Re: [PATCH v1 bpf-next 00/11] bpf: tcp: Add SYN Cookie generation/validation SOCK_OPS hooks.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 10/23/23 18:22, Kuniyuki Iwashima wrote:
>> On 10/23/23 14:35, Martin KaFai Lau wrote:
>>> On 10/20/23 11:48 PM, Kuniyuki Iwashima wrote:
>>>> I think this was doable.  With the diff below, I was able to skip
>>>> validation in cookie_v[46]_check() when if skb->sk is not NULL.
>>>>
>>>> The kfunc allocates req and set req->syncookie to 1, which is usually
>>>> set in TX path, so if it's 1 in RX (inet_steal_sock()), we can see
>>>> that req is allocated by kfunc (at least, req->syncookie &&
>>>> req->rsk_listener never be true in the current TCP stack).
>>>>
>>>> The difference here is that req allocated by kfunc holds refcnt of
>>>> rsk_listener (passing true to inet_reqsk_alloc()) to prevent freeing
>>>> the listener until req reaches cookie_v[46]_check().
>>>
>>> The cookie_v[46]_check() holds the listener sk refcnt now?
>>
>> The caller of cookie_v[46]_check() should hold a refcnt of the listener.
> 
> No, it need not.
> 
> When we handle the default syn cookie, cookie_tcp_reqsk_alloc() passes
> false to inet_reqsk_alloc(), then reqsk does not hold refcnt of the
> listener.
> 
> If inet_csk_reqsk_queue_add() in tcp_get_cookie_sock() succeeds, we know
> the listener is still alive

What I said is the callers of cookie_v[46]_check().
For example, tcp_v4_rcv() will make sure the existence of the sk passing
to tcp_v4_do_rcv() -> tcp_v4_cookie_check(). tcp_v4_rcv() gets the sk
from __inet_lookup_skb().  The sk can be refcounted or not.
For the case of not refcounted, it should be rcu protected
(SOCK_RCU_FREE).

AFAIK, tcp_v4_rcv() is called in a rcu_read_lock() section
(far in ip_local_deliver_finish(), even netif_receive_skb_core()).

tcp_v4_rcv() and cookie_v4_check() also access the content of sk
without increase the refcount of sk. That also indicate these function
believe the sk returned by __inet_lookup_skb() is either refcounted
or protected in someway (RCU here).

What I mean protection is that the sk may be closed but not destroyed.


> 
> 
>> If the listener is destroyed, the callers of cookie_v[46]_check() should
>> fail to lookup a sock for the skb. However, in this case, the kfunc sets
>> a sock to skb->sk, and the lookup function
>> (__inet_lookup_skb()) steals sock from skb. So, there is no guarantee
>> ensuring the listener is still alive.
>>
>> One solution is let the stealing function to lookup the listener if
>> inet_reqsk(skb->sk)->syncookie is true.
> 
> kfunc at least guarantees that the listener is not freed until req
> is freed.  There's two cases where the listener could be close()d
> after kfunc:
> 
>    1. close()d before lookup
>       -> kfree_skb(skb) calls reqsk_put() and releases the last
>          refcnt of the listener
> 
>    2. close()d between lookup and inet_csk_reqsk_queue_add()
>       -> inet_csk_reqsk_queue_add() fails and __reqsk_free()
>          releases the last refcnt of the listener.
> 
> So, we need not look up the listener again in inet_steal_sock().

After thinking about this again, increasing the refcount of the listener
in the kfunc is not necessary. Since the caller of a
bpf program should already hold a refcount of the sk or
rcu protected, we can let inet_csk_reqsk_queue_add() handle it,
just like what you mentioned earlier.

WDYT?


