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    Subject[PATCH 5.10 030/167] mptcp: fix warning in __skb_flow_dissect() when do syn cookie for subflow join
    Date
    From: Jianguo Wu <wujianguo@chinatelecom.cn>

    [ Upstream commit 0c71929b5893e410e0efbe1bbeca6f19a5f19956 ]

    I did stress test with wrk[1] and webfsd[2] with the assistance of
    mptcp-tools[3]:

    Server side:
    ./use_mptcp.sh webfsd -4 -R /tmp/ -p 8099
    Client side:
    ./use_mptcp.sh wrk -c 200 -d 30 -t 4 http://192.168.174.129:8099/

    and got the following warning message:

    [ 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, as skb->dev is set to
    NULL in tcp_v4_rcv(), and skb->sk is never set.

    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);

    So, using seq and transport header derived hash.

    [1] https://github.com/wg/wrk
    [2] https://github.com/ourway/webfsd
    [3] https://github.com/pabeni/mptcp-tools

    Fixes: 9466a1ccebbe ("mptcp: enable JOIN requests even if cookies are in use")
    Suggested-by: Paolo Abeni <pabeni@redhat.com>
    Suggested-by: Florian Westphal <fw@strlen.de>
    Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
    Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/mptcp/syncookies.c | 16 +++++++++++++++-
    1 file changed, 15 insertions(+), 1 deletion(-)

    diff --git a/net/mptcp/syncookies.c b/net/mptcp/syncookies.c
    index abe0fd099746..37127781aee9 100644
    --- a/net/mptcp/syncookies.c
    +++ b/net/mptcp/syncookies.c
    @@ -37,7 +37,21 @@ static spinlock_t join_entry_locks[COOKIE_JOIN_SLOTS] __cacheline_aligned_in_smp

    static u32 mptcp_join_entry_hash(struct sk_buff *skb, struct net *net)
    {
    - u32 i = skb_get_hash(skb) ^ net_hash_mix(net);
    + static u32 mptcp_join_hash_secret __read_mostly;
    + struct tcphdr *th = tcp_hdr(skb);
    + u32 seq, i;
    +
    + net_get_random_once(&mptcp_join_hash_secret,
    + sizeof(mptcp_join_hash_secret));
    +
    + if (th->syn)
    + seq = TCP_SKB_CB(skb)->seq;
    + else
    + seq = TCP_SKB_CB(skb)->seq - 1;
    +
    + i = jhash_3words(seq, net_hash_mix(net),
    + (__force __u32)th->source << 16 | (__force __u32)th->dest,
    + mptcp_join_hash_secret);

    return i % ARRAY_SIZE(join_entries);
    }
    --
    2.30.2


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