linux-kernel - Re: [PATCH V2] exit: trigger panic when global init has exited

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Date:   Fri, 12 Mar 2021 12:23:11 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Qianli Zhao <zhaoqianligood@...il.com>
Cc:     christian@...uner.io, axboe@...nel.dk, oleg@...hat.com,
        tglx@...utronix.de, pcc@...gle.com, linux-kernel@...r.kernel.org,
        zhaoqianli@...omi.com
Subject: Re: [PATCH V2] exit: trigger panic when global init has exited

Qianli Zhao <zhaoqianligood@...il.com> writes:

> From: Qianli Zhao <zhaoqianli@...omi.com>
>
> 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@...omi.com>

The changelog is much better thank you.

As Oleg pointer out we need to do something like the code below.

diff --git a/kernel/exit.c b/kernel/exit.c
index 04029e35e69a..bc676c06ef9a 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -785,15 +785,16 @@ void __noreturn do_exit(long code)
 		sync_mm_rss(tsk->mm);
 	acct_update_integrals(tsk);
 	group_dead = atomic_dec_and_test(&tsk->signal->live);
+	/*
+	 * If the 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);
+	}
 	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);

There is still a race that could lead to the BUG in zap_pid_ns_processes.
We still have a case where the last two threads of a process call
pthread_exit (aka do_exit not do_group_exit in the kernel).

Thread A                            Thread B
do_exit()                           do_exit()

 exit_signals()
   tsk->flags |= PF_EXITING;
 group_dead = false;
                                    exit_signals()
                                      tsk->flags |= PF_EXITING;
 exit_notify()
  forget_original_parent
    find_child_reaper
      reaper = find_alive_thread()
      zap_pid_ns_processes()
         BUG()
                                    group_dead = true;
                                    if (is_global_init())
                                    	panic("Attemted to kill init");

As we are guaranteed to see the panic with my change above I suggest
we augment it by simply removing the BUG in zap_pid_ns_processes.

Or maybe not if there is a better way to write the panic code.  I don't
think having pid namespaces compiled out is a particularly common case.
So whatever we can do to keep the code correct and reduce testing.

Eric