Re: [RFC PATCH bpf-next 00/13] bpf: Introduce BPF namespace

From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Yafang Shao <laoar.shao@gmail.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Song Liu <song@kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, bpf <bpf@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH bpf-next 00/13] bpf: Introduce BPF namespace
Date: Fri, 7 Apr 2023 09:31:52 -0700	[thread overview]
Message-ID: <CAADnVQKwqM6Fnbf+nQetpsoVY8NamQMMhshuAX7tGeHbrHPO_g@mail.gmail.com> (raw)
In-Reply-To: <CALOAHbCwqV2x-HPcq5nr8W_xEKyJMHO6N7TbtDdWfqkC9fsSaw@mail.gmail.com>

On Fri, Apr 7, 2023 at 9:22 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Sat, Apr 8, 2023 at 12:05 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Apr 7, 2023 at 8:59 AM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Thu, Apr 6, 2023 at 6:44 PM Alexei Starovoitov
> > > <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > On Thu, Apr 06, 2023 at 01:22:26PM -0700, Andrii Nakryiko wrote:
> > > > > On Wed, Apr 5, 2023 at 10:44 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > > > >
> > > > > > On Thu, Apr 6, 2023 at 12:24 PM Alexei Starovoitov
> > > > > > <alexei.starovoitov@gmail.com> wrote:
> > > > > > >
> > > > > > > On Wed, Apr 5, 2023 at 8:22 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > > > > > >
> > > > > > > > On Thu, Apr 6, 2023 at 11:06 AM Alexei Starovoitov
> > > > > > > > <alexei.starovoitov@gmail.com> wrote:
> > > > > > > > >
> > > > > > > > > On Wed, Apr 5, 2023 at 7:55 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > > > > > > > >
> > > > > > > > > > It seems that I didn't describe the issue clearly.
> > > > > > > > > > The container doesn't have CAP_SYS_ADMIN, but the CAP_SYS_ADMIN is
> > > > > > > > > > required to run bpftool,  so the bpftool running in the container
> > > > > > > > > > can't get the ID of bpf objects or convert IDs to FDs.
> > > > > > > > > > Is there something that I missed ?
> > > > > > > > >
> > > > > > > > > Nothing. This is by design. bpftool needs sudo. That's all.
> > > > > > > > >
> > > > > > > >
> > > > > > > > Hmm, what I'm trying to do is make bpftool run without sudo.
> > > > > > >
> > > > > > > This is not a task that is worth solving.
> > > > > > >
> > > > > >
> > > > > > Then the container with CAP_BPF enabled can't even iterate its bpf progs ...
> > > > >
> > > > > I'll leave the BPF namespace discussion aside (I agree that it needs
> > > > > way more thought).
> > > > >
> > > > > I am a bit surprised that we require CAP_SYS_ADMIN for GET_NEXT_ID
> > > > > operations. GET_FD_BY_ID is definitely CAP_SYS_ADMIN, as they allow
> > > > > you to take over someone else's link and stuff like this. But just
> > > > > iterating IDs seems like a pretty innocent functionality, so maybe we
> > > > > should remove CAP_SYS_ADMIN for GET_NEXT_ID?
> > > > >
> > > > > By itself GET_NEXT_ID is relatively useless without capabilities, but
> > > > > we've been floating the idea of providing GET_INFO_BY_ID (not by FD)
> > > > > for a while now, and that seems useful in itself, as it would indeed
> > > > > help tools like bpftool to get *some* information even without
> > > > > privileges. Whether those GET_INFO_BY_ID operations should return same
> > > > > full bpf_{prog,map,link,btf}_info or some trimmed down version of them
> > > > > would be up to discussion, but I think getting some info without
> > > > > creating an FD seems useful in itself.
> > > > >
> > > > > Would it be worth discussing and solving this separately from
> > > > > namespacing issues?
> > > >
> > > > Iteration of IDs itself is fine. The set of IDs is not security sensitive,
> > > > but GET_NEXT_BY_ID has to be carefully restricted.
> > > > It returns xlated, jited, BTF, line info, etc
> > > > and with all the restrictions it would need something like
> > > > CAP_SYS_PTRACE and CAP_PERFMON to be useful.
> > > > And with that we're not far from CAP_SYS_ADMIN.
> > > > Why bother then?
> > >
> > > You probably meant that GET_INFO_BY_ID should be carefully restricted?
> >
> > yes.
> >
> > > So yeah, that's what I said that this would have to be discussed
> > > further. I agree that returning func/line info, program dump, etc is
> > > probably a privileged part. But there is plenty of useful info besides
> > > that (e.g., prog name, insns cnt, run stats, etc) that would be useful
> > > for unpriv applications to monitor their own apps that they opened
> > > from BPF FS, or just some observability daemons.
> > >
> > > There is a lot of useful information in bpf_map_info and bpf_link_info
> > > that's way less privileged. I think bpf_link_info is good as is. Same
> > > for bpf_map_info.
> > >
> > > Either way, I'm not insisting, just something that seems pretty simple
> > > to add and useful in some scenarios. We can reuse existing code and
> > > types for GET_INFO_BY_FD and just zero-out (or prevent filling out)
> > > those privileged fields you mentioned. Anyway, something to put on the
> > > backburner, perhaps.
> >
> > Sorry, but I only see negatives. It's an extra code in the kernel
> > that has to be carefully reviewed when initially submitted and
> > then every patch that touches get_info_by_id would have to go
> > through a microscope every time to avoid introducing a security issue.
> > And for what? So that CAP_BPF application can read prog name and run stats?
>
> Per my experience, observability is a very important part for a
> project. If the user can't observe the object directly created by it,
> he will worry about or even mistrust it.

The user can observe the objects just fine. That's what get_info_by_fd is for.
But the kernel will not report JITed instructions to unpriv user who
just loaded a prog and a sole owner of it.
By your definition such a user should not trust the kernel. So be it.