[PATCH] sched/core: Fix error pointer dereference

[Ethan Tidmore <ethantidmore06@gmail.com>]

The function idle_thread_get() can return an error pointer and is not
checked for one. Add check for error pointer.

Detected by Smatch:
kernel/cpu.c:911 finish_cpu() error:
'idle' dereferencing possible ERR_PTR()

Fixes: bf2c59fce4074 ("sched/core: Fix illegal RCU from offline CPUs")
Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
---
 kernel/cpu.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bc4f7a9ba64e..30af888d1bc1 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -908,7 +908,12 @@ static int bringup_cpu(unsigned int cpu)
 static int finish_cpu(unsigned int cpu)
 {
 	struct task_struct *idle = idle_thread_get(cpu);
-	struct mm_struct *mm = idle->active_mm;
+	struct mm_struct *mm;
+
+	if (IS_ERR(idle))
+		return PTR_ERR(idle);
+
+	mm = idle->active_mm;
 
 	/*
 	 * sched_force_init_mm() ensured the use of &init_mm,
-- 
2.53.0