[PATCH v2 13/13] sched_ext: Refuse cross-task select_cpu_from_kfunc calls

Tejun Heo
2026-04-25T00:19:43Z

select_cpu_from_kfunc() skipped pi_lock for @p when called from
ops.select_cpu() or another rq-locked SCX op, assuming the held lock
protects @p. scx_bpf_select_cpu_dfl() / __scx_bpf_select_cpu_and() accept an
arbitrary KF_RCU task_struct, so a caller in e.g. ops.select_cpu(p1) or
ops.enqueue(p1) can pass some other p2 - the held pi_lock / rq lock is p1's,
not p2's - and reading p2->cpus_ptr / nr_cpus_allowed races with
set_cpus_allowed_ptr() and migrate_disable_switch() on another CPU.

Abort the scheduler on cross-task calls in both branches: for
ops.select_cpu() use scx_kf_arg_task_ok() to verify @p is the wake-up
task recorded in current->scx.kf_tasks[] by SCX_CALL_OP_TASK_RET();
for other rq-locked SCX ops compare task_rq(p) against scx_locked_rq().

v2: Per Andrea Righi: switch the in_select_cpu cross-task check from
direct_dispatch_task comparison to scx_kf_arg_task_ok(). The former
spuriously rejects when ops.select_cpu() calls scx_bpf_dsq_insert()
first (mark_direct_dispatch() stamps direct_dispatch_task =
ERR_PTR(-ESRCH)), then calls scx_bpf_select_cpu_*() on the same task.

Fixes: 0022b328504d ("sched_ext: Decouple kfunc unlocked-context check from kf_mask")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andrea Righi <arighi@nvidia.com>
---
kernel/sched/ext_idle.c | 19 +++++++++++++++++--
1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
index c43d62d90e40..7468560a6d80 100644
--- a/kernel/sched/ext_idle.c
+++ b/kernel/sched/ext_idle.c
@@ -927,14 +927,24 @@ static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,
* Accessing p->cpus_ptr / p->nr_cpus_allowed needs either @p's rq
* lock or @p's pi_lock. Three cases:
*
- * - inside ops.select_cpu(): try_to_wake_up() holds @p's pi_lock.
+ * - inside ops.select_cpu(): try_to_wake_up() holds the wake-up
+ * task's pi_lock; the wake-up task is recorded in kf_tasks[0]
+ * by SCX_CALL_OP_TASK_RET().
* - other rq-locked SCX op: scx_locked_rq() points at the held rq.
* - truly unlocked (UNLOCKED ops, SYSCALL, non-SCX struct_ops):
* nothing held, take pi_lock ourselves.
+ *
+ * In the first two cases, BPF schedulers may pass an arbitrary task
+ * that the held lock doesn't cover. Refuse those.
*/
if (this_rq()->scx.in_select_cpu) {
+ if (!scx_kf_arg_task_ok(sch, p))
+ return -EINVAL;
lockdep_assert_held(&p->pi_lock);
- } else if (!scx_locked_rq()) {
+ } else if (scx_locked_rq()) {
+ if (task_rq(p) != scx_locked_rq())
+ goto cross_task;
+ } else {
raw_spin_lock_irqsave(&p->pi_lock, irq_flags);
we_locked = true;
}
@@ -960,6 +970,11 @@ static s32 select_cpu_from_kfunc(struct scx_sched *sch, struct task_struct *p,
raw_spin_unlock_irqrestore(&p->pi_lock, irq_flags);

return cpu;
+
+cross_task:
+ scx_error(sch, "select_cpu kfunc called cross-task on %s[%d]",
+ p->comm, p->pid);
+ return -EINVAL;
}

/**
--
2.53.0