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

[Dave Tucker]

> On 8 Jun 2023, at 00:53, Andrii Nakryiko <andrii at kernel.org> 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.


Hello! Author of a bpfd[1] here.

> 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.

You could do that… but the problem is created due to the pattern of having a
single binary that is responsible for:

- Loading and attaching the BPF program in question
- Interacting with maps

Let’s set aside some of the other fun concerns of eBPF in containers:
 - Requiring mounting of vmlinux, bpffs, traces etc…
 - How fs permissions on host translate into permissions in containers

While your proposal lets you grant a subset of CAP_BPF to some other process,
which I imagine could also be done with SELinux, it doesn’t stop you from needing
other required permissions for attaching tracing programs in such an
environment. 

For example, say container A wants to attach a uprobe to a process in container B.
Container A needs to be able to nsenter into container B’s pidns in order for attachment
to succeed… but then what I can do with CAP_BPF is the least of my concerns since
I’d wager I’d need to mount `/proc` from the host in container A + have elevated privileges
much scarier than CAP_BPF in the first place.

If you move “Loading and attaching” away to somewhere else (i.e a daemon like bpfd)
then with recent kernels your container workload should be fine to run entirely unprivileged,
or worst case with only CAP_BPF since all you need to do is read/write maps.

Policy control - which process can request to load programs that monitor which other
processes - would happen within this system daemon and you wouldn’t need tokens.

Since it’s easy enough to do this in userspace, I’d be strongly against adding more
complexity into BPF to support this usecase.

> 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/
> 

- Dave

[1]: https://github.com/bpfd-dev/bpfd