Re: v5 of seccomp filter c/r patches

[Kees Cook]

On Fri, Oct 2, 2015 at 3:04 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 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.)

Ah-ha! Understood, and yeah, that seems fine.

Speaking of dreams -- what do you think about re-running seccomp in
the face of changed syscalls due to ptrace? Closing the ptrace hole
would be really nice.

-Kees

-- 
Kees Cook
Chrome OS Security
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