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() syn_recv_sock()/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 '''

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_syn_recv_sock() subflow_ulp_fallback() subflow_drop_ctx() mptcp_subflow_ops_undo_override() '''

Then, this subflow can be normally used by sockmap, which replaces the native sk_prot with sockmap's custom sk_prot. The issue occurs when the user executes accept::mptcp_stream_accept::mptcp_fallback_tcp_ops(). Here, it uses sk->sk_prot to compare with the native sk_prot, but this is incorrect when sockmap is used, as we may incorrectly set sk->sk_socket->ops.

This fix uses the more generic sk_family for the comparison instead.

Additionally, this also prevents a PANIC from occurring:

result from ./scripts/decode_stacktrace.sh: ------------[ cut here ]------------

BUG: kernel NULL pointer dereference, address: 00000000000004bb PGD 0 P4D 0 Oops: 0000 [#1] SMP PTI CPU: 0 PID: 400 Comm: test_progs Not tainted 6.1.0+ #16 RIP: 0010:mptcp_stream_accept (./include/linux/list.h:88 net/mptcp/protocol.c:3719)

RSP: 0018:ffffc90000ef3cf0 EFLAGS: 00010246 RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff8880089dcc58 RDX: 0000000000000003 RSI: 0000002c000000b0 RDI: 0000000000000000 RBP: ffffc90000ef3d38 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880089dc600 R13: ffff88800b859e00 R14: ffff88800638c680 R15: 0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00000000000004bb CR3: 000000000b8e8006 CR4: 0000000000770ef0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 PKRU: 55555554 Call Trace: <TASK> ? apparmor_socket_accept (security/apparmor/lsm.c:966) do_accept (net/socket.c:1856) __sys_accept4 (net/socket.c:1897 net/socket.c:1927) __x64_sys_accept (net/socket.c:1941) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)

Fixes: d2f77c53342e ("mptcp: check for plain TCP sock at accept time") Reviewed-by: Jakub Sitnicki jakub@cloudflare.com Reviewed-by: Matthieu Baerts (NGI0) matttbe@kernel.org Signed-off-by: Jiayuan Chen jiayuan.chen@linux.dev --- net/mptcp/protocol.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index 1dbc62537259..13e3510e6c8f 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -79,8 +79,9 @@ static u64 mptcp_wnd_end(const struct mptcp_sock *msk) static bool mptcp_is_tcpsk(struct sock *sk) { struct socket *sock = sk->sk_socket; + unsigned short family = READ_ONCE(sk->sk_family);

- if (unlikely(sk->sk_prot == &tcp_prot)) { + if (unlikely(family == AF_INET)) { /* we are being invoked after mptcp_accept() has * accepted a non-mp-capable flow: sk is a tcp_sk, * not an mptcp one. @@ -91,7 +92,7 @@ static bool mptcp_is_tcpsk(struct sock *sk) sock->ops = &inet_stream_ops; return true; #if IS_ENABLED(CONFIG_MPTCP_IPV6) - } else if (unlikely(sk->sk_prot == &tcpv6_prot)) { + } else if (unlikely(family == AF_INET6)) { sock->ops = &inet6_stream_ops; return true; #endif