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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

> 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.


> 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().


> > 
> >  >
> > > 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<---