[RFC PATCH v2] sched: irq: cpu-hotplug race vs set_cpus_allowed_ptr()

[Sebastian Andrzej Siewior]

syzbot reported a race of assigning a CPU affinity during CPU hotplug
operation.
After the affinity of an IRQ has been changed, the IRQ-core sets
IRQTF_AFFINITY and wakes the relevant interrupt threads which need to
adjust their affinity mask. The thread will then invoke
set_cpus_allowed_ptr() to update the mask. Based on the new mask a CPU
is chosen on which the thread should run.

It is verified that this CPU is online however there is no guarantee
that this CPU remains online while affine_move_task() is moving _this_
task. Since the current task requests the migration the stopper/
migration_cpu_stop() is involved at which point the task pauses for a
while. If the CPU goes offline (or is no longer cpu_online_mask) then
__migrate_task() (due to is_cpu_allowed() reject) will return the rq of
the CPU on which the task is currently running (which does not match
it's task_struct::cpus_mask).

The aftermath:
The IRQ-thread's CPU and task's cpus_mask do not match. A
migrate_disable()-> schedule() will change task_struct::cpus_ptr
ensuring that the following migrate_enable() will update the task to the
requested affinity mask. At this point, affine_move_task() expects
task_struct::migration_pending set but it is NULL because no one
requested an affinity change while the task was in migrate-disable
section.

I see two ways of fixing this:
- Holding the cpus_read_lock while set_cpus_allowed_ptr() is invoked.
  This ensure that the CPU remains in the cpu_online_mask while the
  migration task moving the task over. Should the mask be already
  invalid, then it is rejected otherwise the operation completes.

  sashiko complains about deadlock of cpus_write_lock() in the CPU down
  path and calling irq_migrate_all_off_this_cpu() vs
  irq_thread_check_affinity() doing cpus_read_lock().
  As far as I see, there is no deadlock because the mask is updated, the
  tread is woken, the tread blocks with cpus_read_lock() until the CPU is
  down.
  There is nothing that waits for the interrupt thread to complete,
  there is no synchronize_irq() invocation which would be required for
  the deadlock to happen.

- Should __migrate_task() fail to return the requested rq make sure its
  CPU is part task_struct::cpus_mask. This ensures that the current CPU
  is still part of mask avoiding a possible push by migrate_enable().
  sched_class::set_cpus_allowed did not see this mask. It might be a bit
  inconsistent and feels a bit like select_fallback_rq() without the
  printk.

  Sashiko complains:
| If __migrate_task() fails (e.g., due to a concurrent CPU offline event), this
| fallback code forcibly modifies p->cpus_mask with cpumask_set_cpu(). This
| skips the required p->sched_class->set_cpus_allowed() callback and fails to
| update p->nr_cpus_allowed.
| Can this bypass global EDF real-time invariants for SCHED_DEADLINE (as
| set_cpus_allowed_dl() is skipped), and allow unprivileged tasks to escape
| affinity boundaries, giving them access to CPUs outside their cpusets?

  We did do_set_cpus_allowed() for CPU X and can't migrate there now. We
  also don't know if the invocation was successful.
  
  Looking at this, it is not possible acquire the CPU-hotplug lock here
  and not every caller holds it. This seems to be limited to
  migrate-disable. Should this happen without the lock, then during the
  wake-up the scheduler will just print "process … no longer affine to
  cpu" pick a CPU and be done with it.
  Maybe set the bit followed by do_set_cpus_allowed()?

Both changes are implemented to illustrate, one is enough.
I can reproduce this back on v6.8, therefore I assume we have this since
day #1 of migrate-disable.

Reported-by: syzbot+6835d5c11145e4f77057@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/6a9919ac.94649fcc.25487e.0005.GAE@xxxxxxxxxx/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---

v1…v2: https://lore.kernel.org/all/20260907085825.f-CZ1Q5y@xxxxxxxxxxxxx/
  - Repost with collected notes.

 kernel/irq/manage.c | 6 ++++--
 kernel/sched/core.c | 2 ++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 57eff26fa646a..75b6798c1d97c 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -8,6 +8,7 @@
 
 #define pr_fmt(fmt) "genirq: " fmt
 
+#include <linux/cpuhplock.h>
 #include <linux/irq.h>
 #include <linux/kthread.h>
 #include <linux/module.h>
@@ -1044,8 +1045,9 @@ static void irq_thread_check_affinity(struct irq_desc *desc, struct irqaction *a
 		m = irq_data_get_effective_affinity_mask(&desc->irq_data);
 		cpumask_copy(mask, m);
 	}
-
-	set_cpus_allowed_ptr(current, mask);
+	scoped_guard(cpus_read_lock) {
+		set_cpus_allowed_ptr(current, mask);
+	}
 	free_cpumask_var(mask);
 }
 #else
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 1fe40de6ebe3c..5896be82e6ddc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2674,6 +2674,8 @@ static int migration_cpu_stop(void *data)
 		if (task_on_rq_queued(p)) {
 			update_rq_clock(rq);
 			rq = __migrate_task(rq, &rf, p, arg->dest_cpu);
+			if (rq != cpu_rq(arg->dest_cpu))
+				cpumask_set_cpu(rq->cpu, &p->cpus_mask);
 		} else {
 			p->wake_cpu = arg->dest_cpu;
 		}
-- 
2.55.0