Re: Can AUDIT_LIST_RULES causes kthreadd-spam?

[Steve Grubb <sgrubb@redhat.com>]

On Wednesday, May 10, 2023 9:30:19 AM EDT Tetsuo Handa wrote:
> On 2023/05/10 21:12, Rinat Gadelshin wrote:
> >> Please try to find who is calling audit_send_reply_thread for many
> >> times.
> > 
> > I've rebuilt the kernel with 'dump stack()'.
> 
> Oops, I thought dump_stack() shows pid and comm name, but
> it is dump_stack_print_info() that shows pid and comm name.
> 
> > As far as I can see, it's the exit of `sendto` syscall.
> > It seems that the kernel just creates a new kthreadd for each sendto
> > syscall. But I think that I'm wrong and just missing something.
> 
> Yes, sendto() on netlink socket calls netlink_sendmsg().
> For some reason, audit_send_reply() is called for many times.
> audit_send_reply() is called by audit_receive_msg() for the following
> types.
> 
>   AUDIT_GET
>   AUDIT_SIGNAL_INFO
>   AUDIT_TTY_GET
>   AUDIT_GET_FEATURE

The audit userspace always adds NLM_F_ACK to any netlink communication to 
make sure it did not get discarded before it arrived. It has done this since 
before I started working on audit code.

-Steve

> Would you re-caputure with
> 
> -	dump_stack();
> +	pr_info("%s %s:%d type=%d\n", __func__, current->comm, current->pid,
> type);
> 
> ?
> 
> Regardless of the result of re-caputure, it seems there is no switch that
> can prevent audit_send_reply() from calling
> kthread_run(audit_send_reply_thread).
> 
> But since kthreadd runs with PID=2 and PPID=0, you might be able to use
> PID=2 and/or PPID=0 in your rules in order to let kernel audit subsystem
> ignore kthreadd. (I can't test because I haven't found how to reproduce
> audit_receive_msg() in my environment...)
> 
> # cat /proc/2/status
> Name:   kthreadd
> Umask:  0000
> State:  S (sleeping)
> Tgid:   2
> Ngid:   0
> Pid:    2
> PPid:   0
> 
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