mptcp: ensure snd_nxt is properly initialized on connect

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:
 <TASK>
 __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
 </TASK>

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.

Reported-by: Christoph Paasch <[email protected]>
Tested-by: Christoph Paasch <[email protected]>
Closes: #485
Fixes: 8fd7380 ("mptcp: fallback in case of simultaneous connect")
Signed-off-by: Paolo Abeni <[email protected]>
Message-Id: <3b0ca23eaa8906f967a841f55bafed48d0f895cb.1713367699.git.pabeni@redhat.com>

net/mptcp/protocol.c

@@ -1002,8 +1002,13 @@ static void __mptcp_clean_una(struct sock *sk)
 
 		if (unlikely(dfrag == msk->first_pending)) {
 			/* in recovery mode can see ack after the current snd head */
-			if (WARN_ON_ONCE(!msk->recovery))
+			if (WARN_ON_ONCE(!msk->recovery)) {
+				pr_err("sk state %d:%d snd_una %llx write_seq %llx dfrag end seq %llx frag len %d bytes sent %lld acked %lld",
+				       sk->sk_state, msk->first ? msk->first->sk_state: -1, msk->write_seq,
+				       msk->snd_una, dfrag->data_seq + dfrag->data_len, dfrag->data_len,
+				       msk->bytes_sent, msk->bytes_acked);
 				break;
+			}
 
 			WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
 		}
@@ -3730,6 +3735,9 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_TOKENFALLBACKINIT);
 		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)))
 		MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPCAPABLEACTIVE);