Subject: Re: [PATCH bpf-next 02/17] bpf: Introduce SK_LOOKUP program type with a dedicated attach point

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 08 May 2020 12:45:14 +0200

On Fri, May 08, 2020 at 09:06 AM CEST, Martin KaFai Lau wrote:
> On Wed, May 06, 2020 at 02:54:58PM +0200, Jakub Sitnicki wrote:
>> Add a new program type BPF_PROG_TYPE_SK_LOOKUP and a dedicated attach type
>> called BPF_SK_LOOKUP. The new program kind is to be invoked by the
>> transport layer when looking up a socket for a received packet.
>>
>> When called, SK_LOOKUP program can select a socket that will receive the
>> packet. This serves as a mechanism to overcome the limits of what bind()
>> API allows to express. Two use-cases driving this work are:
>>
>>  (1) steer packets destined to an IP range, fixed port to a socket
>>
>>      192.0.2.0/24, port 80 -> NGINX socket
>>
>>  (2) steer packets destined to an IP address, any port to a socket
>>
>>      198.51.100.1, any port -> L7 proxy socket
>>
>> In its run-time context, program receives information about the packet that
>> triggered the socket lookup. Namely IP version, L4 protocol identifier, and
>> address 4-tuple. Context can be further extended to include ingress
>> interface identifier.
>>
>> To select a socket BPF program fetches it from a map holding socket
>> references, like SOCKMAP or SOCKHASH, and calls bpf_sk_assign(ctx, sk, ...)
>> helper to record the selection. Transport layer then uses the selected
>> socket as a result of socket lookup.
>>
>> This patch only enables the user to attach an SK_LOOKUP program to a
>> network namespace. Subsequent patches hook it up to run on local delivery
>> path in ipv4 and ipv6 stacks.
>>
>> Suggested-by: Marek Majkowski <[email-masked]>
>> Reviewed-by: Lorenz Bauer <[email-masked]>
>> Signed-off-by: Jakub Sitnicki <[email-masked]>
>> ---

[...]

>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index bb1ab7da6103..26d643c171fd 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -2729,6 +2729,8 @@ attach_type_to_prog_type(enum bpf_attach_type attach_type)
>>  	case BPF_CGROUP_GETSOCKOPT:
>>  	case BPF_CGROUP_SETSOCKOPT:
>>  		return BPF_PROG_TYPE_CGROUP_SOCKOPT;
>> +	case BPF_SK_LOOKUP:
> It may be a good idea to enforce the "expected_attach_type ==
> BPF_SK_LOOKUP" during prog load time in bpf_prog_load_check_attach().
> The attr->expected_attach_type could be anything right now if I read
> it correctly.

I'll extend bpf_prog_attach_check_attach_type to enforce it for SK_LOOKUP.

>
>> +		return BPF_PROG_TYPE_SK_LOOKUP;
>>  	default:
>>  		return BPF_PROG_TYPE_UNSPEC;
>>  	}
>> @@ -2778,6 +2780,9 @@ static int bpf_prog_attach(const union bpf_attr *attr)
>>  	case BPF_PROG_TYPE_FLOW_DISSECTOR:
>>  		ret = skb_flow_dissector_bpf_prog_attach(attr, prog);
>>  		break;
>> +	case BPF_PROG_TYPE_SK_LOOKUP:
>> +		ret = sk_lookup_prog_attach(attr, prog);
>> +		break;
>>  	case BPF_PROG_TYPE_CGROUP_DEVICE:
>>  	case BPF_PROG_TYPE_CGROUP_SKB:
>>  	case BPF_PROG_TYPE_CGROUP_SOCK:
>> @@ -2818,6 +2823,8 @@ static int bpf_prog_detach(const union bpf_attr *attr)
>>  		return lirc_prog_detach(attr);
>>  	case BPF_PROG_TYPE_FLOW_DISSECTOR:
>>  		return skb_flow_dissector_bpf_prog_detach(attr);
>> +	case BPF_PROG_TYPE_SK_LOOKUP:
>> +		return sk_lookup_prog_detach(attr);
>>  	case BPF_PROG_TYPE_CGROUP_DEVICE:
>>  	case BPF_PROG_TYPE_CGROUP_SKB:
>>  	case BPF_PROG_TYPE_CGROUP_SOCK:
>> @@ -2867,6 +2874,8 @@ static int bpf_prog_query(const union bpf_attr *attr,
>>  		return lirc_prog_query(attr, uattr);
>>  	case BPF_FLOW_DISSECTOR:
>>  		return skb_flow_dissector_prog_query(attr, uattr);
>> +	case BPF_SK_LOOKUP:
>> +		return sk_lookup_prog_query(attr, uattr);
> "# CONFIG_NET is not set" needs to be taken care.

