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

Tycho Andersen
2015-09-04T22:28:59Z

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]

Tycho