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

Andy Lutomirski
2015-09-05T00:08:52Z

On Fri, Sep 4, 2015 at 4:27 PM, Kees Cook <keescook@chromium.org> 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]
>
> If KCMP_FILE_PRIVATE_DATA isn't desired, I think a global counter id
> is the next best.

The problem is that you can't checkpoint and restore it. We could
have a counter relative to the parent filter, though.

--Andy

>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security

--
Andy Lutomirski
AMA Capital Management, LLC