Re: v5 of seccomp filter c/r patches

[Kees Cook]

On Fri, Oct 2, 2015 at 2:29 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Fri, Oct 2, 2015 at 2:10 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Fri, Oct 2, 2015 at 9:27 AM, Tycho Andersen
>> <tycho.andersen@xxxxxxxxxxxxx> wrote:
>>> Hi all,
>>>
>>> Here's v5 of the seccomp filter c/r set. The individual patch notes have
>>> changes, but two highlights are:
>>>
>>> * This series is now based on http://patchwork.ozlabs.org/patch/525492/ and
>>>   will need to be built with that patch applied. This gets rid of two incorrect
>>>   patches in the previous series and is a nicer API.
>>>
>>> * I couldn't figure out a nice way to have SECCOMP_GET_FILTER_FD return the
>>>   same struct file across calls, so we still need a kcmp command. I've narrowed
>>>   the scope of the one being added to only compare seccomp fds.
>>>
>>> Thoughts welcome,
>>
>> Hi, sorry I've been slow/busy. I'm finally reading through these threads.
>>
>> Happy bit:
>> - avoiding eBPF and just saving the original filters makes things much easier.
>>
>> Sad bit:
>> - inventing a new interface for seccompfds feels like massive overkill to me.
>>
>> While Andy has big dreams, we're not presently doing seccompfd
>> monitoring, etc. There's no driving user for that kind of interface,
>> and accepting the maintenance burden of it only for CRIU seems unwise.
>>
>> So, I'll go back to what I originally proposed at LSS (which it looks
>> like we're half way there now):
>>
>> - save the original filter (done!)
>> - extract filters through a single special-purpose interface (looks
>> like ptrace is the way to go: root-only, stopped process, etc)
>> - compare filter content and issue TSYNCs to merge detected sibling
>> threads, since merging things that weren't merged before creates no
>> problems.
>>
>> This means the parenting logic is heuristic, but it's entirely in
>> userspace, so the complexity burden doesn't live in seccomp which we,
>> by design, want to keep as simple as possible.
>
> This is okay with me with a future-proofing caveat: I think that
> whatever reads out the filter should be clearly documented as
> returning some special error code that indicates that that filter it
> tried to read wasn't in the expected form.  That would happen for
> native eBPF filters, and it would also happen for seccomp monitors
> even if those monitors use classic BPF.

As in, it should have something like "give me BPF" and that'll start
failing when it's only eBPF in the future?

-Kees

-- 
Kees Cook
Chrome OS Security
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