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

[Casey Schaufler <casey@schaufler-ca.com>]

On 6/5/2023 1:41 PM, Andrii Nakryiko wrote:
> On Fri, Jun 2, 2023 at 8:55 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
>> On 6/2/2023 7:59 AM, Andrii Nakryiko wrote:
>>> *Resending with trimmed CC list because original version didn't make it to
>>> the mailing list.*
>>>
>>> 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.
>> Token based privilege has a number of well understood weaknesses,
>> none of which I see addressed here. I also have a real problem with
> Can you please provide some more details about those weaknesses? Hard
> to respond without knowing exactly what we are talking about.

Privileged Process (PP) sends a Token to Trusted Process (TP).
TP sends the Token along to Untrusted Process, which performs nefarious
deeds.

Privileged Process (PP) sends a Token to Trusted Process (TP).
TP uses Token, and then saves it in its toolbox. PP later sends
TP a different Token. TP realizes that with the combination of
Tokens it now has it can do considerably more than what PP
intended in either of the cases it sent Token for. TP performs
nefarious deeds.

Granted, in both cases TP does not deserve to be trusted. 
Because TP does not run with privilege of its own, it is not
treated with the same level of caution as it would be if it did.

Privileged Process (PP) sends a Token to what it thinks is a Trusted
Process (TP) but is in fact an Imposter Process (IP) that has been
enabled on the system using any number of K33L techniques.

I don't see anything that ensures that PP communicates Tokens only
to TP, nor any criteria for "trust" are met.

Those are the issues I'm most familiar with, although I believe
there are others.

>> the notion of "trusted unprivileged" where trust is established by
>> a user space application. Ignoring the possibility of malicious code
>> for the moment, the opportunity for accidental privilege leakage is
>> huge. It would be trivial (and tempting) to create a privileged BPF
>> "shell" that would then be allowed to "trust" any application and
>> run it with privilege by passing it a token.
> Right now most BPF applications are running as real root in
> production. Users have to trust such applications to not do anything
> bad with their full root capabilities. How it is done depends on
> specific production and organizational setups, and could be code
> reviewing, audits, LSM, etc. So in that sense BPF token doesn't make
> things worse. And it actually allows us to improve the situation by
> creating and sharing more restrictive BPF tokens that limit what bpf()
> syscall parts are allowed to be used.
>
>>> 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/
>>>
>>> Andrii Nakryiko (18):
>>>   bpf: introduce BPF token object
>>>   libbpf: add bpf_token_create() API
>>>   selftests/bpf: add BPF_TOKEN_CREATE test
>>>   bpf: move unprivileged checks into map_create() and bpf_prog_load()
>>>   bpf: inline map creation logic in map_create() function
>>>   bpf: centralize permissions checks for all BPF map types
>>>   bpf: add BPF token support to BPF_MAP_CREATE command
>>>   libbpf: add BPF token support to bpf_map_create() API
>>>   selftests/bpf: add BPF token-enabled test for BPF_MAP_CREATE command
>>>   bpf: add BPF token support to BPF_BTF_LOAD command
>>>   libbpf: add BPF token support to bpf_btf_load() API
>>>   selftests/bpf: add BPF token-enabled BPF_BTF_LOAD selftest
>>>   bpf: keep BPF_PROG_LOAD permission checks clear of validations
>>>   bpf: add BPF token support to BPF_PROG_LOAD command
>>>   bpf: take into account BPF token when fetching helper protos
>>>   bpf: consistenly use BPF token throughout BPF verifier logic
>>>   libbpf: add BPF token support to bpf_prog_load() API
>>>   selftests/bpf: add BPF token-enabled BPF_PROG_LOAD tests
>>>
>>>  drivers/media/rc/bpf-lirc.c                   |   2 +-
>>>  include/linux/bpf.h                           |  66 ++-
>>>  include/linux/filter.h                        |   2 +-
>>>  include/uapi/linux/bpf.h                      |  74 +++
>>>  kernel/bpf/Makefile                           |   2 +-
>>>  kernel/bpf/arraymap.c                         |   2 +-
>>>  kernel/bpf/bloom_filter.c                     |   3 -
>>>  kernel/bpf/bpf_local_storage.c                |   3 -
>>>  kernel/bpf/bpf_struct_ops.c                   |   3 -
>>>  kernel/bpf/cgroup.c                           |   6 +-
>>>  kernel/bpf/core.c                             |   3 +-
>>>  kernel/bpf/cpumap.c                           |   4 -
>>>  kernel/bpf/devmap.c                           |   3 -
>>>  kernel/bpf/hashtab.c                          |   6 -
>>>  kernel/bpf/helpers.c                          |   6 +-
>>>  kernel/bpf/inode.c                            |  26 ++
>>>  kernel/bpf/lpm_trie.c                         |   3 -
>>>  kernel/bpf/queue_stack_maps.c                 |   4 -
>>>  kernel/bpf/reuseport_array.c                  |   3 -
>>>  kernel/bpf/stackmap.c                         |   3 -
>>>  kernel/bpf/syscall.c                          | 429 ++++++++++++++----
>>>  kernel/bpf/token.c                            | 141 ++++++
>>>  kernel/bpf/verifier.c                         |  13 +-
>>>  kernel/trace/bpf_trace.c                      |   2 +-
>>>  net/core/filter.c                             |  36 +-
>>>  net/core/sock_map.c                           |   4 -
>>>  net/ipv4/bpf_tcp_ca.c                         |   2 +-
>>>  net/netfilter/nf_bpf_link.c                   |   2 +-
>>>  net/xdp/xskmap.c                              |   4 -
>>>  tools/include/uapi/linux/bpf.h                |  74 +++
>>>  tools/lib/bpf/bpf.c                           |  32 +-
>>>  tools/lib/bpf/bpf.h                           |  24 +-
>>>  tools/lib/bpf/libbpf.map                      |   1 +
>>>  .../selftests/bpf/prog_tests/libbpf_probes.c  |   4 +
>>>  .../selftests/bpf/prog_tests/libbpf_str.c     |   6 +
>>>  .../testing/selftests/bpf/prog_tests/token.c  | 282 ++++++++++++
>>>  .../bpf/prog_tests/unpriv_bpf_disabled.c      |   6 +-
>>>  37 files changed, 1098 insertions(+), 188 deletions(-)
>>>  create mode 100644 kernel/bpf/token.c
>>>  create mode 100644 tools/testing/selftests/bpf/prog_tests/token.c
>>>