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charset="utf-8"; x-default="true" Syzkaller reports a simult-connect race leading to inconsistent fallback status: WARNING: CPU: 3 PID: 33 at net/mptcp/subflow.c:1515 subflow_data_ready+0x40= b/0x7c0 net/mptcp/subflow.c:1515 Modules linked in: CPU: 3 UID: 0 PID: 33 Comm: ksoftirqd/3 Not tainted syzkaller #0 PREEMPT(fu= ll) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16= .3-2~bpo12+1 04/01/2014 RIP: 0010:subflow_data_ready+0x40b/0x7c0 net/mptcp/subflow.c:1515 Code: 89 ee e8 78 61 3c f6 40 84 ed 75 21 e8 8e 66 3c f6 44 89 fe bf 07 00 = 00 00 e8 c1 61 3c f6 41 83 ff 07 74 09 e8 76 66 3c f6 90 <0f> 0b 90 e8 6d 6= 6 3c f6 48 89 df e8 e5 ad ff ff 31 ff 89 c5 89 c6 RSP: 0018:ffffc900006cf338 EFLAGS: 00010246 RAX: 0000000000000000 RBX: ffff888031acd100 RCX: ffffffff8b7f2abf RDX: ffff88801e6ea440 RSI: ffffffff8b7f2aca RDI: 0000000000000005 RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000007 R10: 0000000000000004 R11: 0000000000002c10 R12: ffff88802ba69900 R13: 1ffff920000d9e67 R14: ffff888046f81800 R15: 0000000000000004 FS: 0000000000000000(0000) GS:ffff8880d69bc000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000560fc0ca1670 CR3: 0000000032c3a000 CR4: 0000000000352ef0 Call Trace: tcp_data_queue+0x13b0/0x4f90 net/ipv4/tcp_input.c:5197 tcp_rcv_state_process+0xfdf/0x4ec0 net/ipv4/tcp_input.c:6922 tcp_v6_do_rcv+0x492/0x1740 net/ipv6/tcp_ipv6.c:1672 tcp_v6_rcv+0x2976/0x41e0 net/ipv6/tcp_ipv6.c:1918 ip6_protocol_deliver_rcu+0x188/0x1520 net/ipv6/ip6_input.c:438 ip6_input_finish+0x1e4/0x4b0 net/ipv6/ip6_input.c:489 NF_HOOK include/linux/netfilter.h:318 [inline] NF_HOOK include/linux/netfilter.h:312 [inline] ip6_input+0x105/0x2f0 net/ipv6/ip6_input.c:500 dst_input include/net/dst.h:471 [inline] ip6_rcv_finish net/ipv6/ip6_input.c:79 [inline] NF_HOOK include/linux/netfilter.h:318 [inline] NF_HOOK include/linux/netfilter.h:312 [inline] ipv6_rcv+0x264/0x650 net/ipv6/ip6_input.c:311 __netif_receive_skb_one_core+0x12d/0x1e0 net/core/dev.c:5979 __netif_receive_skb+0x1d/0x160 net/core/dev.c:6092 process_backlog+0x442/0x15e0 net/core/dev.c:6444 __napi_poll.constprop.0+0xba/0x550 net/core/dev.c:7494 napi_poll net/core/dev.c:7557 [inline] net_rx_action+0xa9f/0xfe0 net/core/dev.c:7684 handle_softirqs+0x216/0x8e0 kernel/softirq.c:579 run_ksoftirqd kernel/softirq.c:968 [inline] run_ksoftirqd+0x3a/0x60 kernel/softirq.c:960 smpboot_thread_fn+0x3f7/0xae0 kernel/smpboot.c:160 kthread+0x3c2/0x780 kernel/kthread.c:463 ret_from_fork+0x5d7/0x6f0 arch/x86/kernel/process.c:148 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245 The TCP subflow can process the simult-connect syn-ack packet after transitioning to TCP_FIN1 state, bypassing the MPTCP fallback check, as the sk_state_change() callback is not invoked for * -> FIN_WAIT1 transitions. That will move the msk socket to an inconsistent status and the next incoming data will hit the reported splat. Close the race moving the simult-fallback check at the earliest possible stage - that is at syn-ack generation time. Fixes: 23e89e8ee7be ("tcp: Don't drop SYN+ACK for simultaneous connect().") Fixes: 4fd19a307016 ("mptcp: fix inconsistent state on fastopen race") Fixes: 1e777f39b4d7 ("mptcp: add MSG_FASTOPEN