* [PATCH mptcp-net v2 1/2] mptcp: fallback earlier on simult connection
2025-12-03 18:24 [PATCH mptcp-net v2 0/2] mptcp: fix warn on bad status Paolo Abeni
@ 2025-12-03 18:24 ` Paolo Abeni
2025-12-03 18:24 ` [PATCH mptcp-net v2 2/2] mptcp: ensure context reset on disconnect() Paolo Abeni
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: Paolo Abeni @ 2025-12-03 18:24 UTC (permalink / raw)
To: mptcp
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+0x40b/0x7c0 net/mptcp/subflow.c:1515
Modules linked in:
CPU: 3 UID: 0 PID: 33 Comm: ksoftirqd/3 Not tainted syzkaller #0 PREEMPT(full)
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 66 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:
<TASK>
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
</TASK>
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 <pabeni@redhat.com>
---
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) <mss 1460, sackOK, TS val 100 ecr 0, nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
+0 < S 0:0(0) win 1000 <mss 1460, sackOK, TS val 407 ecr 0, nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
-+0 > S. 0:0(0) ack 1 <mss 1460, sackOK, TS val 330 ecr 407, nop, wscale 8, mpcapable v1 flags[flag_h] nokey>
-+0 < S. 0:0(0) ack 1 win 65535 <mss 1460, sackOK, TS val 507 ecr 100, nop, wscale 8, mpcapable v1 flags[flag_h] key[skey=2]>
++0 > S. 0:0(0) ack 1 <mss 1460, sackOK, TS val 330 ecr 407, nop,wscale 8>
++0 < S. 0:0(0) ack 1 win 65535 <mss 1460, sackOK, TS val 507 ecr 100, nop,wscale 8>
+0 > . 1:1(0) ack 1 <nop, nop, TS val 430 ecr 507, nop, nop, sack 0:1>
---
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 sk_buff *skb,
*/
subflow->snd_isn = 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 = 0;
+ return false;
+ }
+
opts->suboptions = OPTION_MPTCP_MPC_SYN;
opts->csum_reqd = mptcp_is_checksum_enabled(sock_net(sk));
opts->allow_join_id0 = 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(struct sock *sk)
{
struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
- 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 == TCP_SYN_RECV && is_active_ssk(subflow);
}
#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)
__subflow_state_change(sk);
- if (subflow_simultaneous_connect(sk)) {
- WARN_ON_ONCE(!mptcp_try_fallback(sk, MPTCP_MIB_SIMULTCONNFALLBACK));
- subflow->conn_finished = 1;
- mptcp_propagate_state(parent, sk, subflow, NULL);
- }
-
/* as recvmsg() does not acquire the subflow socket for ssk selection
* a fin packet carrying a DSS can be unnoticed if we don't trigger
* the data available machinery here.
--
2.52.0
^ permalink raw reply related [flat|nested] 5+ messages in thread* [PATCH mptcp-net v2 2/2] mptcp: ensure context reset on disconnect()
2025-12-03 18:24 [PATCH mptcp-net v2 0/2] mptcp: fix warn on bad status Paolo Abeni
2025-12-03 18:24 ` [PATCH mptcp-net v2 1/2] mptcp: fallback earlier on simult connection Paolo Abeni
@ 2025-12-03 18:24 ` Paolo Abeni
2025-12-03 19:43 ` [PATCH mptcp-net v2 0/2] mptcp: fix warn on bad status MPTCP CI
2025-12-04 18:56 ` Matthieu Baerts
3 siblings, 0 replies; 5+ messages in thread
From: Paolo Abeni @ 2025-12-03 18:24 UTC (permalink / raw)
To: mptcp
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-g11fc074f6c36 #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:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f88330aefe8 CR3: 000000010ff59000 CR4: 0000000000350ef0
Call Trace:
<TASK>
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 (discriminator 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 <pabeni@redhat.com>
---
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)
lock_sock(ssk);
+ /* Ensure that the MPC subflow will be fully disconnected/reset
+ * by the later mptcp_destroy_common()/__mptcp_close_ssk().
+ */
+ subflow->send_fastclose = 1;
+
/* Some subflow socket states don't allow/need a reset.*/
if ((1 << ssk->sk_state) & (TCPF_LISTEN | TCPF_CLOSE))
goto unlock;
- subflow->send_fastclose = 1;
-
/* Initialize rcv_mss to TCP_MIN_MSS to avoid division by 0
* issue in __tcp_select_window(), see tcp_disconnect().
*/
--
2.52.0
^ permalink raw reply related [flat|nested] 5+ messages in thread