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

[Kees Cook]

On Wed, Jun 25, 2014 at 9:10 AM, Andy Lutomirski <luto at amacapital.net> wrote:
> 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.

The TIF_SECCOMP is needed for the syscall entry path. The check in
secure_computing() is just because the "I am being traced" trigger
includes a call to secure_computing, which filters out tracing
reasons.

Your fast path work would clean a lot of that up, as you say. But it
still doesn't change the ordering check here. TIF_SECCOMP indicates
seccomp.mode must be checked, so that ordering will remain, and if
mode == FILTER, seccomp.filter must be valid.

Isn't there a way we can force the assignment ordering in seccomp_assign_mode()?

-Kees

-- 
Kees Cook
Chrome OS Security