[PATCH v2 sched_ext/for-7.1-fixes] sched_ext: Fix scx_flush_disable_work() UAF race

[PATCH v2 sched_ext/for-7.1-fixes] sched_ext: Fix scx_flush_disable_work() UAF race

[Cheng-Yang Chou]

scx_flush_disable_work() calls irq_work_sync() followed by
kthread_flush_work() to ensure that the disable kthread work has
fully completed before bpf_scx_unreg() frees the SCX scheduler.

However, a concurrent scx_vexit() (e.g., triggered by a watchdog stall)
creates a race window between scx_claim_exit() and irq_work_queue():

  CPU A (scx_vexit (watchdog))        CPU B (bpf_scx_unreg)
  ----                                ----
  scx_claim_exit()
    atomic_try_cmpxchg(NONE->kind)
  stack_trace_save()
  vscnprintf()
                                      scx_disable()
                                        scx_claim_exit() -> FAIL
                                      scx_flush_disable_work()
                                        irq_work_sync()      // no-op: not queued yet
                                        kthread_flush_work() // no-op: not queued yet
                                      kobject_put(&sch->kobj) -> free %sch
  irq_work_queue() -> UAF on %sch
  scx_disable_irq_workfn()
    kthread_queue_work() -> UAF

The root cause is that CPU B's scx_flush_disable_work() returns after
syncing an irq_work that has not yet been queued, while CPU A is still
executing the code between scx_claim_exit() and irq_work_queue().

Loop until exit_kind reaches SCX_EXIT_DONE or SCX_EXIT_NONE, draining
disable_irq_work and disable_work in each pass. This ensures that any
work queued after the previous check is caught, while also correctly
handling cases where no disable was triggered (e.g., the
scx_sub_enable_workfn() abort path).

Fixes: 510a27055446 ("sched_ext: sync disable_irq_work in bpf_scx_unreg()")
Reported-by: https://sashiko.dev/#/patchset/20260424100221.32407-1-icheng%40nvidia.com
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
---
Changes in v2:
- Fold early-return logic into do-while loop (Tejun Heo)
- Link to v1:
  https://lore.kernel.org/r/20260428045529.1670916-1-yphbchou0911@gmail.com/

Thanks for the feedback,
Cheng-Yang

 kernel/sched/ext.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index f7b1b16e81a5..80d34bafe59c 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -6086,8 +6086,13 @@ static void scx_disable(struct scx_sched *sch, enum scx_exit_kind kind)
  */
 static void scx_flush_disable_work(struct scx_sched *sch)
 {
-	irq_work_sync(&sch->disable_irq_work);
-	kthread_flush_work(&sch->disable_work);
+	int kind;
+
+	do {
+		irq_work_sync(&sch->disable_irq_work);
+		kthread_flush_work(&sch->disable_work);
+		kind = atomic_read(&sch->exit_kind);
+	} while (kind != SCX_EXIT_NONE && kind != SCX_EXIT_DONE);
 }
 
 static void dump_newline(struct seq_buf *s)
-- 
2.48.1

Re: [PATCH v2 sched_ext/for-7.1-fixes] sched_ext: Fix scx_flush_disable_work() UAF race

[Tejun Heo]

Hello,

> Cheng-Yang Chou (1):
>   sched_ext: Fix scx_flush_disable_work() UAF race

Applied to sched_ext/for-7.1-fixes.

Thanks.

--
tejun