sendmsg flag support") Reported-by: syzbot+0ff6b771b4f7a5bce83b@syzkaller.appspotmail.com Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/586 Signed-off-by: Paolo Abeni --- v1 -> v2: - re-add is_active_ssk(). MPJ subflow would likely still need more love to handle simult connect correctly, but let's not mix-up things. Note: I kept the subflow_simultaneous_connect() helper around because its name itself is/should be well self-documenting. simult connect pkt drill test will need a paired change; not reporting the full diff to avoid confusing the patch importer: +0 > S 0:0(0) +0 < S 0:0(0) win 1000 -+0 > S. 0:0(0) ack 1 -+0 < S. 0:0(0) ack 1 win 65535 ++0 > S. 0:0(0) ack 1 ++0 < S. 0:0(0) ack 1 win 65535 +0 > . 1:1(0) ack 1 --- net/mptcp/options.c | 10 ++++++++++ net/mptcp/protocol.h | 6 ++---- net/mptcp/subflow.c | 6 ------ 3 files changed, 12 insertions(+), 10 deletions(-) diff --git a/net/mptcp/options.c b/net/mptcp/options.c index ff2b9fc7c01f..ac16e4bd496f 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -408,6 +408,16 @@ bool mptcp_syn_options(struct sock *sk, const struct s= k_buff *skb, */ subflow->snd_isn =3D TCP_SKB_CB(skb)->end_seq; if (subflow->request_mptcp) { + if (unlikely(subflow_simultaneous_connect(sk))) { + WARN_ON_ONCE(!mptcp_try_fallback(sk, MPTCP_MIB_SIMULTCONNFALLBACK)); + + /* Ensure mptcp_finish_connect() will not process the + * MPC handshake. + */ + subflow->request_mptcp =3D 0; + return false; + } + opts->suboptions =3D OPTION_MPTCP_MPC_SYN; opts->csum_reqd =3D mptcp_is_checksum_enabled(sock_net(sk)); opts->allow_join_id0 =3D mptcp_allow_join_id0(sock_net(sk)); diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h index bc470254bd6b..41d80df1ba3e 100644 --- a/net/mptcp/protocol.h +++ b/net/mptcp/protocol.h @@ -1334,10 +1334,8 @@ static inline bool subflow_simultaneous_connect(stru= ct sock *sk) { struct mptcp_subflow_context *subflow =3D mptcp_subflow_ctx(sk); =20 - return (1 << sk->sk_state) & - (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_FIN_WAIT2 | TCPF_CLOSING= ) && - is_active_ssk(subflow) && - !subflow->conn_finished; + /* Note that the sk state implies !subflow->conn_finished. */ + return sk->sk_state =3D=3D TCP_SYN_RECV && is_active_ssk(subflow); } =20 #ifdef CONFIG_SYN_COOKIES diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c index 86ce58ae533d..96d54cb2cd93 100644 --- a/net/mptcp/subflow.c +++ b/net/mptcp/subflow.c @@ -1878,12 +1878,6 @@ static void subflow_state_change(struct sock *sk) =20 __subflow_state_change(sk); 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Wed, 3 Dec 2025 18:24:45 +0000 (UTC) Received: from gerbillo.redhat.com (unknown [10.45.224.2]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id CE33E19560A7 for ; Wed, 3 Dec 2025 18:24:44 +0000 (UTC) From: Paolo Abeni To: mptcp@lists.linux.dev Subject: [PATCH mptcp-net v2 2/2] mptcp: ensure context reset on disconnect() Date: Wed, 3 Dec 2025 19:24:35 +0100 Message-ID: <7789f18691733c076dcc69321963080d30ca27b9.1764786151.git.pabeni@redhat.com> In-Reply-To: References: Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: jpFVGeyMHH71K7iBQm8o_YEpqTXmnbu5-x9xWyWiIG4_1764786286 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8"; x-default="true" After the blamed