Re: v5 of seccomp filter c/r patches

[Andy Lutomirski]

On Fri, Oct 2, 2015 at 3:06 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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.

Yes, absolutely!  We might even want to just move the seccomp check
after ptrace (except for seccomp-induced ptrace).

Unfortunately, I backed us into a corner with two-phase seccomp on
x86, and it's a big mess.  (I wrote the seccomp vs ptrace patches, and
I don't think they're acceptable.)  My big x86 low-level rewrite is an
attempt to get back out of that corner, and I'm hoping to resubmit the
bulk of it today or tomorrow.  Once that happens, I just need to fix
up the 64-bit native case (trivial, I know) and then revert two-phase
seccomp.

One nice outcome of all of this will be that the syscall tables will
contain bona fide C ABI compliant function pointers, which is
currently not the case.

--Andy
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