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

[Stanislav Fomichev <sdf@google.com>]

On Tue, Mar 28, 2023 at 8:03 PM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Wed, Mar 29, 2023 at 1:15 AM Stanislav Fomichev <sdf@google.com> wrote:
> >
> > On 03/28, Yafang Shao wrote:
> > > On Tue, Mar 28, 2023 at 1:28 AM Stanislav Fomichev <sdf@google.com> wrote:
> > > >
> > > > On 03/26, Yafang Shao wrote:
> > > > > Currently only CAP_SYS_ADMIN can iterate BPF object IDs and convert
> > > IDs
> > > > > to FDs, that's intended for BPF's security model[1]. Not only does it
> > > > > prevent non-privilidged users from getting other users' bpf program,
> > > but
> > > > > also it prevents the user from iterating his own bpf objects.
> > > >
> > > > > In container environment, some users want to run bpf programs in their
> > > > > containers. These users can run their bpf programs under CAP_BPF and
> > > > > some other specific CAPs, but they can't inspect their bpf programs
> > > in a
> > > > > generic way. For example, the bpftool can't be used as it requires
> > > > > CAP_SYS_ADMIN. That is very inconvenient.
> > > >
> > > > > Without CAP_SYS_ADMIN, the only way to get the information of a bpf
> > > object
> > > > > which is not created by the process itself is with SCM_RIGHTS, that
> > > > > requires each processes which created bpf object has to implement a
> > > unix
> > > > > domain socket to share the fd of a bpf object between different
> > > > > processes, that is really trivial and troublesome.
> > > >
> > > > > Hence we need a better mechanism to get bpf object info without
> > > > > CAP_SYS_ADMIN.
> > > >
> > > > [..]
> > > >
> > > > > BPF namespace is introduced in this patchset with an attempt to remove
> > > > > the CAP_SYS_ADMIN requirement. The user can create bpf map, prog and
> > > > > link in a specific bpf namespace, then these bpf objects will not be
> > > > > visible to the users in a different bpf namespace. But these bpf
> > > > > objects are visible to its parent bpf namespace, so the sys admin can
> > > > > still iterate and inspect them.
> > > >
> > > > Does it essentially mean unpriv bpf?
> >
> > > Right. With CAP_BPF and some other CAPs enabled.
> >
> > > > Can I, as a non-root, create
> > > > a new bpf namespace and start loading/attaching progs?
> >
> > > No, you can't create a new bpf namespace as a non-root, see also
> > > copy_namespaces().
> > > In the container environment, new namespaces are always created by
> > > containered, which is started by root.
> >
> > Are you talking about "if (!ns_capable(user_ns, CAP_SYS_ADMIN))" part
> > from copy_namespaces? Isn't it trivially bypassed with a new user
> > namespace?
> >
> > IIUC, I can create a new user namespace which gives me CAP_SYS_ADMIN
> > in this particular user-ns. Then I can go on and create a new bpf
> > namespace (with CAP_BPF) and go wild? I won't see anything from the
> > other namespaces, but I'll be able to load/attach bpf programs?
> >
>
> I don't think so. If you create a new userspace, and give the process
> the CAP_BPF or CAP_SYS_ADMIN in this new user namespace but not the
> initial namespace, you can't do that. Because currently only CAP_BPF
> or CAP_SYS_ADMIN in the init user namespace can load/attach bpf
> programs.
>
> > > > Maybe add a paragraph about now vs whatever you're proposing.
> >
> > > What I'm proposing in this patchset is to put bpf objects (map, prog,
> > > link, and btf) into the bpf namespace. Next step I will put bpffs into
> > > the bpf namespace as well.
> > > That said, I'm trying to put  all the objects created in bpf into the
> > > bpf namespace. Below is a simple paragraph to illustrate it.
> >
> > > Regarding the unpriv user with CAP_BPF enabled,
> > >                                                               Now | Future
> > > ------------------------------------------------------------------------
> > > Iterate his BPF IDs                                | N   |  Y  |
> > > Iterate others' BPF IDs                          | N   |  N  |
> > > Convert his BPF IDs to FDs                  | N   |  Y  |
> > > Convert others' BPF IDs to FDs            | N   |  N  |
> > > Get others' object info from pinned file  | Y(*) | N  |
> > > ------------------------------------------------------------------------
> >
> > > (*) It can be improved by,
> > >       1). Different containers has different bpffs
> > >       2). Setting file permission
> > >       That's not perfect, for example, if one single user has two bpf
> > > instances, but we don't want them to inspect each other.
> >
> > I think the question here is what happens to the existing
> > capable(CAP_BPF) checks? Do they become ns_capable(CAP_BPF) eventually?
> >
>
> They won't become ns_capable(CAP_BPF). If it becomes
> ns_capable(CAP_BPF), it will really go wild then.
>
> > And if not, I don't think it integrates well with the user namespaces?
> >
>
> IIUC, it is the CAP_BPF which doesn't integrate with the user
> namespaces, right?

Yeah. And it's probably fine if we don't, I just wanted to see some
explanation on the long-term plan.
If the purpose is to have a bpf namespace and use it for pure
isolation purposes, let's state it clearly in the cover letter.
Otherwise it's not clear whether it's only about isolation or
potentially allowing CAP_BPF in user namespaces.
Maybe clone(CLONE_NEWBPF|CLONE_NEWUSER) should return an explicit
error? (or maybe it already does, haven't looked at the patches)

One other question I have is: should init bpf namespace see
everything? Otherwise it might be hard to chase down the namespaces
that loaded some BPF program...




> --
> Regards
> Yafang