Re: [PATCH v7 3/9] seccomp: introduce writer locking

[Kees Cook]

On Tue, Jun 24, 2014 at 11:30 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> I am puzzled by the usage of smp_load_acquire(),

It was recommended by Andy Lutomirski in preference to ACCESS_ONCE().

> On 06/23, Kees Cook wrote:
>>
>>  static u32 seccomp_run_filters(int syscall)
>>  {
>> -     struct seccomp_filter *f;
>> +     struct seccomp_filter *f = smp_load_acquire(&current->seccomp.filter);
>>       struct seccomp_data sd;
>>       u32 ret = SECCOMP_RET_ALLOW;
>>
>>       /* Ensure unexpected behavior doesn't result in failing open. */
>> -     if (WARN_ON(current->seccomp.filter == NULL))
>> +     if (WARN_ON(f == NULL))
>>               return SECCOMP_RET_KILL;
>>
>>       populate_seccomp_data(&sd);
>> @@ -186,9 +186,8 @@ static u32 seccomp_run_filters(int syscall)
>>        * All filters in the list are evaluated and the lowest BPF return
>>        * value always takes priority (ignoring the DATA).
>>        */
>> -     for (f = current->seccomp.filter; f; f = f->prev) {
>> +     for (; f; f = smp_load_acquire(&f->prev)) {
>>               u32 cur_ret = SK_RUN_FILTER(f->prog, (void *)&sd);
>> -
>>               if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
>>                       ret = cur_ret;
>
> OK, in this case the 1st one is probably fine, altgough it is not
> clear to me why it is better than read_barrier_depends().
>
> But why do we need a 2nd one inside the loop? And if we actually need
> it (I don't think so) then why it is safe to use f->prog without
> load_acquire ?

You're right -- it should not be possible for for any of the ->prev
pointers to change.

>>  void get_seccomp_filter(struct task_struct *tsk)
>>  {
>> -     struct seccomp_filter *orig = tsk->seccomp.filter;
>> +     struct seccomp_filter *orig = smp_load_acquire(&tsk->seccomp.filter);
>>       if (!orig)
>>               return;
>
> This one looks unneeded.
>
> First of all, afaics atomic_inc() should work correctly without any barriers,
> otherwise it is buggy. But even this doesn't matter.
>
> With this changes get_seccomp_filter() must be called under ->siglock, it can't
> race with add-filter and thus tsk->seccomp.filter should be stable.

Excellent point, yes. I'll remove that.

>>       /* Reference count is bounded by the number of total processes. */
>> @@ -361,7 +364,7 @@ void put_seccomp_filter(struct task_struct *tsk)
>>       /* Clean up single-reference branches iteratively. */
>>       while (orig && atomic_dec_and_test(&orig->usage)) {
>>               struct seccomp_filter *freeme = orig;
>> -             orig = orig->prev;
>> +             orig = smp_load_acquire(&orig->prev);
>>               seccomp_filter_free(freeme);
>>       }
>
> This one looks unneeded too. And note that this patch does not add
> smp_load_acquire() to read tsk->seccomp.filter.

Hrm, yes, that should get added.

> atomic_dec_and_test() adds mb(), we do not need more barriers to access
> ->prev ?

Right, same situation as the run_filters loop. Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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