On Thu, Aug 17, 2023 at 9:54 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Mon, Aug 14, 2023 at 7:46 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 15, 2023 at 8:28 AM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> > >
> > >
> > >
> > > On 8/14/23 7:33 AM, Yafang Shao wrote:
> > > > Add a new bpf_current_capable kfunc to check whether the current task
> > > > has a specific capability. In our use case, we will use it in a lsm bpf
> > > > program to help identify if the user operation is permitted.
> > > >
> > > > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > > > ---
> > > > kernel/bpf/helpers.c | 6 ++++++
> > > > 1 file changed, 6 insertions(+)
> > > >
> > > > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > > > index eb91cae..bbee7ea 100644
> > > > --- a/kernel/bpf/helpers.c
> > > > +++ b/kernel/bpf/helpers.c
> > > > @@ -2429,6 +2429,11 @@ __bpf_kfunc void bpf_rcu_read_unlock(void)
> > > > rcu_read_unlock();
> > > > }
> > > >
> > > > +__bpf_kfunc bool bpf_current_capable(int cap)
> > > > +{
> > > > + return has_capability(current, cap);
> > > > +}
> > >
> > > Since you are testing against 'current' capabilities, I assume
> > > that the context should be process. Otherwise, you are testing
> > > against random task which does not make much sense.
> >
> > It is in the process context.
> >
> > >
> > > Since you are testing against 'current' cap, and if the capability
> > > for that task is stable, you do not need this kfunc.
> > > You can test cap in user space and pass it into the bpf program.
> > >
> > > But if the cap for your process may change in the middle of
> > > run, then you indeed need bpf prog to test capability in real time.
> > > Is this your use case and could you describe in in more detail?
> >
> > After we convert the capability of our networking bpf program from
> > CAP_SYS_ADMIN to CAP_BPF+CAP_NET_ADMIN to enhance the security, we
> > encountered the "pointer comparison prohibited" error, because
> > allow_ptr_leaks is enabled only when CAP_PERFMON is set. However, if
> > we enable the CAP_PERFMON for the networking bpf program, it can run
> > tracing bpf prog, perf_event bpf prog and etc, that is not expected by
> > us.
> >
> > Hence we are planning to use a lsm bpf program to disallow it from
> > running other bpf programs. In our lsm bpf program we will check the
> > capability of processes, if the process has cap_net_admin, cap_bpf and
> > cap_perfmon but don't have cap_sys_admin we will refuse it to run
> > tracing and perf_event bpf program. While if a process has cap_bpf
> > and cap_perfmon but doesn't have cap_net_admin, that said it is a bpf
> > program which wants to run trace bpf, we will allow it.
> >
> > We can't use lsm_cgroup because it is supported on cgroup2 only, while
> > we're still using cgroup1.
> >
> > Another possible solution is enable allow_ptr_leaks for cap_net_admin
> > as well, but after I checked the commit which introduces the cap_bpf
> > and cap_perfmon [1], I think we wouldn't like to do it.
>
> Sorry. None of these options are acceptable.
>
> The idea of introducing a bpf_current_capable() kfunc just to work
> around a deficiency in the verifier is not sound.
So what should we do then?
Just enabling the cap_perfmon for it? That does not sound as well ...
>
> Next time instead of sending patch please describe the issue first.
> You should have started with an email that:
> struct iphdr *iph = (void *)(long)skb->data + sizeof(struct ethhdr);
>
> if ((long)(iph + 1) > (long)skb->data_end)
> needs cap_perfmon.
Got it. Thanks for your explanation.
--
Regards
Yafang
