linux-kernel - Re: [PATCH v2.1] audit: Only use the syscall slowpath when syscall audit rules exist

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Date:	Tue, 4 Feb 2014 17:50:15 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-audit@...hat.com, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Steve Grubb <sgrubb@...hat.com>, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH v2.1] audit: Only use the syscall slowpath when syscall
	audit rules exist

On 02/03, Andy Lutomirski wrote:
>
> +void audit_inc_n_rules()
> +{
> +	struct task_struct *p, *g;
> +	unsigned long flags;
> +
> +	read_lock_irqsave(&tasklist_lock, flags);

Confused... read_lock(tasklist) doesn't need to disable irqs.

(ftrace does this for no reason too, perhaps I should resend the patch)

> +	if (audit_n_rules++ == 0) {

probably this can be done outside of read_lock?

> +		do_each_thread(g, p) {

for_each_process_thread ;) do_each_thread will die, I hope.

> +void audit_dec_n_rules()
> +{
> +	struct task_struct *p, *g;
> +	unsigned long flags;
> +
> +	read_lock_irqsave(&tasklist_lock, flags);
> +
> +	--audit_n_rules;
> +	BUG_ON(audit_n_rules < 0);
> +
> +	if (audit_n_rules == 0) {
> +		do_each_thread(g, p) {
> +			clear_tsk_thread_flag(p, TIF_SYSCALL_AUDIT);
> +		} while_each_thread(g, p);
> +	}

The same, and...

On a second thought it seems that audit_dec_n_rules() has a problem.
Note the BUG_ON(context->in_syscall) in __audit_syscall_entry().

Suppose that audit_dec_n_rules() clears TIF_SYSCALL_AUDIT when a task
runs a syscall. In this case (afaics) __audit_syscall_exit() won't be
called. The next audit_inc_n_rules() can set TIF_SYSCALL_AUDIT and
trigger another __audit_syscall_entry() which will hit this BUG_ON().

And in general it doesn't look safe although I know almost nothing
about audit. I mean, currently __audit_syscall_entry() or
__audit_log_bprm_fcaps() assume that __audit_syscall_exit() or
__audit_free() will "cleanup" ->audit_context, perhaps we should not
break the rules?

Once again, I do not pretend I understand this code, this is the
question, not the comment.

But if I am right, then TIF_SYSCALL_AUDIT should be cleared in
__audit_syscall_exit() as you suggested before.

Oleg.

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