[PATCH 6.18.y 2/2] sched_ext: Fix scx_flush_disable_work() UAF race

[Kuba Piecuch]

From: Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>

[ Upstream commit d99f7a32f09dccbe396187370ec1a74a31b73d7e ]

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@xxxxxxxxxx>
Signed-off-by: Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
[ Sync error_irq_work instead of disable_irq_work. ]
Signed-off-by: Kuba Piecuch <jpiecuch@xxxxxxxxxx>
---
 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 d2460a88fba3..78f15ddc0aa5 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -4217,8 +4217,13 @@ 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->error_irq_work);
-	kthread_flush_work(&sch->disable_work);
+	int kind;
+
+	do {
+		irq_work_sync(&sch->error_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)
-- 
2.56.0.385.gac671c819a-goog