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

[Luiz Capitulino <lcapitulino@redhat.com>]

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.

This patch is a RFC because it doesn't actually fix anything. It
just bails out before we busy-wait. Which is better than nothing,
but the system will still give a long pause when terminating all
processes. Besides, we risk papering over the real bug...

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
---
 kernel/audit.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/audit.c b/kernel/audit.c
index 91e53d0..33e9c4c 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -1097,6 +1097,7 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
 	unsigned int		uninitialized_var(serial);
 	int reserve;
 	unsigned long timeout_start = jiffies;
+	unsigned long sleep_time;
 
 	if (audit_initialized != AUDIT_INITIALIZED)
 		return NULL;
@@ -1112,13 +1113,11 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
 
 	while (audit_backlog_limit
 	       && skb_queue_len(&audit_skb_queue) > audit_backlog_limit + reserve) {
-		if (gfp_mask & __GFP_WAIT && audit_backlog_wait_time) {
-			unsigned long sleep_time;
-
 			sleep_time = timeout_start + audit_backlog_wait_time -
 					jiffies;
-			if ((long)sleep_time > 0)
-				wait_for_auditd(sleep_time);
+		if (gfp_mask & __GFP_WAIT && audit_backlog_wait_time
+			&& sleep_time > 0) {
+			wait_for_auditd(sleep_time);
 			continue;
 		}
 		if (audit_rate_check() && printk_ratelimit())
-- 
1.8.1.4