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

[Konstantin Khlebnikov]

Luiz Capitulino wrote:
> On Mon, 09 Sep 2013 19:19:14 +0400
> Konstantin Khlebnikov<khlebnikov@xxxxxxxxxx>  wrote:
>
> > Luiz Capitulino wrote:
> > > 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.
> >
> > Yep, this is normal behaviour -- default audit_backlog_wait_time is 60 seconds.
>
> Yes, I know that's the cause but I don't call it normal behavior to
> be unable to use my system during 60 seconds.

This stuff was designed for auditing, so it shouldn't loose messages too easily.
Meanwhile 'readahead' reuses it for a different purpose without proper tuning.
I think kernel should provide interface for tuning that timeout and let 'readahead'
set it to zero or some small value.

> > But this logic is really strange, I don't see any interface for tuning that timeout
> > and seems like kernel set it to zero after first disaster and never recovers it back.

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