Re: [PATCH 3/6] ebpf: add a way to dump an eBPF program

Andy Lutomirski
2015-09-09T23:44:46Z

On Wed, Sep 9, 2015 at 3:34 PM, Tycho Andersen
<tycho.andersen@canonical.com> wrote:
> On Fri, Sep 04, 2015 at 06:27:27PM -0600, Tycho Andersen wrote:
>> On Fri, Sep 04, 2015 at 04:08:53PM -0700, Andy Lutomirski wrote:
>> > On Fri, Sep 4, 2015 at 3:28 PM, Tycho Andersen
>> > <tycho.andersen@canonical.com> wrote:
>> > > On Fri, Sep 04, 2015 at 02:48:03PM -0700, Andy Lutomirski wrote:
>> > >> On Fri, Sep 4, 2015 at 1:45 PM, Tycho Andersen
>> > >> <tycho.andersen@canonical.com> wrote:
>> > >> > On Fri, Sep 04, 2015 at 01:17:30PM -0700, Kees Cook wrote:
>> > >> >> On Fri, Sep 4, 2015 at 9:04 AM, Tycho Andersen
>> > >> >> <tycho.andersen@canonical.com> wrote:
>> > >> >> > This commit adds a way to dump eBPF programs. The initial implementation
>> > >> >> > doesn't support maps, and therefore only allows dumping seccomp ebpf
>> > >> >> > programs which themselves don't currently support maps.
>> > >> >> >
>> > >> >> > We export the GPL bit as well as a unique ID for the program so that
>> > >> >>
>> > >> >> This unique ID appears to be the heap address for the prog. That's a
>> > >> >> huge leak, and should not be done. We don't want to introduce new
>> > >> >> kernel address leaks while we're trying to fix the remaining ones.
>> > >> >> Shouldn't the "unique ID" be the fd itself? I imagine KCMP_FILE
>> > >> >> could be used, for example.
>> > >> >
>> > >> > No; we acquire the fd per process, so if a task installs a filter and
>> > >> > then forks N times, we'll grab N (+1) copies of the filter from N (+1)
>> > >> > different file descriptors. Ideally, we'd have some way to figure out
>> > >> > that these were all the same. Some sort of prog_id is one way,
>> > >> > although there may be others.
>> > >>
>> > >> I disagree a bit. I think we want the actual hierarchy to be a
>> > >> well-defined thing, because I have plans to make the hierarchy
>> > >> actually do something. That means that we'll need to have a more
>> > >> exact way to dump the hierarchy than "these two filters are identical"
>> > >> or "these two filters are not identical".
>> > >
>> > > Can you elaborate on what this would look like? I think with the
>> > > "these two filters are the same" primitive (the same in the sense that
>> > > they were inherited during a fork, not just that
>> > > memcmp(filter1->insns, filter2->insns) == 0) you can infer the entire
>> > > hierarchy, however clunky it may be to do so.
>> > >
>> > > Another issue is that KCMP_FILE won't work in this case, as it
>> > > effectively compares the struct file *, which will be different since
>> > > we need to call anon_inode_getfd() for each call of
>> > > ptrace(PTRACE_SECCOMP_GET_FILTER_FD). We could add a KCMP_BPF (or just
>> > > a KCMP_FILE_PRIVATE_DATA, since that's effectively what it would be).
>> > > Does that make sense? [added Cyrill]
>> > >
>> >
>> > I don't really know what it would look like. I think we want a way to
>> > compare struct seccomp_filter pointers.
>
> Here's a thought,
>
> The set I'm currently proposing effectively separates the ref-counting
> of the struct seccomp_filter from the struct bpf_prog (by necessity,
> since we're referring to filters from fds). What if we went a little
> futher, and made a copy of each seccomp_filter on fork(), keeping it
> pointed at the same bpf_prog but adding some metadata about how it was
> inherited (tsk->seccomp.filter->inheritence_count++ perhaps). This
> would still require this change:

Won't that break the tsync mechanism?

--Andy