FAILED: patch "[PATCH] mm/damon/sysfs-schemes: delete tried region in" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 441f92f7d386b85bad16de49db95a307cba048a2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026060426-crept-cursive-9df8@gregkh' --subject-prefix 'PATCH 6.6.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 441f92f7d386b85bad16de49db95a307cba048a2 Mon Sep 17 00:00:00 2001
From: SeongJae Park <[email protected]>
Date: Mon, 18 May 2026 08:25:58 -0700
Subject: [PATCH] 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.

index 04746cbb3327..a8014780edae 100644
@@ -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--;
 	}

@@ -2928,14 +2927,15 @@ void damos_sysfs_populate_region_dir(struct damon_sysfs_schemes *sysfs_schemes,
 	if (!region)
 		return;
 	region->sz_filter_passed = sz_filter_passed;
-	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);
+		return;
 	}
+	list_add_tail(&region->list, &sysfs_regions->regions_list);
+	sysfs_regions->nr_regions++;
 }
 
 int damon_sysfs_schemes_clear_regions(

From: SeongJae Park <[email protected]>

[ Upstream commit 441f92f7d386b85bad16de49db95a307cba048a2 ]

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.

---
 mm/damon/sysfs-schemes.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index e8bf79c0d344ec..62df3ff8de178e 100644
@@ -78,7 +78,6 @@ 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);
 }
 

@@ -151,7 +150,7 @@ 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--;
 	}

@@ -1772,14 +1771,15 @@ static int damon_sysfs_before_damos_apply(struct damon_ctx *ctx,
 	region = damon_sysfs_scheme_region_alloc(r);
 	if (!region)
 		return 0;
-	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",
 				damon_sysfs_schemes_region_idx++)) {
 		kobject_put(&region->kobj);
+		return 0;
 	}
+	list_add_tail(&region->list, &sysfs_regions->regions_list);
+	sysfs_regions->nr_regions++;
 	return 0;
 }
 

2.53.0