[PATCH v2] umh: fix out of scope usage when the process is being killed

[Schspa Shi]

When the process is killed, wait_for_completion_state will return with
-ERESTARTSYS, and the completion variable in the stack will be unavailable,
even freed. If the user-mode thread is complete at the same time, there
will be a race to use a unavailable variable.

Please refer to the following scenarios.
            T1                                  T2
------------------------------------------------------------------
call_usermodehelper_exec
                                   call_usermodehelper_exec_async
                                   << do something >>
                                   umh_complete(sub_info);
                                   comp = xchg(&sub_info->complete, NULL);
                                   /* we got the completion */
                                   << context switch >>

    << Being killed >>
	retval = wait_for_completion_state(sub_info->complete, state);
	if (!retval)
		goto wait_done;

	if (wait & UMH_KILLABLE) {
		/* umh_complete() will see NULL and free sub_info */
		if (xchg(&sub_info->complete, NULL))
			goto unlock;
        << we can't got the completion, because T2 take it already >>
	}
	....
	return retval;
}

/**
 * the completion variable in stack is end of life cycle.
 * and maybe freed due to process is recycled.
 */
                                   -------- BUG here----------
                                   if (comp)
                                       complete(comp);

To fix it, we can add an additional wait_for_completion to ensure the
completion object is completely unused. And this is what
kthread_create_on_node does to handle this race.

Reported-by: syzbot+10d19d528d9755d9af22@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+70d5d5d83d03db2c813d@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+83cb0411d0fcf0a30fc1@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+c92c6a251d49ceceb625@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Schspa Shi <schspa@xxxxxxxxx>
---

v1->v2:
  - Use a new way to fix the race as kthread_create_on_node do.
  - Optimize comments and use more accurate words to describe the problem.

 kernel/umh.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/umh.c b/kernel/umh.c
index 850631518665..d8350a195c7f 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -452,6 +452,10 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 		/* umh_complete() will see NULL and free sub_info */
 		if (xchg(&sub_info->complete, NULL))
 			goto unlock;
+		/*
+		 * umh_complete will call complete() shortly.
+		 */
+		wait_for_completion(&done);
 	}
 
 wait_done:
-- 
2.37.3