Re: [PATCH] mptcp: fix warning in __skb_flow_dissect() when do syn cookie for subflow join

[Paolo Abeni <pabeni@redhat.com>]

Hello,

On Sat, 2021-06-05 at 11:24 +0800, Jianguo Wu wrote:
> From: Jianguo Wu <wujianguo@chinatelecom.cn>
> 
> I got the following warning message while doing the test:
> 
> [   55.552626] TCP: request_sock_subflow: Possible SYN flooding on port 8099. Sending cookies.  Check SNMP counters.
> [   55.553024] ------------[ cut here ]------------
> [   55.553027] WARNING: CPU: 0 PID: 10 at net/core/flow_dissector.c:984 __skb_flow_dissect+0x280/0x1650
> ...
> [   55.553117] CPU: 0 PID: 10 Comm: ksoftirqd/0 Not tainted 5.12.0+ #18
> [   55.553121] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 02/27/2020
> [   55.553124] RIP: 0010:__skb_flow_dissect+0x280/0x1650
> ...
> [   55.553133] RSP: 0018:ffffb79580087770 EFLAGS: 00010246
> [   55.553137] RAX: 0000000000000000 RBX: ffffffff8ddb58e0 RCX: ffffb79580087888
> [   55.553139] RDX: ffffffff8ddb58e0 RSI: ffff8f7e4652b600 RDI: 0000000000000000
> [   55.553141] RBP: ffffb79580087858 R08: 0000000000000000 R09: 0000000000000008
> [   55.553143] R10: 000000008c622965 R11: 00000000d3313a5b R12: ffff8f7e4652b600
> [   55.553146] R13: ffff8f7e465c9062 R14: 0000000000000000 R15: ffffb79580087888
> [   55.553149] FS:  0000000000000000(0000) GS:ffff8f7f75e00000(0000) knlGS:0000000000000000
> [   55.553152] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [   55.553154] CR2: 00007f73d1d19000 CR3: 0000000135e10004 CR4: 00000000003706f0
> [   55.553160] Call Trace:
> [   55.553166]  ? __sha256_final+0x67/0xd0
> [   55.553173]  ? sha256+0x7e/0xa0
> [   55.553177]  __skb_get_hash+0x57/0x210
> [   55.553182]  subflow_init_req_cookie_join_save+0xac/0xc0
> [   55.553189]  subflow_check_req+0x474/0x550
> [   55.553195]  ? ip_route_output_key_hash+0x67/0x90
> [   55.553200]  ? xfrm_lookup_route+0x1d/0xa0
> [   55.553207]  subflow_v4_route_req+0x8e/0xd0
> [   55.553212]  tcp_conn_request+0x31e/0xab0
> [   55.553218]  ? selinux_socket_sock_rcv_skb+0x116/0x210
> [   55.553224]  ? tcp_rcv_state_process+0x179/0x6d0
> [   55.553229]  tcp_rcv_state_process+0x179/0x6d0
> [   55.553235]  tcp_v4_do_rcv+0xaf/0x220
> [   55.553239]  tcp_v4_rcv+0xce4/0xd80
> [   55.553243]  ? ip_route_input_rcu+0x246/0x260
> [   55.553248]  ip_protocol_deliver_rcu+0x35/0x1b0
> [   55.553253]  ip_local_deliver_finish+0x44/0x50
> [   55.553258]  ip_local_deliver+0x6c/0x110
> [   55.553262]  ? ip_rcv_finish_core.isra.19+0x5a/0x400
> [   55.553267]  ip_rcv+0xd1/0xe0
> ...
> 
> After debugging, I found in __skb_flow_dissect(), skb->dev and skb->sk are both NULL,
> then net is NULL, and trigger WARN_ON_ONCE(!net), actually net is always NULL in this
> code path.
> 
> Code snippet in __skb_flow_dissect() that trigger warning:
>   975         if (skb) {
>   976                 if (!net) {
>   977                         if (skb->dev)
>   978                                 net = dev_net(skb->dev);
>   979                         else if (skb->sk)
>   980                                 net = sock_net(skb->sk);
>   981                 }
>   982         }
>   983
>   984         WARN_ON_ONCE(!net);
> 
> skb->dev is set to NULL in tcp_v4_rcv(), and skb->sk is never set in this code path.
> But for sk_state is TCP_NEW_SYN_RECV or TCP_TIME_WAIT, skb->dev is not set to NULL,
> so we can remove skb->dev = NULL in tcp_v4_rcv()?

Thank you for the detailed report!

I'm unable to find easily any negative side effect to remove such
assignment, still I'm more than a bit uneasy for such change.

The issue is clearly an mptcp-specific one, so I think event the fix
should be mptcp-specific.

If the skb->hash is not available - !skb_get_hash_raw(skb) == 0, then
we could fallback using an sk-derived hash, e.g. doing something alike
what inet_ehashfn() does - with different seeds/secrets.

Cheers,

Paolo