[PATCH v3] sched/fair: Clear ->h_load_next when unregistering cgroup

[Peng Wang]

An invalid pointer dereference bug was reported on arm64 cpu, and has
not yet been seen on x86. A partial oops looks like:

 Call trace:
  update_cfs_rq_h_load+0x80/0xb0
  wake_affine+0x158/0x168
  select_task_rq_fair+0x364/0x3a8
  try_to_wake_up+0x154/0x648
  wake_up_q+0x68/0xd0
  futex_wake_op+0x280/0x4c8
  do_futex+0x198/0x1c0
  __arm64_sys_futex+0x11c/0x198

Link: https://lore.kernel.org/all/20251013071820.1531295-1-CruzZhao@xxxxxxxxxxxxxxxxx/

We found that the task_group corresponding to the problematic se
is not in the parent task_group’s children list, indicating that
h_load_next points to an invalid address. Consider the following
cgroup and task hierarchy:

         A
        / \
       /   \
      B     E
     / \    |
    /   \   t2
   C     D
   |     |
   t0    t1

Here follows a timing sequence that may be responsible for triggering
the problem:

CPU X                   CPU Y                   CPU Z
wakeup t0
set list A->B->C
traverse A->B->C
t0 exits
destroy C
                        wakeup t2
                        set list A->E           wakeup t1
                                                set list A->B->D
                        traverse A->B->C
                        panic

CPU Z sets ->h_load_next list to A->B->D, but due to arm64 weaker memory
ordering, Y may observe A->B before it sees B->D, then in this time window,
it can traverse A->B->C and reach an invalid se.

We can avoid stale pointer accesses by clearing ->h_load_next when
unregistering cgroup.

Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Fixes: 685207963be9 ("sched: Move h_load calculation to task_h_load()")
Cc: <stable@xxxxxxxxxxxxxxx>
Co-developed-by: Cruz Zhao <CruzZhao@xxxxxxxxxxxxxxxxx>
Signed-off-by: Cruz Zhao <CruzZhao@xxxxxxxxxxxxxxxxx>
Signed-off-by: Peng Wang <peng_wang@xxxxxxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cee1793e8277..32b466605925 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13418,6 +13418,8 @@ void unregister_fair_sched_group(struct task_group *tg)
 		struct rq *rq = cpu_rq(cpu);
 
 		if (se) {
+			struct cfs_rq *parent_cfs_rq = cfs_rq_of(se);
+
 			if (se->sched_delayed) {
 				guard(rq_lock_irqsave)(rq);
 				if (se->sched_delayed) {
@@ -13427,6 +13429,13 @@ void unregister_fair_sched_group(struct task_group *tg)
 				list_del_leaf_cfs_rq(cfs_rq);
 			}
 			remove_entity_load_avg(se);
+
+			/*
+			 * Clear parent's h_load_next if it points to the
+			 * sched_entity being freed to avoid stale pointer.
+			 */
+			if (READ_ONCE(parent_cfs_rq->h_load_next) == se)
+				WRITE_ONCE(parent_cfs_rq->h_load_next, NULL);
 		}
 
 		/*
-- 
2.27.0