Re: v5 of seccomp filter c/r patches

[Andy Lutomirski]

On Fri, Oct 2, 2015 at 3:02 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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?

Yes, but it might also start failing when if my dreams come true, it's
still classic BPF, but it's no longer a classic seccomp bpf filter
layer with the semantics we expect today.  (E.g. if it's classic bpf
but has a monitor attached, then the read should fail because
restoring it without restoring the monitor will cause all kinds of
mess.)

--Andy
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