Re: [PATCH] seccomp: fix the usage of get/put_seccomp_filter() in seccomp_get_filter()

[Kees Cook]

On Thu, Sep 21, 2017 at 3:57 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> On 09/20, Kees Cook wrote:
>>
>> On Wed, Sep 20, 2017 at 5:56 AM, Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>> > @@ -908,13 +912,13 @@ long seccomp_get_filter(struct task_struct *task, unsigned long filter_off,
>> >         if (!data)
>> >                 goto out;
>> >
>> > -       get_seccomp_filter(task);
>> > +       refcount_inc(&filter->usage);
>> >         spin_unlock_irq(&task->sighand->siglock);
>> >
>> >         if (copy_to_user(data, fprog->filter, bpf_classic_proglen(fprog)))
>> >                 ret = -EFAULT;
>> >
>> > -       put_seccomp_filter(task);
>> > +       __put_seccomp_filter(filter);
>> >         return ret;
>>
>> Given how reference counting is done for filters, I'd be happier with
>> leaving the get_seccomp_filter() as-is,
>
> No, please note that filter != tsk->seccomp.filter, get_seccomp_filter()
> won't work.

Ah yes, sorry, you're right.

>> (i.e. don't open-code
>> the refcount_inc()).
>
> agreed, probably another __get_seccomp_filter(filter) makes sense, especially
> if we do other changes like get_nth().
>
> But imo not in this fix.

Regardless, whatever lands will need backport adjustment for
refcount_*/atomic_* in -stable.

Can you resend the two patches; I can send the backport to -stable manually...

-Kees

-- 
Kees Cook
Pixel Security