tcp: fix mptcp DSS corruption due to large pmtu xmit.

Syzkaller was able to trigger a DSS corruption:

TCP: request_sock_subflow_v4: Possible SYN flooding on port [::]:20002. Sending cookies.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 5227 at net/mptcp/protocol.c:695 __mptcp_move_skbs_from_subflow+0x20a9/0x21f0 net/mptcp/protocol.c:695
Modules linked in:
CPU: 0 UID: 0 PID: 5227 Comm: syz-executor350 Not tainted 6.11.0-syzkaller-08829-gaf9c191ac2a0 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
RIP: 0010:__mptcp_move_skbs_from_subflow+0x20a9/0x21f0 net/mptcp/protocol.c:695
Code: 0f b6 dc 31 ff 89 de e8 b5 dd ea f5 89 d8 48 81 c4 50 01 00 00 5b 41 5c 41 5d 41 5e 41 5f 5d c3 cc cc cc cc e8 98 da ea f5 90 <0f> 0b 90 e9 47 ff ff ff e8 8a da ea f5 90 0f 0b 90 e9 99 e0 ff ff
RSP: 0018:ffffc90000006db8 EFLAGS: 00010246
RAX: ffffffff8ba9df18 RBX: 00000000000055f0 RCX: ffff888030023c00
RDX: 0000000000000100 RSI: 00000000000081e5 RDI: 00000000000055f0
RBP: 1ffff110062bf1ae R08: ffffffff8ba9cf12 R09: 1ffff110062bf1b8
R10: dffffc0000000000 R11: ffffed10062bf1b9 R12: 0000000000000000
R13: dffffc0000000000 R14: 00000000700cec61 R15: 00000000000081e5
FS:  000055556679c380(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000020287000 CR3: 0000000077892000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <IRQ>
 move_skbs_to_msk net/mptcp/protocol.c:811 [inline]
 mptcp_data_ready+0x29c/0xa90 net/mptcp/protocol.c:854
 subflow_data_ready+0x34a/0x920 net/mptcp/subflow.c:1490
 tcp_data_queue+0x20fd/0x76c0 net/ipv4/tcp_input.c:5283
 tcp_rcv_established+0xfba/0x2020 net/ipv4/tcp_input.c:6237
 tcp_v4_do_rcv+0x96d/0xc70 net/ipv4/tcp_ipv4.c:1915
 tcp_v4_rcv+0x2dc0/0x37f0 net/ipv4/tcp_ipv4.c:2350
 ip_protocol_deliver_rcu+0x22e/0x440 net/ipv4/ip_input.c:205
 ip_local_deliver_finish+0x341/0x5f0 net/ipv4/ip_input.c:233
 NF_HOOK+0x3a4/0x450 include/linux/netfilter.h:314
 NF_HOOK+0x3a4/0x450 include/linux/netfilter.h:314
 __netif_receive_skb_one_core net/core/dev.c:5662 [inline]
 __netif_receive_skb+0x2bf/0x650 net/core/dev.c:5775
 process_backlog+0x662/0x15b0 net/core/dev.c:6107
 __napi_poll+0xcb/0x490 net/core/dev.c:6771
 napi_poll net/core/dev.c:6840 [inline]
 net_rx_action+0x89b/0x1240 net/core/dev.c:6962
 handle_softirqs+0x2c5/0x980 kernel/softirq.c:554
 do_softirq+0x11b/0x1e0 kernel/softirq.c:455
 </IRQ>
 <TASK>
 __local_bh_enable_ip+0x1bb/0x200 kernel/softirq.c:382
 local_bh_enable include/linux/bottom_half.h:33 [inline]
 rcu_read_unlock_bh include/linux/rcupdate.h:919 [inline]
 __dev_queue_xmit+0x1764/0x3e80 net/core/dev.c:4451
 dev_queue_xmit include/linux/netdevice.h:3094 [inline]
 neigh_hh_output include/net/neighbour.h:526 [inline]
 neigh_output include/net/neighbour.h:540 [inline]
 ip_finish_output2+0xd41/0x1390 net/ipv4/ip_output.c:236
 ip_local_out net/ipv4/ip_output.c:130 [inline]
 __ip_queue_xmit+0x118c/0x1b80 net/ipv4/ip_output.c:536
 __tcp_transmit_skb+0x2544/0x3b30 net/ipv4/tcp_output.c:1466
 tcp_transmit_skb net/ipv4/tcp_output.c:1484 [inline]
 tcp_mtu_probe net/ipv4/tcp_output.c:2547 [inline]
 tcp_write_xmit+0x641d/0x6bf0 net/ipv4/tcp_output.c:2752
 __tcp_push_pending_frames+0x9b/0x360 net/ipv4/tcp_output.c:3015
 tcp_push_pending_frames include/net/tcp.h:2107 [inline]
 tcp_data_snd_check net/ipv4/tcp_input.c:5714 [inline]
 tcp_rcv_established+0x1026/0x2020 net/ipv4/tcp_input.c:6239
 tcp_v4_do_rcv+0x96d/0xc70 net/ipv4/tcp_ipv4.c:1915
 sk_backlog_rcv include/net/sock.h:1113 [inline]
 __release_sock+0x214/0x350 net/core/sock.c:3072
 release_sock+0x61/0x1f0 net/core/sock.c:3626
 mptcp_push_release net/mptcp/protocol.c:1486 [inline]
 __mptcp_push_pending+0x6b5/0x9f0 net/mptcp/protocol.c:1625
 mptcp_sendmsg+0x10bb/0x1b10 net/mptcp/protocol.c:1903
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg+0x1a6/0x270 net/socket.c:745
 ____sys_sendmsg+0x52a/0x7e0 net/socket.c:2603
 ___sys_sendmsg net/socket.c:2657 [inline]
 __sys_sendmsg+0x2aa/0x390 net/socket.c:2686
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb06e9317f9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe2cfd4f98 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fb06e97f468 RCX: 00007fb06e9317f9
RDX: 0000000000000000 RSI: 0000000020000080 RDI: 0000000000000005
RBP: 00007fb06e97f446 R08: 0000555500000000 R09: 0000555500000000
R10: 0000555500000000 R11: 0000000000000246 R12: 00007fb06e97f406
R13: 0000000000000001 R14: 00007ffe2cfd4fe0 R15: 0000000000000003
 </TASK>

Additionally syzkaller provided a nice reproducer. The repro enables
pmtu on the loopback device, leading to tcp_mtu_probe() generating
very large probe packets.

tcp_can_coalesce_send_queue_head() currently does not check for
mptcp-level invariants, and allowed the creation of cross-DSS probes,
leading to the mentioned corruption.

Address the issue teaching tcp_can_coalesce_send_queue_head() about
mptcp using the tcp_skb_can_collapse(), also reducing the code
duplication.

Fixes: 8571248 ("tcp: coalesce/collapse must respect MPTCP extensions")
Reported-by: [email protected]
Closes: multipath-tcp/mptcp_net-next#513
Signed-off-by: Paolo Abeni <[email protected]>

net/ipv4/tcp_output.c

@@ -2342,10 +2342,7 @@ static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
 		if (len <= skb->len)
 			break;
 
-		if (unlikely(TCP_SKB_CB(skb)->eor) ||
-		    tcp_has_tx_tstamp(skb) ||
-		    !skb_pure_zcopy_same(skb, next) ||
-		    skb_frags_readable(skb) != skb_frags_readable(next))
+		if (tcp_has_tx_tstamp(skb) || !tcp_skb_can_collapse(skb, next))
 			return false;
 
 		len -= skb->len;