[PATCH V2] exit: trigger panic when global init has exited

From: Qianli Zhao
Date: Thu Mar 11 2021 - 22:35:47 EST


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.
And every thread status is abnormal after exit(PF_EXITING set,task->mm=NULL etc),
which makes it difficult to parse coredump from fulldump normally.
In order to fix the above problem, when any one init has been set to SIGNAL_GROUP_EXIT,
trigger panic immediately, and prevent other init threads from continuing to 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>
---
V2:
- Changelog update
- Remove wrong useage of SIGNAL_UNKILLABLE.
- Add thread_group_empty() test to handle single init thread exit

---
kernel/exit.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 04029e3..9d2716b 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -767,6 +767,17 @@ void __noreturn do_exit(long code)
validate_creds_for_do_exit(tsk);

/*
+ * If global init has exited,
+ * panic immediately to get a useable coredump.
+ */
+ if (unlikely(is_global_init(tsk) &&
+ (thread_group_empty(tsk) ||
+ (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.
*/
@@ -786,14 +797,6 @@ void __noreturn do_exit(long code)
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);
-
#ifdef CONFIG_POSIX_TIMERS
hrtimer_cancel(&tsk->signal->real_timer);
exit_itimers(tsk->signal);
--
1.9.1