[RFC bpf-next 07/11] bpf: Add helper to retrieve socket in BPF

[Joe Stringer <joe@wand.net.nz>]

This patch adds a new BPF helper function, sk_lookup() which allows BPF
programs to find out if there is a socket listening on this host, and
returns a socket pointer which the BPF program can then access to
determine, for instance, whether to forward or drop traffic. sk_lookup()
takes a reference on the socket, so when a BPF program makes use of this
function, it must subsequently pass the returned pointer into the newly
added sk_release() to return the reference.

By way of example, the following pseudocode would filter inbound
connections at XDP if there is no corresponding service listening for
the traffic:

  struct bpf_sock_tuple tuple;
  struct bpf_sock_ops *sk;

  populate_tuple(ctx, &tuple); // Extract the 5tuple from the packet
  sk = bpf_sk_lookup(ctx, &tuple, sizeof tuple, netns, 0);
  if (!sk) {
    // Couldn't find a socket listening for this traffic. Drop.
    return TC_ACT_SHOT;
  }
  bpf_sk_release(sk, 0);
  return TC_ACT_OK;

Signed-off-by: Joe Stringer <joe@wand.net.nz>
---
 include/uapi/linux/bpf.h                  |  39 +++++++++++-
 kernel/bpf/verifier.c                     |   8 ++-
 net/core/filter.c                         | 102 ++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h            |  40 +++++++++++-
 tools/testing/selftests/bpf/bpf_helpers.h |   7 ++
 5 files changed, 193 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d615c777b573..29f38838dbca 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -1828,6 +1828,25 @@ union bpf_attr {
  * 	Return
  * 		0 on success, or a negative error in case of failure.
  *
+ * struct bpf_sock_ops *bpf_sk_lookup(ctx, tuple, tuple_size, netns, flags)
+ * 	Decription
+ * 		Look for socket matching 'tuple'. The return value must be checked,
+ * 		and if non-NULL, released via bpf_sk_release().
+ * 		@ctx: pointer to ctx
+ * 		@tuple: pointer to struct bpf_sock_tuple
+ * 		@tuple_size: size of the tuple
+ * 		@flags: flags value
+ * 	Return
+ * 		pointer to socket ops on success, or
+ * 		NULL in case of failure
+ *
+ *  int bpf_sk_release(sock, flags)
+ * 	Description
+ * 		Release the reference held by 'sock'.
+ * 		@sock: Pointer reference to release. Must be found via bpf_sk_lookup().
+ * 		@flags: flags value
+ * 	Return
+ * 		0 on success, or a negative error in case of failure.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -1898,7 +1917,9 @@ union bpf_attr {
 	FN(xdp_adjust_tail),		\
 	FN(skb_get_xfrm_state),		\
 	FN(get_stack),			\
-	FN(skb_load_bytes_relative),
+	FN(skb_load_bytes_relative),	\
+	FN(sk_lookup),			\
+	FN(sk_release),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -2060,6 +2081,22 @@ struct bpf_sock {
 				 */
 };
 
+struct bpf_sock_tuple {
+	union {
+		__be32 ipv6[4];
+		__be32 ipv4;
+	} saddr;
+	union {
+		__be32 ipv6[4];
+		__be32 ipv4;
+	} daddr;
+	__be16 sport;
+	__be16 dport;
+	__u32 dst_if;
+	__u8 family;
+	__u8 proto;
+};
+
 #define XDP_PACKET_HEADROOM 256
 
 /* User return codes for XDP prog type.
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 92b9a5dc465a..579012c483e4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -153,6 +153,12 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
  * PTR_TO_MAP_VALUE, PTR_TO_SOCKET_OR_NULL becomes PTR_TO_SOCKET when the type
  * passes through a NULL-check conditional. For the branch wherein the state is
  * changed to CONST_IMM, the verifier releases the reference.
+ *
+ * For each helper function that allocates a reference, such as bpf_sk_lookup(),
+ * there is a corresponding release function, such as bpf_sk_release(). When
+ * a reference type passes into the release function, the verifier also releases
+ * the reference. If any unchecked or unreleased reference remains at the end of
+ * the program, the verifier rejects it.
  */
 