> 
> 
>>>
>>>   >
>>>> The cookie generation at least should be done at tc/xdp.  The
>>>> valdation can be done earlier as well on tc/xdp, but it could
>>>> add another complexity, listener's life cycle if we allocate
>>>> req there.
>>>
>>> I think your code below looks pretty close already.
>>>
>>> It seems the only concern/complexity is the extra rsk_listener refcnt (btw the
>>> concern is on performance for the extra refcnt? or there is correctness issue?).
> 
> Yes, that's the only concern and I think it's all ok now.
> 
> [ I was seeing a weird refcnt warning, but I missed *refcounted was true
>    in inet_steal_sock() for reqsk and forgot to flipping it to false :S ]
> 
> 
>>>
>>> Asking because bpf_sk_assign() can already assign a listener to skb->sk and it
>>> also does not take a refcnt on the listener. The same no refcnt needed on
>>> req->rsk_listener should be doable also. sock_pfree may need to be smarter to
>>> check req->syncookie. What else may need to change?
> 
> I was wondering if we are in the same RCU period between tc and
> cookie_v[46]_check(), but yeah, probably sock_pfree() can check
> req->syncookie and set NULL to rsk_listener so that reqsk_put()
> will not touch the listener.
> 
> 
>>>
>>>>
>>>> I'm wondering which place to add the validation capability, and
>>>> I think SOCK_OPS is simpler than tc.
>>>>
>>>>     #1 validate cookie and allocate req at tc, and skip validation
>>>>
>>>>     #2 validate cookie (and update bpf map at xdp/tc, and look up bpf
>>>>        map) and allocate req at SOCK_OPS hook
>>>>
>>>> Given SYN proxy is usually on the other node and incoming cookie
>>>> is almost always valid, we might need not validate it in the early
>>>> stage in the stack.
>>>>
>>>> What do you think ?
>>>
>>> Yeah, supporting validation in sock_ops is an open option if the tc side is too
>>> hard but I feel you are pretty close on the tc side.
> 
> Now I think I can go v2 with tc.
> 
> Thanks for your guide!
> 
> 
>>>
>>>>
>>>> ---8<---
>>>> diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
>>>> index 3ecfeadbfa06..e5e4627bf270 100644
>>>> --- a/include/net/inet_hashtables.h
>>>> +++ b/include/net/inet_hashtables.h
>>>> @@ -462,9 +462,19 @@ struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff,
>>>>    	if (!sk)
>>>>    		return NULL;
>>>>    
>>>> -	if (!prefetched || !sk_fullsock(sk))
>>>> +	if (!prefetched)
>>>>    		return sk;
>>>>    
>>>> +	if (!sk_fullsock(sk)) {
>>>> +		if (sk->sk_state == TCP_NEW_SYN_RECV && inet_reqsk(sk)->syncookie) {
>>>> +			skb->sk = sk;
>>>> +			skb->destructor = sock_pfree;
>>>> +			sk = inet_reqsk(sk)->rsk_listener;
>>>> +		}
>>>> +
>>>> +		return sk;
>>>> +	}
>>>> +
>>>>    	if (sk->sk_protocol == IPPROTO_TCP) {
>>>>    		if (sk->sk_state != TCP_LISTEN)
>>>>    			return sk;
>>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>>> index cc2e4babc85f..bca491ddf42c 100644
>>>> --- a/net/core/filter.c
>>>> +++ b/net/core/filter.c
>>>> @@ -11800,6 +11800,71 @@ __bpf_kfunc int bpf_sock_addr_set_sun_path(struct bpf_sock_addr_kern *sa_kern,
>>>>    
>>>>    	return 0;
>>>>    }
>>>> +
>>>> +__bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct sk_buff *skb, struct sock *sk,
>>>> +					struct tcp_options_received *tcp_opt,
>>>> +					int tcp_opt__sz, u16 mss)
>>>> +{
>>>> +	const struct tcp_request_sock_ops *af_ops;
