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Message-ID: <649dc1dc-ca80-4686-ae37-62d7c81dde8b@linux.dev>
Date: Wed, 27 Mar 2024 15:16:16 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
Paolo Abeni <pabeni@...hat.com>
Cc: Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
MPTCP Upstream <mptcp@...ts.linux.dev>, Matthieu Baerts
<matttbe@...nel.org>, Mat Martineau <martineau@...nel.org>,
Jakub Kicinski <kuba@...nel.org>, Martin KaFai Lau <martin.lau@...nel.org>
Subject: Re: mptcp splat
On 3/27/24 11:50 AM, Alexei Starovoitov wrote:
> On Wed, Mar 27, 2024 at 11:33 AM Paolo Abeni <pabeni@...hat.com> wrote:
>>
>> On Wed, 2024-03-27 at 10:00 -0700, Alexei Starovoitov wrote:
>>> On Wed, Mar 27, 2024 at 9:56 AM Paolo Abeni <pabeni@...hat.com> wrote:
>>>>
>>>> On Wed, 2024-03-27 at 09:43 -0700, Alexei Starovoitov wrote:
>>>>> I ffwded bpf tree with the recent net fixes and caught this:
>>>>>
>>>>> [ 48.386337] WARNING: CPU: 32 PID: 3276 at net/mptcp/subflow.c:1430
>>>>> subflow_data_ready+0x147/0x1c0
>>>>> [ 48.392012] Modules linked in: dummy bpf_testmod(O) [last unloaded:
>>>>> bpf_test_no_cfi(O)]
>>>>> [ 48.396609] CPU: 32 PID: 3276 Comm: test_progs Tainted: G
>>>>> O 6.8.0-12873-g2c43c33bfd23 #1014
>>>>> #[ 48.467143] Call Trace:
>>>>> [ 48.469094] <TASK>
>>>>> [ 48.472159] ? __warn+0x80/0x180
>>>>> [ 48.475019] ? subflow_data_ready+0x147/0x1c0
>>>>> [ 48.478068] ? report_bug+0x189/0x1c0
>>>>> [ 48.480725] ? handle_bug+0x36/0x70
>>>>> [ 48.483061] ? exc_invalid_op+0x13/0x60
>>>>> [ 48.485809] ? asm_exc_invalid_op+0x16/0x20
>>>>> [ 48.488754] ? subflow_data_ready+0x147/0x1c0
>>>>> [ 48.492159] mptcp_set_rcvlowat+0x79/0x1d0
>>>>> [ 48.495026] sk_setsockopt+0x6c0/0x1540
>>>>>
>>>>> It doesn't reproduce all the time though.
>>>>> Some race?
>>>>> Known issue?
>>>>
>>>> It was not known to me. Looks like something related to not so recent
>>>> changes (rcvlowat support).
>>>>
>>>> Definitely looks lie a race.
>>>>
>>>> If you could share more info about the running context and/or a full
>>>> decoded splat it could help, thanks!
>>>
>>> This is just running bpf selftests in parallel:
>>> test_progs -j
>>>
>>> The end of the splat:
>>> [ 48.500075] __bpf_setsockopt+0x6f/0x90
>>> [ 48.503124] bpf_sock_ops_setsockopt+0x3c/0x90
>>> [ 48.506053] bpf_prog_509ce5db2c7f9981_bpf_test_sockopt_int+0xb4/0x11b
>>> [ 48.510178] bpf_prog_dce07e362d941d2b_bpf_test_socket_sockopt+0x12b/0x132
>>> [ 48.515070] bpf_prog_348c9b5faaf10092_skops_sockopt+0x954/0xe86
>>> [ 48.519050] __cgroup_bpf_run_filter_sock_ops+0xbc/0x250
>>> [ 48.523836] tcp_connect+0x879/0x1160
>>> [ 48.527239] ? ktime_get_with_offset+0x8d/0x140
>>> [ 48.531362] tcp_v6_connect+0x50c/0x870
>>> [ 48.534609] ? mptcp_connect+0x129/0x280
>>> [ 48.538483] mptcp_connect+0x129/0x280
>>> [ 48.542436] __inet_stream_connect+0xce/0x370
>>> [ 48.546664] ? rcu_is_watching+0xd/0x40
>>> [ 48.549063] ? lock_release+0x1c4/0x280
>>> [ 48.553497] ? inet_stream_connect+0x22/0x50
>>> [ 48.557289] ? rcu_is_watching+0xd/0x40
>>> [ 48.560430] inet_stream_connect+0x36/0x50
>>> [ 48.563604] bpf_trampoline_6442491565+0x49/0xef
>>> [ 48.567770] ? security_socket_connect+0x34/0x50
>>> [ 48.575400] inet_stream_connect+0x5/0x50
>>> [ 48.577721] __sys_connect+0x63/0x90
>>> [ 48.580189] ? bpf_trace_run2+0xb0/0x1a0
>>> [ 48.583171] ? rcu_is_watching+0xd/0x40
>>> [ 48.585802] ? syscall_trace_enter+0xfb/0x1e0
>>> [ 48.588836] __x64_sys_connect+0x14/0x20
>>
>> Ouch, it looks bad. BPF should not allow any action on mptcp subflows
>> that go through sk_socket. They touch the mptcp main socket, which is
>> _not_ protected by the subflow socket lock.
>>
>> AFICS currently the relevant set of racing sockopt allowed by bpf boils
>> down to SO_RCVLOWAT only - sk_setsockopt(SO_RCVLOWAT) will call sk-
>>> sk_socket->ops->set_rcvlowat()
>>
>> So something like the following (completely untested) should possibly
>> address the issue at hand, but I think it would be better/safer
>> completely disable ebpf on mptcp subflows, WDYT?
>>
>> Thanks,
>>
>> Paolo
>>
>> ---
>> diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
>> index dcd1c76d2a3b..6e5e64c2cf89 100644
>> --- a/net/mptcp/sockopt.c
>> +++ b/net/mptcp/sockopt.c
>> @@ -1493,6 +1493,9 @@ int mptcp_set_rcvlowat(struct sock *sk, int val)
>> struct mptcp_subflow_context *subflow;
>> int space, cap;
>>
>> + if (has_current_bpf_ctx())
>> + return -EINVAL;
>> +
>
> Looks fine to me.
>
> Martin,
>
> Do you have any better ideas?
>
> The splat explains the race.
> In this case setget_sockopt test happen to run in parallel
> with mptcp/bpf test and the former one was TCP connect request
> but it was for subflow.
>
> We can disable that callback when tcp flow is a subflow,
> but that doesn't feel right.
I am also not sure if we can disable all set/getsockopt for tcp subflows. Not
clear if there is use case that depends on this to setsockopt the subflow.
> I think Paolo's targeted fix is cleaner.
I will also go with Paolo's fix. The radius is smaller. I don't have a better idea.
Unrelated, is there a way to tell if a tcp_sock is a subflow? bpf prog can use
it to decide if it wants to setsockopt on a subflow or not.
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