linux-kernel - Re: [PATCH v3] audit: Turn off TIF_SYSCALL

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Message-ID: <CALCETrX0GtSaLUB9UBeKsxOoVhiVyMzWrZTYt3O5He1Wdvz+1Q@mail.gmail.com>
Date:	Mon, 10 Feb 2014 11:01:36 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-audit@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Steve Grubb <sgrubb@...hat.com>, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH v3] audit: Turn off TIF_SYSCALL_AUDIT when there are no rules

On Mon, Feb 10, 2014 at 9:29 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Feb 10, 2014 at 8:57 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> On 02/08, Andy Lutomirski wrote:
>>>
>>> +void audit_inc_n_rules()
>>> +{
>>> +     struct task_struct *p, *t;
>>> +
>>> +     read_lock(&tasklist_lock);
>>> +     audit_n_rules++;
>>> +     smp_wmb();
>>> +     if (audit_n_rules == 1) {
>>> +             /*
>>> +              * We now have a rule; we need to hook syscall entry.
>>> +              */
>>> +             for_each_process_thread(p, t) {
>>> +                     if (t->audit_context)
>>> +                             set_tsk_thread_flag(t, TIF_SYSCALL_AUDIT);
>>> +             }
>>> +     }
>>> +     read_unlock(&tasklist_lock);
>>> +}
>>> +
>>> +void audit_dec_n_rules()
>>> +{
>>> +     read_lock(&tasklist_lock);
>>> +     --audit_n_rules;
>>> +     BUG_ON(audit_n_rules < 0);
>>> +
>>> +     /*
>>> +      * If audit_n_rules == 0, then __audit_syscall_exit will clear
>>> +      * TIF_SYSCALL_AUDIT.
>>> +      */
>>> +
>>> +     read_unlock(&tasklist_lock);
>>> +}
>>
>> To be honest, I do not understand why _dec_ takes tasklist_lock...
>> And why _inc_ increments audit_n_rules under tasklist.
>
> Bah, incorrect leftover from last time.
>
>>
>>> @@ -1528,6 +1562,25 @@ void __audit_syscall_exit(int success, long return_code)
>>>               context->filterkey = NULL;
>>>       }
>>>       tsk->audit_context = context;
>>> +
>>> +     if (ACCESS_ONCE(audit_n_rules) == 0) {
>>> +             /*
>>> +              * Either this is the very first syscall by this process or
>>> +              * audit_dec_n_rules recently set audit_n_rules to zero.
>>> +              */
>>> +             smp_rmb();
>>
>> rmb() looks wrong, we need mb() to serialize ACCESS_ONCE() and
>> clear_tsk_thread_flag().
>
> I clearly need to review the rules.  I think you're right, though --
> no barrier should be needed.
>
>>
>> But, otoh, I think we do not need any barrier at all, we can rely on
>> control dependency. See the recent 18c03c61444a21 "Documentation/
>> memory-barriers.txt: Prohibit speculative writes".
>>
>>> +             /* audit_inc_n_rules could increment audit_n_rules here... */
>>> +
>>> +             clear_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
>>> +
>>> +             smp_rmb();
>>
>> Again, I guess this should be mb() or smp_mb__after_clear_bit().
>>
>>
>> And I still think this needs more changes. Once again, I do not think
>> that, say, __audit_log_bprm_fcaps() should populate context->aux if
>> !TIF_SYSCALL_AUDIT, this list can grow indefinitely. Or __audit_signal_info()...
>>
>> Perhaps __audit_syscall_exit() should also set context->dummy?
>
> That would work.
>
> I'm still torn between trying to make it possible for things like
> __audit_log_bprm_fcaps to start a syscall audit record in the middle
> of a syscall or to just try to tighten up the current approach to the
> point where it will work correctly.

This is worse than I thought.  Things like signal auditing can enter
the audit system from outside of a syscall.  I don't think there's
currently any way to tell whether you're in a syscall (when
TIF_SYSCALL_AUDIT is clear) so getting this to work right would
require arch help.

I'll ask what people on the Fedora list think about just changing the
default to -t task,never.

--Andy
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