Re: [RFC] audit: avoid soft lockup in audit_log_start()

From: Luiz Capitulino
Date: Mon Sep 09 2013 - 10:54:53 EST


On Mon, 09 Sep 2013 18:32:13 +0400
Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx> wrote:

> Luiz Capitulino wrote:
> > I'm getting the following soft lockup:
> >
> > CPU: 6 PID: 2278 Comm: killall5 Tainted: GF 3.11.0-rc7+ #1
> > Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> > 0000000000000099 ffff88011fd83de8 ffffffff815324df 0000000000002800
> > ffffffff817d48f9 ffff88011fd83e68 ffffffff8152e669 ffff88011fd83e68
> > ffffffff00000008 ffff88011fd83e78 ffff88011fd83e18 0000004081dac040
> > Call Trace:
> > <IRQ> [<ffffffff815324df>] dump_stack+0x46/0x58
> > [<ffffffff8152e669>] panic+0xbb/0x1c4
> > [<ffffffff810d03c3>] watchdog_timer_fn+0x163/0x170
> > [<ffffffff8106c691>] __run_hrtimer+0x81/0x1c0
> > [<ffffffff810d0260>] ? watchdog+0x30/0x30
> > [<ffffffff8106cea7>] hrtimer_interrupt+0x107/0x240
> > [<ffffffff8102f61b>] local_apic_timer_interrupt+0x3b/0x60
> > [<ffffffff81542465>] smp_apic_timer_interrupt+0x45/0x60
> > [<ffffffff8154124a>] apic_timer_interrupt+0x6a/0x70
> > <EOI> [<ffffffff810c2f5f>] ? audit_log_start+0xbf/0x430
> > [<ffffffff810c2fe7>] ? audit_log_start+0x147/0x430
> > [<ffffffff81079030>] ? try_to_wake_up+0x2a0/0x2a0
> > [<ffffffff810c86be>] audit_log_exit+0x6ae/0xc30
> > [<ffffffff81188662>] ? __alloc_fd+0x42/0x100
> > [<ffffffff810c98e7>] __audit_syscall_exit+0x257/0x2b0
> > [<ffffffff81540794>] sysret_audit+0x17/0x21
> >
> > The reproducer is somewhat unusual:
> >
> > 1. Install RHEL6.5 (maybe a similar older user-space will do)
> > 2. Boot the just installed system
> > 3. In this first boot you'll meet the firstboot script, which
> > will do some setup and (depending on your answers) it will
> > reboot the machine
> > 4. During that first reboot the system hangs while terminating
> > all processes:
> >
> > Sending all processes the TERM signal...
> >
> > It's when the soft lockup above happens. And yes, I managed
> > to get this with latest upstream kernel (HEAD fa8218def1b1)
> >
> > I'm reproducing it on a VM, but the first report was on bare-metal.
> >
> > This is what is happening:
> >
> > 1. audit_log_start() is called
> > 2. As we have SKBs waiting in audit_skb_queue and all conditions
> > evaluate to true, we sleep in wait_for_auditd()
> > 3. Go to 2, until sleep_time gets negative and audit_log_start()
> > just busy-waits
> >
> > Now, *why* this is happening is a mistery to me. I tried debugging
> > it, but all I could find is that at some point the kauditd thread
> > never wakes up after having called schedule(). I even tried waking
> > it up before calling wait_for_auditd(), but it didn't.
>
> We run into the same problem in rhel6 kernel.
>
> "readahead-collector" uses audit interface and sometimes stuck in 'stopped' state.

Yes, please also see:

http://marc.info/?l=linux-kernel&m=137818375024600&w=2

> After commit 829199197a430dade2519d54f5545c4a094393b8
> (which was backported by RH into their kernel)
> audit emiters will block forever if userspace daemon cannot handle backlog.
> That commit just breaks timeout condition, after timeout waiting loop turns
> into busy loop until deamon dies or returns back to work.
>
> this trivial patch should fix that problem
>
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -1215,9 +1215,10 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
>
> sleep_time = timeout_start + audit_backlog_wait_time -
> jiffies;
> - if ((long)sleep_time > 0)
> + if ((long)sleep_time > 0) {
> wait_for_auditd(sleep_time);
> - continue;
> + continue;
> + }
> }


Chuck Anderson posted a similar fix:

http://marc.info/?l=linux-kernel&m=137817994623832&w=2

I still get a hang for around a minute with Chuck's fix, I believe I'll
get the same thing with yours.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/