Re: [PATCH v2 bpf-next 00/18] BPF token

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Jun 8, 2023 at 11:49 AM Stanislav Fomichev <sdf@google.com> wrote:
>
> On 06/07, Andrii Nakryiko wrote:
> > This patch set introduces new BPF object, BPF token, which allows to delegate
> > a subset of BPF functionality from privileged system-wide daemon (e.g.,
> > systemd or any other container manager) to a *trusted* unprivileged
> > application. Trust is the key here. This functionality is not about allowing
> > unconditional unprivileged BPF usage. Establishing trust, though, is
> > completely up to the discretion of respective privileged application that
> > would create a BPF token.
> >
> > The main motivation for BPF token is a desire to enable containerized
> > BPF applications to be used together with user namespaces. This is currently
> > impossible, as CAP_BPF, required for BPF subsystem usage, cannot be namespaced
> > or sandboxed, as a general rule. E.g., tracing BPF programs, thanks to BPF
> > helpers like bpf_probe_read_kernel() and bpf_probe_read_user() can safely read
> > arbitrary memory, and it's impossible to ensure that they only read memory of
> > processes belonging to any given namespace. This means that it's impossible to
> > have namespace-aware CAP_BPF capability, and as such another mechanism to
> > allow safe usage of BPF functionality is necessary. BPF token and delegation
> > of it to a trusted unprivileged applications is such mechanism. Kernel makes
> > no assumption about what "trusted" constitutes in any particular case, and
> > it's up to specific privileged applications and their surrounding
> > infrastructure to decide that. What kernel provides is a set of APIs to create
> > and tune BPF token, and pass it around to privileged BPF commands that are
> > creating new BPF objects like BPF programs, BPF maps, etc.
> >
> > Previous attempt at addressing this very same problem ([0]) attempted to
> > utilize authoritative LSM approach, but was conclusively rejected by upstream
> > LSM maintainers. BPF token concept is not changing anything about LSM
> > approach, but can be combined with LSM hooks for very fine-grained security
> > policy. Some ideas about making BPF token more convenient to use with LSM (in
> > particular custom BPF LSM programs) was briefly described in recent LSF/MM/BPF
> > 2023 presentation ([1]). E.g., an ability to specify user-provided data
> > (context), which in combination with BPF LSM would allow implementing a very
> > dynamic and fine-granular custom security policies on top of BPF token. In the
> > interest of minimizing API surface area discussions this is going to be
> > added in follow up patches, as it's not essential to the fundamental concept
> > of delegatable BPF token.
> >
> > It should be noted that BPF token is conceptually quite similar to the idea of
> > /dev/bpf device file, proposed by Song a while ago ([2]). The biggest
> > difference is the idea of using virtual anon_inode file to hold BPF token and
> > allowing multiple independent instances of them, each with its own set of
> > restrictions. BPF pinning solves the problem of exposing such BPF token
> > through file system (BPF FS, in this case) for cases where transferring FDs
> > over Unix domain sockets is not convenient. And also, crucially, BPF token
> > approach is not using any special stateful task-scoped flags. Instead, bpf()
> > syscall accepts token_fd parameters explicitly for each relevant BPF command.
> > This addresses main concerns brought up during the /dev/bpf discussion, and
> > fits better with overall BPF subsystem design.
> >
> > This patch set adds a basic minimum of functionality to make BPF token useful
> > and to discuss API and functionality. Currently only low-level libbpf APIs
> > support passing BPF token around, allowing to test kernel functionality, but
> > for the most part is not sufficient for real-world applications, which
> > typically use high-level libbpf APIs based on `struct bpf_object` type. This
> > was done with the intent to limit the size of patch set and concentrate on
> > mostly kernel-side changes. All the necessary plumbing for libbpf will be sent
> > as a separate follow up patch set kernel support makes it upstream.
> >
> > Another part that should happen once kernel-side BPF token is established, is
> > a set of conventions between applications (e.g., systemd), tools (e.g.,
> > bpftool), and libraries (e.g., libbpf) about sharing BPF tokens through BPF FS
> > at well-defined locations to allow applications take advantage of this in
> > automatic fashion without explicit code changes on BPF application's side.
> > But I'd like to postpone this discussion to after BPF token concept lands.
> >
> >   [0] https://lore.kernel.org/bpf/20230412043300.360803-1-andrii@kernel.org/
> >   [1] http://vger.kernel.org/bpfconf2023_material/Trusted_unprivileged_BPF_LSFMM2023.pdf
> >   [2] https://lore.kernel.org/bpf/20190627201923.2589391-2-songliubraving@fb.com/
> >
> > v1->v2:
> >   - fix build failures on Kconfig with CONFIG_BPF_SYSCALL unset;
> >   - drop BPF_F_TOKEN_UNKNOWN_* flags and simplify UAPI (Stanislav).
>
> I went through v2, everything makes sense, the only thing that is
> slightly confusing to me is the bpf_token_capable() call.
> The name somehow implies that the token is capable of something
> where in reality the function does "return token || capable(x)".

heh, "bpf_token_" part is sort of like namespace/object prefix. The
intent here was to have a token-aware capable check. And yes, if we
get a token during prog/map/etc construction, the assumption is that
it provides all relevant permissions.

>
> IMO, it would be less confusing if we do something like the following,
> explicitly, instead of calling a function:
>
> if (token || {bpf_,perfmon_,}capable(x)) ...
>
> (or rename to something like bpf_token_or_capable(x))

I'd rather not open-code `if (token || ...)` checks everywhere, but I
can rename to `bpf_token_or_capable()` if people prefer. I erred on
the side of succinctness, but if it's confusing, then best to rename?

>
> Up to you on whether to take any action on that. OTOH, once you
> grasp what bpf_token_capable really does, it's not really a problem.

Cool, thanks for taking a look!