Sorry, embarassing mistake. Will add stubs returning -EINVAL like
flow_dissector and cgroup_bpf progs have.

>
>>  	default:
>>  		return -EINVAL;
>>  	}
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index bc25bb1085b1..a00bdc70041c 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -9054,6 +9054,253 @@ const struct bpf_verifier_ops sk_reuseport_verifier_ops = {
>>
>>  const struct bpf_prog_ops sk_reuseport_prog_ops = {
>>  };
>> +
>> +static DEFINE_MUTEX(sk_lookup_prog_mutex);
>> +
>> +int sk_lookup_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>> +{
>> +	struct net *net = current->nsproxy->net_ns;
>> +	int ret;
>> +
>> +	if (unlikely(attr->attach_flags))
>> +		return -EINVAL;
>> +
>> +	mutex_lock(&sk_lookup_prog_mutex);
>> +	ret = bpf_prog_attach_one(&net->sk_lookup_prog,
>> +				  &sk_lookup_prog_mutex, prog,
>> +				  attr->attach_flags);
>> +	mutex_unlock(&sk_lookup_prog_mutex);
>> +
>> +	return ret;
>> +}
>> +
>> +int sk_lookup_prog_detach(const union bpf_attr *attr)
>> +{
>> +	struct net *net = current->nsproxy->net_ns;
>> +	int ret;
>> +
>> +	if (unlikely(attr->attach_flags))
>> +		return -EINVAL;
>> +
>> +	mutex_lock(&sk_lookup_prog_mutex);
>> +	ret = bpf_prog_detach_one(&net->sk_lookup_prog,
>> +				  &sk_lookup_prog_mutex);
>> +	mutex_unlock(&sk_lookup_prog_mutex);
>> +
>> +	return ret;
>> +}
>> +
>> +int sk_lookup_prog_query(const union bpf_attr *attr,
>> +			 union bpf_attr __user *uattr)
>> +{
>> +	struct net *net;
>> +	int ret;
>> +
>> +	net = get_net_ns_by_fd(attr->query.target_fd);
>> +	if (IS_ERR(net))
>> +		return PTR_ERR(net);
>> +
>> +	ret = bpf_prog_query_one(&net->sk_lookup_prog, attr, uattr);
>> +
>> +	put_net(net);
>> +	return ret;
>> +}
>> +
>> +BPF_CALL_3(bpf_sk_lookup_assign, struct bpf_sk_lookup_kern *, ctx,
>> +	   struct sock *, sk, u64, flags)
>> +{
>> +	if (unlikely(flags != 0))
>> +		return -EINVAL;
>> +	if (unlikely(!sk_fullsock(sk)))
> May be ARG_PTR_TO_SOCKET instead?

I had ARG_PTR_TO_SOCKET initially, then switched to SOCK_COMMON to match
the TC bpf_sk_assign proto. Now that you point it out, it makes more
sense to be more specific in the helper proto.

>
>> +		return -ESOCKTNOSUPPORT;
>> +
>> +	/* Check if socket is suitable for packet L3/L4 protocol */
>> +	if (sk->sk_protocol != ctx->protocol)
>> +		return -EPROTOTYPE;
>> +	if (sk->sk_family != ctx->family &&
>> +	    (sk->sk_family == AF_INET || ipv6_only_sock(sk)))
>> +		return -EAFNOSUPPORT;
>> +
>> +	/* Select socket as lookup result */
>> +	ctx->selected_sk = sk;
> Could sk be a TCP_ESTABLISHED sk?

Yes, and what's worse, it could be ref-counted. This is a bug. I should
be rejecting ref counted sockets here.

Callers of __inet_lookup_listener() and inet6_lookup_listener() expect
an RCU-freed socket on return.

For UDP lookup, returning a TCP_ESTABLISHED (connected) socket is okay.

Thank you for valuable comments. Will fix all of the above in v2.