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

Stanislav Fomichev
2023-06-08T18:49:14Z

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)".

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))

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.