[PATCH 6.18.y 2/2] sched_ext: Avoid UAF in scx_root_enable_workfn() init failure path

[Sasha Levin <sashal@kernel.org>]

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 9a415cc53711f2238e0f0ca8a6bcc796c003b127 ]

In scx_root_enable_workfn(), put_task_struct(p) is called before scx_error()
dereferences p->comm and p->pid. If the iterator's reference is the last
drop, the task is freed synchronously and the deref becomes a UAF.

Move put_task_struct() past scx_error().

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260511214031.AF5E9C2BCB0@smtp.kernel.org/
Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Cc: stable@vger.kernel.org # v6.12+
Signed-off-by: Tejun Heo <tj@kernel.org>
[ kept `scx_init_task()` call site instead of `__scx_init_task()`/`task_rq_lock` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/sched/ext.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 54621f6226886..5c1547df50130 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4814,10 +4814,10 @@ static void scx_enable_workfn(struct kthread_work *work)
 
 		ret = scx_init_task(p, task_group(p), false);
 		if (ret) {
-			put_task_struct(p);
 			scx_task_iter_stop(&sti);
 			scx_error(sch, "ops.init_task() failed (%d) for %s[%d]",
 				  ret, p->comm, p->pid);
+			put_task_struct(p);
 			goto err_disable_unlock_all;
 		}
 
-- 
2.53.0