Re: [PATCH v1 bpf-next 06/11] bpf: tcp: Add SYN Cookie validation SOCK_OPS hook.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Stanislav Fomichev <sdf@google.com>
Date: Mon, 16 Oct 2023 13:38:25 -0700
> On 10/13, Kuniyuki Iwashima wrote:
> > This patch adds a new SOCK_OPS hook to validate arbitrary SYN Cookie.
> > 
> > When the kernel receives ACK for SYN Cookie, the hook is invoked with
> > bpf_sock_ops.op == BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB if the listener has
> > BPF_SOCK_OPS_SYNCOOKIE_CB_FLAG set by bpf_sock_ops_cb_flags_set().
> > 
> > The BPF program can access the following information to validate ISN:
> > 
> >   bpf_sock_ops.sk      : 4-tuple
> >   bpf_sock_ops.skb     : TCP header
> >   bpf_sock_ops.args[0] : ISN
> > 
> > The program must decode MSS and set it to bpf_sock_ops.replylong[0].
> > 
> > By default, the kernel validates SYN Cookie before allocating reqsk, but
> > the hook is invoked after allocating reqsk to keep the user interface
> > consistent with BPF_SOCK_OPS_GEN_SYNCOOKIE_CB.
> > 
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > ---
> >  include/net/tcp.h              | 12 ++++++
> >  include/uapi/linux/bpf.h       | 20 +++++++---
> >  net/ipv4/syncookies.c          | 73 +++++++++++++++++++++++++++-------
> >  net/ipv6/syncookies.c          | 44 +++++++++++++-------
> >  tools/include/uapi/linux/bpf.h | 20 +++++++---
> >  5 files changed, 130 insertions(+), 39 deletions(-)
> > 
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index 676618c89bb7..90d95acdc34a 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -2158,6 +2158,18 @@ static inline __u32 cookie_init_sequence(const struct tcp_request_sock_ops *ops,
> >  	__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESSENT);
> >  	return ops->cookie_init_seq(skb, mss);
> >  }
> > +
> > +#ifdef CONFIG_CGROUP_BPF
> > +int bpf_skops_cookie_check(struct sock *sk, struct request_sock *req,
> > +			   struct sk_buff *skb);
> > +#else
> > +static inline int bpf_skops_cookie_check(struct sock *sk, struct request_sock *req,
> > +					 struct sk_buff *skb)
> > +{
> > +	return 0;
> > +}
> > +#endif
> > +
> >  #else
> >  static inline __u32 cookie_init_sequence(const struct tcp_request_sock_ops *ops,
> >  					 const struct sock *sk, struct sk_buff *skb,
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index d3cc530613c0..e6f1507d7895 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -6738,13 +6738,16 @@ enum {
> >  	 * options first before the BPF program does.
> >  	 */
> >  	BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG = (1<<6),
> > -	/* Call bpf when the kernel generates SYN Cookie (ISN) for SYN+ACK.
> > +	/* Call bpf when the kernel generates SYN Cookie (ISN) for SYN+ACK
> > +	 * and validates ACK for SYN Cookie.
> >  	 *
> > -	 * The bpf prog will be called to encode MSS into SYN Cookie with
> > -	 * sock_ops->op == BPF_SOCK_OPS_GEN_SYNCOOKIE_CB.
> > +	 * The bpf prog will be first called to encode MSS into SYN Cookie
> > +	 * with sock_ops->op == BPF_SOCK_OPS_GEN_SYNCOOKIE_CB.  Then, the
> > +	 * bpf prog will be called to decode MSS from SYN Cookie with
> > +	 * sock_ops->op == BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB.
> >  	 *
> > -	 * Please refer to the comment in BPF_SOCK_OPS_GEN_SYNCOOKIE_CB for
> > -	 * input and output.
> > +	 * Please refer to the comment in BPF_SOCK_OPS_GEN_SYNCOOKIE_CB and
> > +	 * BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB for input and output.
> >  	 */
> >  	BPF_SOCK_OPS_SYNCOOKIE_CB_FLAG = (1<<7),
> >  /* Mask of all currently supported cb flags */
> > @@ -6868,6 +6871,13 @@ enum {
> >  					 *
> >  					 * replylong[0]: ISN
> >  					 */
> > +	BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB,/* Validate SYN Cookie and set
> > +					 * MSS.
> > +					 *
> > +					 * args[0]: ISN
> > +					 *
> > +					 * replylong[0]: MSS
> > +					 */
> >  };
> >  
> >  /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect
> > diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
> > index 514f1a4abdee..b1dd415863ff 100644
> > --- a/net/ipv4/syncookies.c
> > +++ b/net/ipv4/syncookies.c
> > @@ -317,6 +317,37 @@ struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
> >  }
> >  EXPORT_SYMBOL_GPL(cookie_tcp_reqsk_alloc);
> >  
> > +#if IS_ENABLED(CONFIG_CGROUP_BPF) && IS_ENABLED(CONFIG_SYN_COOKIES)
> > +int bpf_skops_cookie_check(struct sock *sk, struct request_sock *req, struct sk_buff *skb)
> > +{
> > +	struct bpf_sock_ops_kern sock_ops;
> > +
> > +	memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
> > +
> > +	sock_ops.op = BPF_SOCK_OPS_CHECK_SYNCOOKIE_CB;
> > +	sock_ops.sk = req_to_sk(req);
> > +	sock_ops.args[0] = tcp_rsk(req)->snt_isn;
> > +
> > +	bpf_skops_init_skb(&sock_ops, skb, tcp_hdrlen(skb));
> > +
> > +	if (BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk))
> > +		goto err;
> > +
> > +	if (!sock_ops.replylong[0])
> > +		goto err;
> > +
> > +	__NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV);
> 
> I don't see LINUX_MIB_SYNCOOKIESSENT being incremented in the
> previous patch, so maybe also don't touch the mib here? The bpf
> program can do the counting if needed?
> 
> Or, alternatively, add LINUX_MIB_SYNCOOKIESSENT to
> the BPF_SOCK_OPS_GEN_SYNCOOKIE_CB path?

Good catch!

I skipped calling tcp_synq_overflow() in the previous patch but should
have incremented LINUX_MIB_SYNCOOKIESSENT.  Will fix in v2.

Thanks!