Re: [PATCH v2 seccomp 3/6] seccomp/cache: Add "emulator" to check if filter is arg-dependent

Andy Lutomirski
2020-09-25T19:51:28Z

> On Sep 25, 2020, at 12:42 PM, Kees Cook <keescook@chromium.org> wrote:
>
> ﻿On Fri, Sep 25, 2020 at 11:45:05AM -0500, YiFei Zhu wrote:
>> On Thu, Sep 24, 2020 at 10:04 PM YiFei Zhu <zhuyifei1999@gmail.com> wrote:
>>>> Why do the prepare here instead of during attach? (And note that it
>>>> should not be written to fail.)
>>>
>>> Right.
>>
>> During attach a spinlock (current->sighand->siglock) is held. Do we
>> really want to put the emulator in the "atomic section"?
>
> It's a good point, but I had some other ideas around it that lead to me
> a different conclusion. Here's what I've got in my head:
>
> I don't view filter attach (nor the siglock) as fastpath: the lock is
> rarely contested and the "long time" will only be during filter attach.
>
> When performing filter emulation, all the syscalls that are already
> marked as "must run filter" on the previous filter can be skipped for
> the new filter, since it cannot change the outcome, which makes the
> emulation step faster.
>
> The previous filter's bitmap isn't "stable" until siglock is held.
>
> If we do the emulation step before siglock, we have to always do full
> evaluation of all syscalls, and then merge the bitmap during attach.
> That means all filters ever attached will take maximal time to perform
> emulation.
>
> I prefer the idea of the emulation step taking advantage of the bitmap
> optimization, since the kernel spends less time doing work over the life
> of the process tree. It's certainly marginal, but it also lets all the
> bitmap manipulation stay in one place (as opposed to being split between
> "prepare" and "attach").
>
> What do you think?
>
>

I’m wondering if we should be much much lazier. We could potentially wait until someone actually tries to do a given syscall before we try to evaluate whether the result is fixed.