[bpf-next,0/2] Fix missing synack in BPF cgroup_skb filters

Message

Kui-Feng Lee June 12, 2023, 7:16 p.m. UTC

TCP SYN/ACK packets of connections from processes/sockets outside a
cgroup on the same host are not received by the cgroup's installed
cgroup_skb filters.

There were two BPF cgroup_skb programs attached to a cgroup named
"my_cgroup".

    SEC("cgroup_skb/ingress")
    int ingress(struct __sk_buff *skb)
    {
        /* .... process skb ... */
        return 1;
    }

    SEC("cgroup_skb/egress")
    int egress(struct __sk_buff *skb)
    {
        /* .... process skb ... */
        return 1;
    
    }

We discovered that when running the command "nc -6 -l 8000" in
"my_group" and connecting to it from outside of "my_cgroup" with the
command "nc -6 localhost 8000", the egress filter did not detect the
SYN/ACK packet. However, we did observe the SYN/ACK packet at the
ingress when connecting from a socket in "my_cgroup" to a socket
outside of it.

We came across BPF_CGROUP_RUN_PROG_INET_EGRESS(). This macro is
responsible for calling BPF programs that are attached to the egress
hook of a cgroup and it skips programs if the sending socket is not the
owner of the sk_buff. Specifically, in our situation, the SYN/ACK
sk_buff is owned by a struct request_sock instance, but the sending
socket is the listener socket we use to receive incoming
connections. The request_sock is created to manage an incoming
connection.

It has been determined that checking the owner of a sk_buff against
the sending socket is not required. Removing this check will allow the
filters to receive SYN/ACK packets.

To ensure that cgroup_skb filters can receive all signaling packets,
including SYN, SYN/ACK, ACK, FIN, and FIN/ACK. A new self-test has
been added as well.

Kui-Feng Lee (2):
  net: bpf: Always call BPF cgroup filters for egress.
  selftests/bpf: Verify that the cgroup_skb filters receive expected
    packets.

 include/linux/bpf-cgroup.h                    |   2 +-
 tools/testing/selftests/bpf/cgroup_helpers.c  |  12 +
 tools/testing/selftests/bpf/cgroup_helpers.h  |   1 +
 tools/testing/selftests/bpf/cgroup_tcp_skb.h  |  35 ++
 .../selftests/bpf/prog_tests/cgroup_tcp_skb.c | 363 ++++++++++++++++++
 .../selftests/bpf/progs/cgroup_tcp_skb.c      | 340 ++++++++++++++++
 6 files changed, 752 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/cgroup_tcp_skb.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/cgroup_tcp_skb.c
 create mode 100644 tools/testing/selftests/bpf/progs/cgroup_tcp_skb.c