[tip: sched/urgent] sched/fair: Clear ->h_load_next when unregistering a cgroup

["tip-bot2 for Peng Wang" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     f85fd15b0da76f08e2d5bb4179c54993f7e07853
Gitweb:        https://git.kernel.org/tip/f85fd15b0da76f08e2d5bb4179c54993f7e07853
Author:        Peng Wang <peng_wang@linux.alibaba.com>
AuthorDate:    Fri, 24 Oct 2025 15:23:16 +08:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 03 Dec 2025 17:44:01 +01:00

sched/fair: Clear ->h_load_next when unregistering a cgroup

An invalid pointer dereference bug was reported on ARM64 CPUs, 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

See: https://lore.kernel.org/all/20251013071820.1531295-1-CruzZhao@linux.alibaba.com/

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 weaker memory
ordering on ARM64, 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 a cgroup.

Fixes: 685207963be9 ("sched: Move h_load calculation to task_h_load()")
Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Co-developed-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
Signed-off-by: Peng Wang <peng_wang@linux.alibaba.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/bf93d41ff9f2da19ef2c1cfb505362e0b48c39de.1761290330.git.peng_wang@linux.alibaba.com
---
 kernel/sched/fair.c |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 769d7b7..00a32c9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -13687,6 +13687,8 @@ 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) {
@@ -13696,6 +13698,13 @@ 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);
 		}
 
 		/*