FAILED: patch "[PATCH] sched_ext: Remove redundant css_put() in scx_cgroup_init()" failed to apply to 6.12-stable tree

The patch below does not apply to the 6.12-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-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 1336b579f6079fb8520be03624fcd9ba443c930b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026031630-character-subsector-36b1@gregkh' --subject-prefix 'PATCH 6.12.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 1336b579f6079fb8520be03624fcd9ba443c930b Mon Sep 17 00:00:00 2001
From: Cheng-Yang Chou <[email protected]>
Date: Tue, 3 Mar 2026 22:35:30 +0800
Subject: [PATCH] sched_ext: Remove redundant css_put() in scx_cgroup_init()

The iterator css_for_each_descendant_pre() walks the cgroup hierarchy under cgroup_lock(). It does not increment the reference counts on yielded css structs.

According to the cgroup documentation, css_put() should only be used to release a reference obtained via css_get() or css_tryget_online(). Since the iterator does not use either of these to acquire a reference, calling css_put() in the error path of scx_cgroup_init() causes a refcount underflow.

Remove the unbalanced css_put() to prevent a potential Use-After-Free (UAF) vulnerability.

index 2ba69b302368..eab6e09b6442 100644
@@ -3589,7 +3589,6 @@ static int scx_cgroup_init(struct scx_sched *sch)
 		ret = SCX_CALL_OP_RET(sch, SCX_KF_UNLOCKED, cgroup_init, NULL,
 				      css->cgroup, &args);
 		if (ret) {
-			css_put(css);
 			scx_error(sch, "ops.cgroup_init() failed (%d)", ret);
 			return ret;
 		}

The iterator css_for_each_descendant_pre() walks the cgroup hierarchy under cgroup_lock(). It does not increment the reference counts on yielded css structs.

According to the cgroup documentation, css_put() should only be used to release a reference obtained via css_get() or css_tryget_online(). Since the iterator does not use either of these to acquire a reference, calling css_put() in the error path of scx_cgroup_init() causes a refcount underflow.

Remove the unbalanced css_put() to prevent a potential Use-After-Free (UAF) vulnerability.

---
 kernel/sched/ext.c | 1 -
 1 file changed, 1 deletion(-)

index 7e79f39c7bcf..da64e242ddbf 100644
@@ -4319,7 +4319,6 @@ static int scx_cgroup_init(void)
 		ret = SCX_CALL_OP_RET(SCX_KF_UNLOCKED, cgroup_init,
 				      css->cgroup, &args);
 		if (ret) {
-			css_put(css);
 			scx_ops_error("ops.cgroup_init() failed (%d)", ret);
 			return ret;
 		}

2.48.1