PATCH] mm/damon/sysfs-schemes: delete tried region in regions_rmdirs()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Moving `list_del()` out of the kobject release callback causes a Use-After-Free when `kobject_init_and_add()` fails in `damos_sysfs_populate_region_dir()`.
--

commit 97136b36d31f71cb6972114db84fe86abd95246f
Author: SeongJae Park <sj@kernel.org>
Subject: 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. Deletion of the container
region object from the linked list is now done inside the removal function
instead of the kobj release callback.

[1] https://lore.kernel.org/20260513011920.119183-1-sj@kernel.org

> diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> index 622c3799db87b..aefeb5d22fb7d 100644
> --- a/mm/damon/sysfs-schemes.c
> +++ b/mm/damon/sysfs-schemes.c
> @@ -88,7 +88,6 @@ static void damon_sysfs_scheme_region_release(struct kobject *kobj)
>  	struct damon_sysfs_scheme_region *region = container_of(kobj,
>  			struct damon_sysfs_scheme_region, kobj);
>  
> -	list_del(&region->list);
>  	kfree(region);
>  }

Does removing list_del() here cause a use-after-free if kobject_init_and_add()
fails in damos_sysfs_populate_region_dir()?

If kobject_init_and_add() fails, the error path drops the reference:

mm/damon/sysfs-schemes.c:damos_sysfs_populate_region_dir() {
    ...
	list_add_tail(&region->list, &sysfs_regions->regions_list);
	sysfs_regions->nr_regions++;
	if (kobject_init_and_add(&region->kobj,
				&damon_sysfs_scheme_region_ktype,
				&sysfs_regions->kobj, "%d",
				sysfs_regions->nr_regions++)) {
		kobject_put(&region->kobj);
	}
    ...
}

Because region is added to the list before kobject_init_and_add(), dropping
the reference will free the region, but it remains in regions_list since
the release callback no longer unlinks it.

Should we add an explicit list_del(&region->list) in the error path of
damos_sysfs_populate_region_dir() to prevent list corruption?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260516211436.1883-1-sj@kernel.org?part=1

Re: [RFC PATCH] mm/damon/sysfs-schemes: delete tried region in regions_rmdirs()

[SeongJae Park]

On Sat, 16 May 2026 21:39:58 +0000 sashiko-bot@kernel.org wrote:

> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Moving `list_del()` out of the kobject release callback causes a Use-After-Free when `kobject_init_and_add()` fails in `damos_sysfs_populate_region_dir()`.
> --
> 
> commit 97136b36d31f71cb6972114db84fe86abd95246f
> Author: SeongJae Park <sj@kernel.org>
> Subject: 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. Deletion of the container
> region object from the linked list is now done inside the removal function
> instead of the kobj release callback.
> 
> [1] https://lore.kernel.org/20260513011920.119183-1-sj@kernel.org
> 
> > diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> > index 622c3799db87b..aefeb5d22fb7d 100644
> > --- a/mm/damon/sysfs-schemes.c
> > +++ b/mm/damon/sysfs-schemes.c
> > @@ -88,7 +88,6 @@ static void damon_sysfs_scheme_region_release(struct kobject *kobj)
> >  	struct damon_sysfs_scheme_region *region = container_of(kobj,
> >  			struct damon_sysfs_scheme_region, kobj);
> >  
> > -	list_del(&region->list);
> >  	kfree(region);
> >  }
> 
> Does removing list_del() here cause a use-after-free if kobject_init_and_add()
> fails in damos_sysfs_populate_region_dir()?
> 
> If kobject_init_and_add() fails, the error path drops the reference:
> 
> mm/damon/sysfs-schemes.c:damos_sysfs_populate_region_dir() {
>     ...
> 	list_add_tail(&region->list, &sysfs_regions->regions_list);
> 	sysfs_regions->nr_regions++;
> 	if (kobject_init_and_add(&region->kobj,
> 				&damon_sysfs_scheme_region_ktype,
> 				&sysfs_regions->kobj, "%d",
> 				sysfs_regions->nr_regions++)) {
> 		kobject_put(&region->kobj);
> 	}
>     ...
> }
> 
> Because region is added to the list before kobject_init_and_add(), dropping
> the reference will free the region, but it remains in regions_list since
> the release callback no longer unlinks it.

Good finding.  I wrote this patch on a tree having the data attributes
monitoring, which moved the list_add_tail() call to be done after
kobject_init_and_add() success, so missed this.

> 
> Should we add an explicit list_del(&region->list) in the error path of
> damos_sysfs_populate_region_dir() to prevent list corruption?

I will move list_add_tail() call to be done after kobject_init_and_add()
success.

> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260516211436.1883-1-sj@kernel.org?part=1


Thanks,
SJ