[PATCH V4] exit: trigger panic when global init has exited

[Qianli Zhao]

From: Qianli Zhao <zhaoqianli@xxxxxxxxxx>

When init sub-threads running on different CPUs exit at the same time,
zap_pid_ns_processe()->BUG() may be happened(timing is as below),move
panic() before set PF_EXITING to fix this problem.

In addition,if panic() after other sub-threads finish do_exit(),
some key variables (task->mm,task->nsproxy etc) of sub-thread will be lost,
which makes it difficult to parse coredump from fulldump,checking SIGNAL_GROUP_EXIT
to prevent init sub-threads exit.

[   24.705376] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00
[   24.705382] CPU: 4 PID: 552 Comm: init Tainted: G S         O    4.14.180-perf-g4483caa8ae80-dirty #1
[   24.705390] kernel BUG at include/linux/pid_namespace.h:98!

PID: 552   CPU: 4   COMMAND: "init"
PID: 1     CPU: 7   COMMAND: "init"
core4                           core7
...                             sys_exit_group()
                                do_group_exit()
                                   - sig->flags = SIGNAL_GROUP_EXIT
                                   - zap_other_threads()
                                do_exit() //PF_EXITING is set
ret_to_user()
do_notify_resume()
get_signal()
    - signal_group_exit
    - goto fatal;
do_group_exit()
do_exit() //PF_EXITING is set
    - panic("Attempted to kill init! exitcode=0x%08x\n")
                                exit_notify()
                                find_alive_thread() //no alive sub-threads
                                zap_pid_ns_processes()//CONFIG_PID_NS is not set
                                BUG()

Signed-off-by: Qianli Zhao <zhaoqianli@xxxxxxxxxx>
---
V4:
- Changelog update

V3:
- Use group_dead instead of thread_group_empty() to test single init exit.

V2:
- Changelog update
- Remove wrong useage of SIGNAL_UNKILLABLE. 
- Add thread_group_empty() test to handle single init thread exit

---
 kernel/exit.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 04029e3..f95f8dc 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -766,6 +766,17 @@ void __noreturn do_exit(long code)
 
 	validate_creds_for_do_exit(tsk);
 
+	group_dead = atomic_dec_and_test(&tsk->signal->live);
+	/*
+	 * If global init has exited,
+	 * panic immediately to get a useable coredump.
+	 */
+	if (unlikely(is_global_init(tsk) &&
+	    (group_dead || (tsk->signal->flags & SIGNAL_GROUP_EXIT)))) {
+			panic("Attempted to kill init! exitcode=0x%08x\n",
+				tsk->signal->group_exit_code ?: (int)code);
+	}
+
 	/*
 	 * We're taking recursive faults here in do_exit. Safest is to just
 	 * leave this task alone and wait for reboot.
@@ -784,16 +795,8 @@ void __noreturn do_exit(long code)
 	if (tsk->mm)
 		sync_mm_rss(tsk->mm);
 	acct_update_integrals(tsk);
-	group_dead = atomic_dec_and_test(&tsk->signal->live);
-	if (group_dead) {
-		/*
-		 * If the last thread of global init has exited, panic
-		 * immediately to get a useable coredump.
-		 */
-		if (unlikely(is_global_init(tsk)))
-			panic("Attempted to kill init! exitcode=0x%08x\n",
-				tsk->signal->group_exit_code ?: (int)code);
 
+	if (group_dead) {
 #ifdef CONFIG_POSIX_TIMERS
 		hrtimer_cancel(&tsk->signal->real_timer);
 		exit_itimers(tsk->signal);
-- 
1.9.1