[PATCH bpf v2 0/6] Fix attach / detach uapi for sockmap and flow_dissector

[Lorenz Bauer <lmb@cloudflare.com>]

Both sockmap and flow_dissector ingnore various arguments passed to
BPF_PROG_ATTACH and BPF_PROG_DETACH. We can fix the attach case by
checking that the unused arguments are zero. I considered requiring
target_fd to be -1 instead of 0, but this leads to a lot of churn
in selftests. There is also precedent in that bpf_iter already
expects 0 for a similar field. I think that we can come up with a
work around for fd 0 should we need to in the future.

The detach case is more problematic: both cgroups and lirc2 verify
that attach_bpf_fd matches the currently attached program. This
way you need access to the program fd to be able to remove it.
Neither sockmap nor flow_dissector do this. flow_dissector even
has a check for CAP_NET_ADMIN because of this. The patch set
addresses this by implementing the desired behaviour.

There is a possibility for user space breakage: any callers that
don't provide the correct fd will fail with ENOENT. For sockmap
the risk is low: even the selftests assume that sockmap works
the way I described. For flow_dissector the story is less
straightforward, and the selftests use a variety of arguments.

I've includes fixes tags for the oldest commits that allow an easy
backport, however the behaviour dates back to when sockmap and
flow_dissector were introduced. What is the best way to handle these?

This set is based on top of Jakub's work "bpf, netns: Prepare
for multi-prog attachment" available at
https://lore.kernel.org/bpf/87k0zwmhtb.fsf@cloudflare.com/T/

Since v1:
- Adjust selftests
- Implement detach behaviour

Lorenz Bauer (6):
  bpf: flow_dissector: check value of unused flags to BPF_PROG_ATTACH
  bpf: flow_dissector: check value of unused flags to BPF_PROG_DETACH
  bpf: sockmap: check value of unused args to BPF_PROG_ATTACH
  bpf: sockmap: require attach_bpf_fd when detaching a program
  selftests: bpf: pass program and target_fd in flow_dissector_reattach
  selftests: bpf: pass program to bpf_prog_detach in flow_dissector

 include/linux/bpf-netns.h                     |  5 +-
 include/linux/bpf.h                           | 13 ++++-
 include/linux/skmsg.h                         | 13 +++++
 kernel/bpf/net_namespace.c                    | 22 ++++++--
 kernel/bpf/syscall.c                          |  6 +--
 net/core/sock_map.c                           | 53 +++++++++++++++++--
 .../selftests/bpf/prog_tests/flow_dissector.c |  4 +-
 .../bpf/prog_tests/flow_dissector_reattach.c  | 12 ++---
 8 files changed, 103 insertions(+), 25 deletions(-)

-- 
2.25.1