netdev - [PATCH net] mptcp: prevent BPF accessing lowat from a subflow socket.

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Message-ID: <d8cb7d8476d66cb0812a6e29cd1e626869d9d53e.1711738080.git.pabeni@redhat.com>
Date: Fri, 29 Mar 2024 19:50:36 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: netdev@...r.kernel.org
Cc: Matthieu Baerts <matttbe@...nel.org>,
	Mat Martineau <martineau@...nel.org>,
	Geliang Tang <geliang@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	mptcp@...ts.linux.dev,
	bpf@...r.kernel.org
Subject: [PATCH net] mptcp: prevent BPF accessing lowat from a subflow socket.

Alexei reported the following splat:

 WARNING: CPU: 32 PID: 3276 at net/mptcp/subflow.c:1430 subflow_data_ready+0x147/0x1c0
 Modules linked in: dummy bpf_testmod(O) [last unloaded: bpf_test_no_cfi(O)]
 CPU: 32 PID: 3276 Comm: test_progs Tainted: GO       6.8.0-12873-g2c43c33bfd23
 Call Trace:
  <TASK>
  mptcp_set_rcvlowat+0x79/0x1d0
  sk_setsockopt+0x6c0/0x1540
  __bpf_setsockopt+0x6f/0x90
  bpf_sock_ops_setsockopt+0x3c/0x90
  bpf_prog_509ce5db2c7f9981_bpf_test_sockopt_int+0xb4/0x11b
  bpf_prog_dce07e362d941d2b_bpf_test_socket_sockopt+0x12b/0x132
  bpf_prog_348c9b5faaf10092_skops_sockopt+0x954/0xe86
  __cgroup_bpf_run_filter_sock_ops+0xbc/0x250
  tcp_connect+0x879/0x1160
  tcp_v6_connect+0x50c/0x870
  mptcp_connect+0x129/0x280
  __inet_stream_connect+0xce/0x370
  inet_stream_connect+0x36/0x50
  bpf_trampoline_6442491565+0x49/0xef
  inet_stream_connect+0x5/0x50
  __sys_connect+0x63/0x90
  __x64_sys_connect+0x14/0x20

The root cause of the issue is that bpf allows accessing mptcp-level
proto_ops from a tcp subflow scope.

Fix the issue detecting the problematic call and preventing any action.

Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/482
Fixes: 5684ab1a0eff ("mptcp: give rcvlowat some love")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 net/mptcp/sockopt.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index dcd1c76d2a3b..73fdf423de44 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -1493,6 +1493,10 @@ int mptcp_set_rcvlowat(struct sock *sk, int val)
 	struct mptcp_subflow_context *subflow;
 	int space, cap;
 
+	/* bpf can land here with a wrong sk type */
+	if (sk->sk_protocol == IPPROTO_TCP)
+		return -EINVAL;
+
 	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
 		cap = sk->sk_rcvbuf >> 1;
 	else
-- 
2.43.2