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

[Luiz Capitulino]

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.

> 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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