commit below, if the MPC subflow is already in TCP_CLOSE status at mptcp_disconnect() time, mptcp_do_fastclose() skips setting the `send_fastclose flag` and the later __mptcp_close_ssk() does not reset anymore the related subflow context. Any later connection will be created with both the `request_mptcp` flag and the msk-level fallback status off, leading to a warning in subflow_data_ready(): WARNING: CPU: 26 PID: 8996 at net/mptcp/subflow.c:1519 subflow_data_ready = (net/mptcp/subflow.c:1519 (discriminator 13)) Modules linked in: CPU: 26 UID: 0 PID: 8996 Comm: syz.22.39 Not tainted 6.18.0-rc7-05427-g11f= c074f6c36 #1 PREEMPT(voluntary) Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 RIP: 0010:subflow_data_ready (net/mptcp/subflow.c:1519 (discriminator 13)) Code: 90 0f 0b 90 90 e9 04 fe ff ff e8 b7 1e f5 fe 89 ee bf 07 00 00 00 e8= db 19 f5 fe 83 fd 07 0f 84 35 ff ff ff e8 9d 1e f5 fe 90 <0f> 0b 90 e9 27 = ff ff ff e8 8f 1e f5 fe 4c 89 e7 48 89 de e8 14 09 RSP: 0018:ffffc9002646fb30 EFLAGS: 00010293 RAX: 0000000000000000 RBX: ffff88813b218000 RCX: ffffffff825c8435 RDX: ffff8881300b3580 RSI: ffffffff825c8443 RDI: 0000000000000005 RBP: 000000000000000b R08: ffffffff825c8435 R09: 000000000000000b R10: 0000000000000005 R11: 0000000000000007 R12: ffff888131ac0000 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 FS: 00007f88330af6c0(0000) GS:ffff888a93dd2000(0000) knlGS:00000000000000= 00 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f88330aefe8 CR3: 000000010ff59000 CR4: 0000000000350ef0 Call Trace: tcp_data_ready (net/ipv4/tcp_input.c:5356) tcp_data_queue (net/ipv4/tcp_input.c:5445) tcp_rcv_state_process (net/ipv4/tcp_input.c:7165) tcp_v4_do_rcv (net/ipv4/tcp_ipv4.c:1955) __release_sock (include/net/sock.h:1158 (discriminator 6) net/core/sock.c= :3180 (discriminator 6)) release_sock (net/core/sock.c:3737) mptcp_sendmsg (net/mptcp/protocol.c:1763 net/mptcp/protocol.c:1857) inet_sendmsg (net/ipv4/af_inet.c:853 (discriminator 7)) __sys_sendto (net/socket.c:727 (discriminator 15) net/socket.c:742 (discr= iminator 15) net/socket.c:2244 (discriminator 15)) __x64_sys_sendto (net/socket.c:2247) do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1) arch/x86/= entry/syscall_64.c:94 (discriminator 1)) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130) RIP: 0033:0x7f883326702d Address the issue ensuring `send_fastclose` is always set for all subflows after mptcp_do_fastclose(). Fixes: ae155060247b ("mptcp: fix duplicate reset on fastclose") Signed-off-by: Paolo Abeni --- net/mptcp/protocol.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index e484c6391b48..cd5a19ab3ba1 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -2891,12 +2891,15 @@ static void mptcp_do_fastclose(struct sock *sk) =20 lock_sock(ssk); =20 + /* Ensure that the MPC subflow will be fully disconnected/reset + * by the later mptcp_destroy_common()/__mptcp_close_ssk(). + */ + subflow->send_fastclose =3D 1; + /* Some subflow socket states don't allow/need a reset.*/ if ((1 << ssk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE)) goto unlock; =20 - subflow->send_fastclose =3D 1; - /* Initialize rcv_mss to TCP_MIN_MSS to avoid division by 0 * issue in __tcp_select_window(), see tcp_disconnect(). */ --=20 2.52.0