>>>> +	const struct request_sock_ops *ops;
>>>> +	struct inet_request_sock *ireq;
>>>> +	struct tcp_request_sock *treq;
>>>> +	struct request_sock *req;
>>>> +
>>>> +	if (!sk)
>>>> +		return -EINVAL;
>>>> +
>>>> +	if (!skb_at_tc_ingress(skb))
>>>> +		return -EINVAL;
>>>> +
>>>> +	if (dev_net(skb->dev) != sock_net(sk))
>>>> +		return -ENETUNREACH;
>>>> +
>>>> +	switch (sk->sk_family) {
>>>> +	case AF_INET:  /* TODO: MPTCP */
>>>> +		ops = &tcp_request_sock_ops;
>>>> +		af_ops = &tcp_request_sock_ipv4_ops;
>>>> +		break;
>>>> +#if IS_ENABLED(CONFIG_IPV6)
>>>> +	case AF_INET6:
>>>> +		ops = &tcp6_request_sock_ops;
>>>> +		af_ops = &tcp_request_sock_ipv6_ops;
>>>> +		break;
>>>> +#endif
>>>> +	default:
>>>> +		return -EINVAL;
>>>> +	}
>>>> +
>>>> +	if (sk->sk_type != SOCK_STREAM || sk->sk_state != TCP_LISTEN)
>>>> +		return -EINVAL;
>>>> +
>>>> +	req = inet_reqsk_alloc(ops, sk, true);
>>>> +	if (!req)
>>>> +		return -ENOMEM;
>>>> +
>>>> +	ireq = inet_rsk(req);
>>>> +	treq = tcp_rsk(req);
>>>> +
>>>> +	refcount_set(&req->rsk_refcnt, 1);
>>>> +	req->syncookie = 1;
>>>> +	req->mss = mss;
>>>> +	req->ts_recent = tcp_opt->saw_tstamp ? tcp_opt->rcv_tsval : 0;
>>>> +
>>>> +	ireq->snd_wscale = tcp_opt->snd_wscale;
>>>> +	ireq->sack_ok = tcp_opt->sack_ok;
>>>> +	ireq->wscale_ok = tcp_opt->wscale_ok;
>>>> +	ireq->tstamp_ok	= tcp_opt->saw_tstamp;
>>>> +
>>>> +	tcp_rsk(req)->af_specific = af_ops;
>>>> +	tcp_rsk(req)->ts_off = tcp_opt->rcv_tsecr - tcp_ns_to_ts(tcp_clock_ns());
>>>> +
>>>> +	skb_orphan(skb);
>>>> +	skb->sk = req_to_sk(req);
>>>> +	skb->destructor = sock_pfree;
>>>> +
>>>> +	return 0;
>>>> +}
>>>> +
>>>>    __diag_pop();
>>>>    
>>>>    int bpf_dynptr_from_skb_rdonly(struct sk_buff *skb, u64 flags,
>>>> @@ -11828,6 +11893,10 @@ BTF_SET8_START(bpf_kfunc_check_set_sock_addr)
>>>>    BTF_ID_FLAGS(func, bpf_sock_addr_set_sun_path)
>>>>    BTF_SET8_END(bpf_kfunc_check_set_sock_addr)
>>>>    
>>>> +BTF_SET8_START(bpf_kfunc_check_set_tcp_reqsk)
>>>> +BTF_ID_FLAGS(func, bpf_sk_assign_tcp_reqsk)
>>>> +BTF_SET8_END(bpf_kfunc_check_set_tcp_reqsk)
>>>> +
>>>>    static const struct btf_kfunc_id_set bpf_kfunc_set_skb = {
>>>>    	.owner = THIS_MODULE,
>>>>    	.set = &bpf_kfunc_check_set_skb,
>>>> @@ -11843,6 +11912,11 @@ static const struct btf_kfunc_id_set bpf_kfunc_set_sock_addr = {
>>>>    	.set = &bpf_kfunc_check_set_sock_addr,
>>>>    };
>>>>    
>>>> +static const struct btf_kfunc_id_set bpf_kfunc_set_tcp_reqsk = {
>>>> +	.owner = THIS_MODULE,
>>>> +	.set = &bpf_kfunc_check_set_tcp_reqsk,
>>>> +};
>>>> +
>>>>    static int __init bpf_kfunc_init(void)
>>>>    {
>>>>    	int ret;
>>>> @@ -11858,8 +11932,10 @@ static int __init bpf_kfunc_init(void)
>>>>    	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_LWT_SEG6LOCAL, &bpf_kfunc_set_skb);
>>>>    	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_NETFILTER, &bpf_kfunc_set_skb);
>>>>    	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &bpf_kfunc_set_xdp);
>>>> -	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
>>>> -						&bpf_kfunc_set_sock_addr);
>>>> +	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_CGROUP_SOCK_ADDR,
>>>> +					       &bpf_kfunc_set_sock_addr);
>>>> +	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &bpf_kfunc_set_tcp_reqsk);
>>>> +	return ret;
>>>>    }
>>>>    late_initcall(bpf_kfunc_init);
>>>>    
>>>> ---8<---