bpf, capability: Introduce CAP_BPF

Split BPF operations that are allowed under CAP_SYS_ADMIN into
combination of CAP_BPF, CAP_PERFMON, CAP_NET_ADMIN.
For backward compatibility include them in CAP_SYS_ADMIN as well.

The end result provides simple safety model for applications that use BPF:
- to load tracing program types
  BPF_PROG_TYPE_{KPROBE, TRACEPOINT, PERF_EVENT, RAW_TRACEPOINT, etc}
  use CAP_BPF and CAP_PERFMON
- to load networking program types
  BPF_PROG_TYPE_{SCHED_CLS, XDP, SK_SKB, etc}
  use CAP_BPF and CAP_NET_ADMIN

There are few exceptions from this rule:
- bpf_trace_printk() is allowed in networking programs, but it's using
  tracing mechanism, hence this helper needs additional CAP_PERFMON
  if networking program is using this helper.
- BPF_F_ZERO_SEED flag for hash/lru map is allowed under CAP_SYS_ADMIN only
  to discourage production use.
- BPF HW offload is allowed under CAP_SYS_ADMIN.
- bpf_probe_write_user() is allowed under CAP_SYS_ADMIN only.

CAPs are not checked at attach/detach time with two exceptions:
- loading BPF_PROG_TYPE_CGROUP_SKB is allowed for unprivileged users,
  hence CAP_NET_ADMIN is required at attach time.
- flow_dissector detach doesn't check prog FD at detach,
  hence CAP_NET_ADMIN is required at detach time.

CAP_SYS_ADMIN is required to iterate BPF objects (progs, maps, links) via get_next_id
command and convert them to file descriptor via GET_FD_BY_ID command.
This restriction guarantees that mutliple tasks with CAP_BPF are not able to
affect each other. That leads to clean isolation of tasks. For example:
task A with CAP_BPF and CAP_NET_ADMIN loads and attaches a firewall via bpf_link.
task B with the same capabilities cannot detach that firewall unless
task A explicitly passed link FD to task B via scm_rights or bpffs.
CAP_SYS_ADMIN can still detach/unload everything.

Two networking user apps with CAP_SYS_ADMIN and CAP_NET_ADMIN can
accidentely mess with each other programs and maps.
Two networking user apps with CAP_NET_ADMIN and CAP_BPF cannot affect each other.

CAP_NET_ADMIN + CAP_BPF allows networking programs access only packet data.
Such networking progs cannot access arbitrary kernel memory or leak pointers.

bpftool, bpftrace, bcc tools binaries should NOT be installed with
CAP_BPF and CAP_PERFMON, since unpriv users will be able to read kernel secrets.
But users with these two permissions will be able to use these tracing tools.

CAP_PERFMON is least secure, since it allows kprobes and kernel memory access.
CAP_NET_ADMIN can stop network traffic via iproute2.
CAP_BPF is the safest from security point of view and harmless on its own.

Having CAP_BPF and/or CAP_NET_ADMIN is not enough to write into arbitrary map
and if that map is used by firewall-like bpf prog.
CAP_BPF allows many bpf prog_load commands in parallel. The verifier
may consume large amount of memory and significantly slow down the system.

Existing unprivileged BPF operations are not affected.
In particular unprivileged users are allowed to load socket_filter and cg_skb
program types and to create array, hash, prog_array, map-in-map map types.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200513230355.7858-2-alexei.starovoitov@gmail.com

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