The sockmap feature allows bpf syscall from userspace, or based on bpf
sockops, replacing the sk_prot of sockets during protocol stack processing
with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
  subflow_syn_recv_sock()
    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot
'''
Consider two scenarios:

1. When the server has MPTCP enabled and the client also requests MPTCP,
   the sk passed to the BPF program is a subflow sk. Since subflows only
   handle partial data, replacing their sk_prot is meaningless and will
   cause traffic disruption.

2. When the server has MPTCP enabled but the client sends a TCP SYN
   without MPTCP, subflow_syn_recv_sock() performs a fallback on the
   subflow, replacing the subflow sk's sk_prot with the native sk_prot.
   '''
   subflow_ulp_fallback()
    subflow_drop_ctx()
      mptcp_subflow_ops_undo_override()
   '''
   Subsequently, accept::mptcp_stream_accept::mptcp_fallback_tcp_ops()
   converts the subflow to plain TCP.

For the first case, we should prevent it from being combined with sockmap
by setting sk_prot->psock_update_sk_prot to NULL, which will be blocked by
sockmap's own flow.

For the second case, since subflow_syn_recv_sock() has already restored
sk_prot to native tcp_prot/tcpv6_prot, no further action is needed.

Fixes: d2f77c53342e ("mptcp: check for plain TCP sock at accept time")
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/mptcp/subflow.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 2159b5f9988f..c922bbb12bd8 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1936,6 +1936,10 @@ void __init mptcp_subflow_init(void)
 
 	tcp_prot_override = tcp_prot;
 	tcp_prot_override.release_cb = tcp_release_cb_override;
+#ifdef CONFIG_BPF_SYSCALL
+	/* Disable sockmap processing for subflows */
+	tcp_prot_override.psock_update_sk_prot = NULL;
+#endif
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 	subflow_request_sock_ipv6_ops = tcp_request_sock_ipv6_ops;
@@ -1957,6 +1961,10 @@ void __init mptcp_subflow_init(void)
 
 	tcpv6_prot_override = tcpv6_prot;
 	tcpv6_prot_override.release_cb = tcp_release_cb_override;
+#ifdef CONFIG_BPF_SYSCALL
+	/* Disable sockmap processing for subflows */
+	tcpv6_prot_override.psock_update_sk_prot = NULL;
+#endif
 #endif
 
 	mptcp_diag_subflow_init(&subflow_ulp_ops);
-- 
2.43.0