linux-kernel - Re: [PATCH] bpf: lirc program type should not require SYS_CAP

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Date:   Fri, 14 Apr 2023 11:58:49 +0100
From:   Sean Young <sean@...s.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH] bpf: lirc program type should not require SYS_CAP_ADMIN

On Thu, Apr 13, 2023 at 04:54:21PM -0700, Alexei Starovoitov wrote:
> On Thu, Apr 13, 2023 at 1:28 AM Sean Young <sean@...s.org> wrote:
> >
> > On Wed, Apr 12, 2023 at 04:14:05PM -0700, Alexei Starovoitov wrote:
> > > On Tue, Apr 11, 2023 at 8:45 AM Sean Young <sean@...s.org> wrote:
> > > >
> > > > Make it possible to load lirc program type with just CAP_BPF.
> > >
> > > Is it safe?
> > > If the user can load with just CAP_BPF the FD to the prog and target_fd
> > > will allow attach as well.
> >
> > Exactly, that's the $1m question of course.
> >
> > I think it's safe from a lirc perspective because you need to be able to
> > open the /dev/lirc0 device in the first place; if you can open it, you
> > alter all sorts of lirc receiving options already. Changing the IR protocol
> > decoder is no different in that perspective.
> >
> > The other side of course, is it save to load a bpf lirc program as a normal
> > user. I don't see any issue with this; I guess this depends on whether the
> > subset of functions in lirc_mode2_func_proto() is safe. I am hoping that
> > the expert opinion everyone here can help answer that question.
> 
> That part is fine under CAP_BPF.
> I don't know how lirc devices are typically setup.
> If they need root to open them
> then why bother relaxing bpf loading part?

I'd like to get a point where /dev/lircN can have the same permissions as
for example /dev/videoN devices: group read/write, so that local users
don't have to become root to use them.

Without relaxing the bpf side, this seems like a chicken and egg problem
(tiktaalik and egg?).

Also - the CAP_NET_ADMIN requirement seems completely arbitrary compared
to other program types.


Sean