Re: [PATCH v8 5/9] seccomp: split mode set routines

[Kees Cook]

On Wed, Jun 25, 2014 at 6:51 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 06/24, Kees Cook wrote:
>>
>> +static inline void seccomp_assign_mode(struct task_struct *task,
>> +                                    unsigned long seccomp_mode)
>> +{
>> +     BUG_ON(!spin_is_locked(&task->sighand->siglock));
>> +
>> +     task->seccomp.mode = seccomp_mode;
>> +     set_tsk_thread_flag(task, TIF_SECCOMP);
>> +}
>
> OK, but unless task == current this can race with secure_computing().
> I think this needs smp_mb__before_atomic() and secure_computing() needs
> rmb() after test_bit(TIF_SECCOMP).
>
> Otherwise, can't __secure_computing() hit BUG() if it sees the old
> mode == SECCOMP_MODE_DISABLED ?
>
> Or seccomp_run_filters() can see ->filters == NULL and WARN(),
> smp_load_acquire() only serializes that LOAD with the subsequent memory
> operations.

Hm, actually, now I'm worried about smp_load_acquire() being too slow
in run_filters().

The ordering must be:
- task->seccomp.filter must be valid before
- task->seccomp.mode is set, which must be valid before
- TIF_SECCOMP is set

But I don't want to impact secure_computing(). What's the best way to
make sure this ordering is respected?

-Kees

-- 
Kees Cook
Chrome OS Security
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