Re: [PATCH 1/6] ebpf: add a seccomp program type

Kees Cook
2015-09-04T21:08:40Z

On Fri, Sep 4, 2015 at 2:06 PM, Tycho Andersen
<tycho.andersen@canonical.com> wrote:
> On Fri, Sep 04, 2015 at 01:34:12PM -0700, Kees Cook wrote:
>> On Fri, Sep 4, 2015 at 9:04 AM, Tycho Andersen
>> <tycho.andersen@canonical.com> wrote:
>> > +static const struct bpf_func_proto *
>> > +seccomp_func_proto(enum bpf_func_id func_id)
>> > +{
>> > + /* Right now seccomp eBPF loading doesn't support maps; seccomp filters
>> > + * are considered to be read-only after they're installed, so map fds
>> > + * probably need to be invalidated when a seccomp filter with maps is
>> > + * installed.
>> > + *
>> > + * The rest of these might be reasonable to call from seccomp, so we
>> > + * export them.
>> > + */
>> > + switch (func_id) {
>> > + case BPF_FUNC_ktime_get_ns:
>> > + return &bpf_ktime_get_ns_proto;
>> > + case BPF_FUNC_trace_printk:
>> > + return bpf_get_trace_printk_proto();
>> > + case BPF_FUNC_get_prandom_u32:
>> > + return &bpf_get_prandom_u32_proto;
>> > + case BPF_FUNC_get_smp_processor_id:
>> > + return &bpf_get_smp_processor_id_proto;
>> > + case BPF_FUNC_tail_call:
>> > + return &bpf_tail_call_proto;
>> > + case BPF_FUNC_get_current_pid_tgid:
>> > + return &bpf_get_current_pid_tgid_proto;
>> > + case BPF_FUNC_get_current_uid_gid:
>> > + return &bpf_get_current_uid_gid_proto;
>> > + case BPF_FUNC_get_current_comm:
>> > + return &bpf_get_current_comm_proto;
>> > + default:
>> > + return NULL;
>> > + }
>> > +}
>>
>> While this list is probably fine, I don't want to mix the addition of
>> eBPF functions to the seccomp ABI with the CRIU changes. No function
>> calls are currently possible and it should stay that way.
>
> Ok, I can remove them.
>
>> I was expecting to see a validator, similar to the existing BPF
>> validator that is called when creating seccomp filters currently. Can
>> we add a similar validator for new BPF_PROG_TYPE_SECCOMP?
>
> That's effectively what this patch does; when the eBPF is loaded via
> bpf(), you tell bpf() you want a BPF_PROG_TYPE_SECCOMP, and it invokes
> this validation/translation code, i.e. it uses
> seccomp_is_valid_access() to check and make sure access are aligned
> and inside struct seccomp_data.

What about limiting the possible instructions?

-Kees

--
Kees Cook
Chrome OS Security