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

[Andy Lutomirski]

On Wed, Jun 25, 2014 at 7:51 AM, Kees Cook <keescook at chromium.org> wrote:
> On Wed, Jun 25, 2014 at 6:51 AM, Oleg Nesterov <oleg at redhat.com> 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?

Remove the ordering requirement, perhaps?

What if you moved mode into seccomp.filter?  Then there would be
little reason to check TIF_SECCOMP from secure_computing; instead, you
could smp_load_acquire (or read_barrier_depends, maybe) seccomp.filter
from secure_computing and pass the result as a parameter to
__secure_computing.  Or you could even remove the distinction between
secure_computing and __secure_computing -- it's essentially useless
anyway to split entry hook approaches like my x86 fastpath prototype.

--Andy