[PATCH v2] sched: fix migration to invalid cpu in __set_cpus_allowed_ptr

[KeMeng Shi]

Oops occur when running qemu on arm64:
 Unable to handle kernel paging request at virtual address ffff000008effe40
 Internal error: Oops: 96000007 [#1] SMP
 Process migration/0 (pid: 12, stack limit = 0x00000000084e3736)
 pstate: 20000085 (nzCv daIf -PAN -UAO)
 pc : __ll_sc___cmpxchg_case_acq_4+0x4/0x20
 lr : move_queued_task.isra.21+0x124/0x298
 ...
 Call trace:
  __ll_sc___cmpxchg_case_acq_4+0x4/0x20
  __migrate_task+0xc8/0xe0
  migration_cpu_stop+0x170/0x180
  cpu_stopper_thread+0xec/0x178
  smpboot_thread_fn+0x1ac/0x1e8
  kthread+0x134/0x138
  ret_from_fork+0x10/0x18

__set_cpus_allowed_ptr will choose an active dest_cpu in affinity mask to
migrage the process if process is not currently running on any one of the
CPUs specified in affinity mask. __set_cpus_allowed_ptr will choose an
invalid dest_cpu (dest_cpu >= nr_cpu_ids, 1024 in my virtual machine) if 
CPUS in an affinity mask are deactived by cpu_down after cpumask_intersects
check. cpumask_test_cpu of dest_cpu afterwards is overflow and may pass if
corresponding bit is coincidentally set. As a consequence, kernel will
access an invalid rq address associate with the invalid cpu in
migration_cpu_stop->__migrate_task->move_queued_task and the Oops occurs.

Process as follows may trigger the Oops:
1) A process repeatedly binds itself to cpu0 and cpu1 in turn by calling
sched_setaffinity.
2) A shell script repeatedly "echo 0 > /sys/devices/system/cpu/cpu1/online"
and "echo 1 > /sys/devices/system/cpu/cpu1/online" in turn.
3) Oops appears if the invalid cpu is set in memory after tested cpumask.

Signed-off-by: KeMeng Shi <shikemeng@xxxxxxxxxx>
Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
---
Changes in v2:
-solve format problems in log

 kernel/sched/core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3c7b90bcbe4e..087f4ac30b60 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1656,7 +1656,8 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 	if (cpumask_equal(p->cpus_ptr, new_mask))
 		goto out;
 
-	if (!cpumask_intersects(new_mask, cpu_valid_mask)) {
+	dest_cpu = cpumask_any_and(cpu_valid_mask, new_mask);
+	if (dest_cpu >= nr_cpu_ids) {
 		ret = -EINVAL;
 		goto out;
 	}
@@ -1677,7 +1678,6 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
 	if (cpumask_test_cpu(task_cpu(p), new_mask))
 		goto out;
 
-	dest_cpu = cpumask_any_and(cpu_valid_mask, new_mask);
 	if (task_running(rq, p) || p->state == TASK_WAKING) {
 		struct migration_arg arg = { p, dest_cpu };
 		/* Need help from migration thread: drop lock and wait. */
-- 
2.19.1