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

[Tejun Heo <tj@kernel.org>]

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: check @p against
direct_dispatch_task (the task currently being selected) for
ops.select_cpu(), and task_rq(p) against scx_locked_rq() for other rq-locked
SCX ops.

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..ff4d1b97437d 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 (stashed in direct_dispatch_task;
+	 *    mark_direct_dispatch() invalidates it post-dispatch).
 	 *  - 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 (p != __this_cpu_read(direct_dispatch_task))
+			goto cross_task;
 		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