Re: audit: rcu_read_lock() used illegally while idle

Andy Lutomirski
2014-12-03T20:07:01Z

On 12/03/2014 11:29 AM, Paul E. McKenney wrote:
> On Wed, Dec 03, 2014 at 01:19:22PM -0500, Dave Jones wrote:
>> I'm not sure why this only just started complaining, because this code
>> hasn't changed in years, but I don't recall seeing this before.
>> This gets spewed during bootup since I put 3.18-rc7 on my firewall.
>> Previously it was running rc4 where I didn't see this.
>
> That is quite strange. I wonder if NO_HZ_FULL has told RCU that the
> CPU is idle before the sysret_audit hook is called.
>
> Adding Frederic for his thoughts on this.
>
>> Did something in RCU change recently ?
>
> Not since -rc1, as far as I know, anyway.

There was a cute little bug that probably prevented sysret_audit from
being called if TIF_NOHZ was set:

b5e212a3051b x86, syscall: Fix _TIF_NOHZ handling in
syscall_trace_enter_phase1

IOW, between 3.18-rc1 and 3.18-rc6 (approximately), sysret_audit would
never happen in NO_HZ_FULL mode due to that bug.

Looking at the current code, I'm not sure I understand why we don't just
infinite-loop. Hmm, maybe we do -- could this be the lockup that no one
understands yet? (I doubt it, but maybe.)

The loop would be:

sysret_check:

Load ti.flags into edx (assume that _TIF_SYSCALL_AUDIT and some other
_TIF_ALLWORK_MASK but is set).

Jump to sysret_careful. Eventually get to sysret_signal. Notice that
TIF_SYSCALL_AUDIT is set in edx. Jump to sysret_audit.

sysret_audit then clears the _TIF_SYSCALL_AUDIT bit from edi. edi is
not edx. Hmm.

Jump to sysret_check, where edx is the same as before. Repeat.

This doesn't explain how we end up screwing up context tracking. But I
don't understand why we don't execute the audit exit hook *twice* if
TIF_NOHZ is set.

I have patches to delete this whole fscking sysret fast but not really
fast path. I'll resend them for 3.19. In the mean time, can you test
this patch by itself:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/entry&id=1072a16a8d4ad1b11b8062f76e3236b9771b0fb6

Applying just that patch will result in a fairly large performance hit
if auditing is on, but given the shear number of bugs that the syscall
audit hooks seem to cause, I'm think it may be a reasonable tradeoff. I
don't really like it as a last-minute fix, though. (To get the
performance back, you need the two patches before it, but those are
*definitely* not last-minute material.)

On another note, all those TIF_FOO_BAR_BAZ masks are incomprehensible
and probably wrong in various interesting ways.

--Andy

>
> Thanx, Paul
>
>> ===============================
>> [ INFO: suspicious RCU usage. ]
>> 3.18.0-rc7+ #93 Not tainted
>> -------------------------------
>> include/linux/rcupdate.h:883 rcu_read_lock() used illegally while idle!
>>
>> other info that might help us debug this:
>>
>>
>> RCU used illegally from idle CPU!
>> rcu_scheduler_active = 1, debug_locks = 0
>> RCU used illegally from extended quiescent state!
>> 1 lock held by systemd-sysctl/557:
>> #0: (rcu_read_lock){......}, at: [<ffffffff890f1320>] audit_filter_type+0x0/0x240
>>
>> stack backtrace:
>> CPU: 0 PID: 557 Comm: systemd-sysctl Not tainted 3.18.0-rc7+ #93
>> 0000000000000000 0000000063c50efc ffff88021f51fd28 ffffffff895abec3
>> 0000000000000000 ffff880234758000 ffff88021f51fd58 ffffffff890a5280
>> ffff880221d9c548 00007fd651f320d0 0000000000000514 00000000ffff9012
>> Call Trace:
>> [<ffffffff895abec3>] dump_stack+0x4e/0x68
>> [<ffffffff890a5280>] lockdep_rcu_suspicious+0xf0/0x110
>> [<ffffffff890f150e>] audit_filter_type+0x1ee/0x240
>> [<ffffffff890f1320>] ? audit_filter_user+0x340/0x340
>> [<ffffffff890ecd39>] audit_log_start+0x49/0x4a0
>> [<ffffffff8908f4f5>] ? local_clock+0x25/0x30
>> [<ffffffff890a397f>] ? lock_release_holdtime.part.30+0xf/0x190
>> [<ffffffff890f1973>] audit_log_exit+0x53/0xcf0
>> [<ffffffff8908f4f5>] ? local_clock+0x25/0x30
>> [<ffffffff890a397f>] ? lock_release_holdtime.part.30+0xf/0x190
>> [<ffffffff895b4f0c>] ? sysret_signal+0x5/0x43
>> [<ffffffff890f4a65>] __audit_syscall_exit+0x245/0x2a0
>> [<ffffffff895b4f61>] sysret_audit+0x17/0x21
>>
>