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@xxxxxxxxxx/ > [1] http://vger.kernel.org/bpfconf2023_material/Trusted_unprivileged_BPF_LSFMM2023.pdf > [2] https://lore.kernel.org/bpf/20190627201923.2589391-2-songliubraving@xxxxxx/ > > 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.
