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.
[1]. https://lore.kernel.org/bpf/20200513230355.7858-3-alexei.starovoitov@xxxxxxxxx/
--
Regards
Yafang
