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From: "Matthieu Baerts (NGI0)" <>
To:, Mat Martineau <>,
	 Geliang Tang <>,
	"David S. Miller" <>,
	 Eric Dumazet <>,
	Jakub Kicinski <>,
	 Paolo Abeni <>,
	Davide Caratti <>
Cc:,,, Christoph Paasch <>,
	 "Matthieu Baerts (NGI0)" <>
Subject: [PATCH net] mptcp: ensure snd_nxt is properly initialized on connect
Date: Mon, 29 Apr 2024 20:00:31 +0200	[thread overview]
Message-ID: <> (raw)

From: Paolo Abeni <>

Christoph reported a splat hinting at a corrupted snd_una:

  WARNING: CPU: 1 PID: 38 at net/mptcp/protocol.c:1005 __mptcp_clean_una+0x4b3/0x620 net/mptcp/protocol.c:1005
  Modules linked in:
  CPU: 1 PID: 38 Comm: kworker/1:1 Not tainted 6.9.0-rc1-gbbeac67456c9 #59
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
  Workqueue: events mptcp_worker
  RIP: 0010:__mptcp_clean_una+0x4b3/0x620 net/mptcp/protocol.c:1005
  Code: be 06 01 00 00 bf 06 01 00 00 e8 a8 12 e7 fe e9 00 fe ff ff e8
  	8e 1a e7 fe 0f b7 ab 3e 02 00 00 e9 d3 fd ff ff e8 7d 1a e7 fe
  	<0f> 0b 4c 8b bb e0 05 00 00 e9 74 fc ff ff e8 6a 1a e7 fe 0f 0b e9
  RSP: 0018:ffffc9000013fd48 EFLAGS: 00010293
  RAX: 0000000000000000 RBX: ffff8881029bd280 RCX: ffffffff82382fe4
  RDX: ffff8881003cbd00 RSI: ffffffff823833c3 RDI: 0000000000000001
  RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000
  R10: 0000000000000000 R11: fefefefefefefeff R12: ffff888138ba8000
  R13: 0000000000000106 R14: ffff8881029bd908 R15: ffff888126560000
  FS:  0000000000000000(0000) GS:ffff88813bd00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f604a5dae38 CR3: 0000000101dac002 CR4: 0000000000170ef0
  Call Trace:
   __mptcp_clean_una_wakeup net/mptcp/protocol.c:1055 [inline]
   mptcp_clean_una_wakeup net/mptcp/protocol.c:1062 [inline]
   __mptcp_retrans+0x7f/0x7e0 net/mptcp/protocol.c:2615
   mptcp_worker+0x434/0x740 net/mptcp/protocol.c:2767
   process_one_work+0x1e0/0x560 kernel/workqueue.c:3254
   process_scheduled_works kernel/workqueue.c:3335 [inline]
   worker_thread+0x3c7/0x640 kernel/workqueue.c:3416
   kthread+0x121/0x170 kernel/kthread.c:388
   ret_from_fork+0x44/0x50 arch/x86/kernel/process.c:147
   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:243

When fallback to TCP happens early on a client socket, snd_nxt
is not yet initialized and any incoming ack will copy such value
into snd_una. If the mptcp worker (dumbly) tries mptcp-level
re-injection after such ack, that would unconditionally trigger a send
buffer cleanup using 'bad' snd_una values.

We could easily disable re-injection for fallback sockets, but such
dumb behavior already helped catching a few subtle issues and a very
low to zero impact in practice.

Instead address the issue always initializing snd_nxt (and write_seq,
for consistency) at connect time.

Fixes: 8fd738049ac3 ("mptcp: fallback in case of simultaneous connect")
Reported-by: Christoph Paasch <>
Tested-by: Christoph Paasch <>
Signed-off-by: Paolo Abeni <>
Reviewed-by: Mat Martineau <>
Signed-off-by: Matthieu Baerts (NGI0) <>
 net/mptcp/protocol.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 7e74b812e366..965eb69dc5de 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3723,6 +3723,9 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 		mptcp_subflow_early_fallback(msk, subflow);
+	WRITE_ONCE(msk->write_seq, subflow->idsn);
+	WRITE_ONCE(msk->snd_nxt, subflow->idsn);
 	if (likely(!__mptcp_check_fallback(msk)))

base-commit: ba1cb99b559e3b12db8b65ca9ff03358ea318064
change-id: 20240429-upstream-net-20240429-mptcp-snd_nxt-init-connect-d9844d880f48

Best regards,
Matthieu Baerts (NGI0) <>

             reply	other threads:[~2024-04-29 18:01 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-29 18:00 Matthieu Baerts (NGI0) [this message]
2024-05-01  2:10 ` [PATCH net] mptcp: ensure snd_nxt is properly initialized on connect patchwork-bot+netdevbpf

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