sched/core: Fix oops in sched_show_task()

When CONFIG_THREAD_INFO_IN_TASK=y, it is possible that an exited thread
remains in the task list after its stack pointer was already set to NULL.

Therefore, thread_saved_pc() and stack_not_used() in sched_show_task()
will trigger NULL pointer dereference if an attempt to dump such thread's
traces (e.g. SysRq-t, khungtaskd) is made.

Since show_stack() in sched_show_task() calls try_get_task_stack() and
sched_show_task() is called from interrupt context, calling
try_get_task_stack() from sched_show_task() will be safe as well.

Signed-off-by: Tetsuo Handa <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Tetsuo Handa

kernel/sched/core.c

@@ -5192,6 +5192,8 @@ void sched_show_task(struct task_struct *p)
 	int ppid;
 	unsigned long state = p->state;
 
+	if (!try_get_task_stack(p))
+		return;
 	if (state)
 		state = __ffs(state) + 1;
 	printk(KERN_INFO "%-15.15s %c", p->comm,
@@ -5221,6 +5223,7 @@ void sched_show_task(struct task_struct *p)
 
 	print_worker_info(KERN_INFO, p);
 	show_stack(p, NULL);
+	put_task_stack(p);
 }
 
 void show_state_filter(unsigned long state_filter)