Re: [PATCH] audit: Only use the syscall slowpath when syscall audit rules exist
On Mon, Feb 3, 2014 at 10:11 AM, Oleg Nesterov <oleg(a)redhat.com> wrote:
On 02/03, Andy Lutomirski wrote:
>
> @@ -911,6 +918,47 @@ static inline struct audit_context *audit_alloc_context(enum
audit_state state)
> return context;
> }
>
> +void audit_inc_n_rules()
> +{
> + struct task_struct *p, *g;
> +
> + write_lock(&n_rules_lock);
> +
> + if (audit_n_rules++ != 0)
> + goto out; /* The overall state isn't changing. */
> +
> + read_lock(&tasklist_lock);
> + do_each_thread(g, p) {
> + if (p->audit_context)
> + set_tsk_thread_flag(p, TIF_SYSCALL_AUDIT);
> + } while_each_thread(g, p);
> + read_unlock(&tasklist_lock);
Cosmetic, but I'd suggest to use for_each_process_thread() instead
of do_each_thread/while_each_thread.
I didn't notice that option :)
And I am not sure why n_rules_lock is rwlock_t... OK, to make
audit_alloc() more scalable, I guess. Please see below.
Yes. I should probably also use the irqsave variant.
> @@ -942,8 +995,14 @@ int audit_alloc(struct task_struct *tsk)
> }
> context->filterkey = key;
>
> + read_lock(&n_rules_lock);
> tsk->audit_context = context;
> - set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
> + if (audit_n_rules)
> + set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
> + else
> + clear_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
> + read_unlock(&n_rules_lock);
Perhaps this is fine, but n_rules_lock can't prevent the race with
audit_inc/dec_n_rules(). The problem is, this is called before the
new task is visible to for_each_process_thread().
Hmm. I missed that.
If we want to fix this race, we need something like audit_sync_flags()
called after copy_process() drops tasklist, or from tasklist_lock
protected section (in this case it doesn't need n_rules_lock).
Or perhaps audit_alloc() should not try to clear TIF_SYSCALL_AUDIT at all.
In both cases n_rules_lock can be spinlock_t.
Here are two options:
1. Sync the flags inside tasklist_lock, as I think you're suggesting.
This seems simple.
2. Make this whole thing lazy -- always set TIF_SYSCALL_AUDIT for new
tasks, but clear it on the first syscall.
I'm leaning toward number 1.
Thanks for the instant review!
--Andy