Re: v5 of seccomp filter c/r patches

[Andy Lutomirski]

On Fri, Oct 2, 2015 at 3:44 PM, Tycho Andersen
<tycho.andersen@xxxxxxxxxxxxx> wrote:
> On Fri, Oct 02, 2015 at 02:10:24PM -0700, Kees Cook 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.
>
> Ok, how about,
>
> struct sock_filter insns[BPF_MAXINSNS];
> insn_cnt = ptrace(PTRACE_SECCOMP_GET_FILTER, pid, insns, i);
>
> when asking for the ith filter? It returns either the number of
> instructions, -EINVAL if something was wrong (i, pid,
> CONFIG_CHECKPOINT_RESTORE isn't enabled). While it would always
> succeed now, if/when the underlying filter was not created from a bpf
> classic filter, we can return -EMEDIUMTYPE? (Suggestions welcome, I
> picked this mostly based on what sounds nice.)
>

Are we still requiring global permissions or that the caller isn't
seccomped at all?  I've not lost track of how we're resolving the case
where the caller and the tracee have exactly the same seccomp state
(or the tracee is derived from the caller's state or they're totally
unrelated states).

--Andy
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