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

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Date:	Mon, 10 Feb 2014 09:29:19 -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 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.

Grr.  Why is all this crap tied up with syscall auditing anyway?  ISTM
it would have been a lot nicer if audit calls just immediately emitted
audit records, completely independently of the syscall machinery.

--Andy
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