Re: v5 of seccomp filter c/r patches

[Tycho Andersen]

On Fri, Oct 02, 2015 at 03:52:03PM -0700, Andy Lutomirski wrote:
> 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).

At least for now, I think requiring real root and no seccomp is fine,
so I'll do that.

Tycho
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