 /* verifier_state + insn_idx are pushed to stack when branch is encountered */
@@ -277,7 +283,7 @@ static bool arg_type_is_refcounted(enum bpf_arg_type type)
  */
 static bool is_release_function(enum bpf_func_id func_id)
 {
-	return false;
+	return func_id == BPF_FUNC_sk_release;
 }
 
 /* string representation of 'enum bpf_reg_type' */
diff --git a/net/core/filter.c b/net/core/filter.c
index 4c35152fb3a8..751c255d17d3 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -58,8 +58,12 @@
 #include <net/busy_poll.h>
 #include <net/tcp.h>
 #include <net/xfrm.h>
+#include <net/udp.h>
 #include <linux/bpf_trace.h>
 #include <net/xdp_sock.h>
+#include <net/inet_hashtables.h>
+#include <net/inet6_hashtables.h>
+#include <net/net_namespace.h>
 
 /**
  *	sk_filter_trim_cap - run a packet through a socket filter
@@ -4032,6 +4036,96 @@ static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
 };
 #endif
 
+struct sock *
+sk_lookup(struct net *net, struct bpf_sock_tuple *tuple) {
+	int dst_if = (int)tuple->dst_if;
+	struct in6_addr *src6;
+	struct in6_addr *dst6;
+
+	if (tuple->family == AF_INET6) {
+		src6 = (struct in6_addr *)&tuple->saddr.ipv6;
+		dst6 = (struct in6_addr *)&tuple->daddr.ipv6;
+	} else if (tuple->family != AF_INET) {
+		return ERR_PTR(-EOPNOTSUPP);
+	}
+
+	if (tuple->proto == IPPROTO_TCP) {
+		if (tuple->family == AF_INET)
+			return inet_lookup(net, &tcp_hashinfo, NULL, 0,
+					   tuple->saddr.ipv4, tuple->sport,
+					   tuple->daddr.ipv4, tuple->dport,
+					   dst_if);
+		else
+			return inet6_lookup(net, &tcp_hashinfo, NULL, 0,
+					    src6, tuple->sport,
+					    dst6, tuple->dport, dst_if);
+	} else if (tuple->proto == IPPROTO_UDP) {
+		if (tuple->family == AF_INET)
+			return udp4_lib_lookup(net, tuple->saddr.ipv4,
+					       tuple->sport, tuple->daddr.ipv4,
+					       tuple->dport, dst_if);
+		else
+			return udp6_lib_lookup(net, src6, tuple->sport,
+					       dst6, tuple->dport, dst_if);
+	} else {
+		return ERR_PTR(-EOPNOTSUPP);
+	}
+
+	return NULL;
+}
+
+BPF_CALL_5(bpf_sk_lookup, struct sk_buff *, skb,
+	   struct bpf_sock_tuple *, tuple, u32, len, u32, netns_id, u64, flags)
+{
+	struct net *caller_net = dev_net(skb->dev);
+	struct sock *sk = NULL;
+	struct net *net;
+
+	/* XXX: Perform verification-time checking of tuple size? */
+	if (unlikely(len != sizeof(struct bpf_sock_tuple) || flags))
+		goto out;
+
+	net = get_net_ns_by_id(caller_net, netns_id);
+	if (unlikely(!net))
+		goto out;
+
+	sk = sk_lookup(net, tuple);
+	put_net(net);
+	if (IS_ERR_OR_NULL(sk))
+		sk = NULL;
+	else
+		sk = sk_to_full_sk(sk);
+out:
+	return (unsigned long) sk;
+}
+
+static const struct bpf_func_proto bpf_sk_lookup_proto = {
+	.func		= bpf_sk_lookup,
+	.gpl_only	= false,
+	.ret_type	= RET_PTR_TO_SOCKET_OR_NULL,
+	.arg1_type	= ARG_PTR_TO_CTX,
+	.arg2_type	= ARG_PTR_TO_MEM,
+	.arg3_type	= ARG_CONST_SIZE,
+	.arg4_type	= ARG_ANYTHING,
+	.arg5_type	= ARG_ANYTHING,
+};
+
+BPF_CALL_2(bpf_sk_release, struct sock *, sk, u64, flags)
+{
+	sock_gen_put(sk);
+	if (unlikely(flags))
+		return -EINVAL;
+	return 0;
+}
+
+static const struct bpf_func_proto bpf_sk_release_proto = {
+	.func		= bpf_sk_release,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+	.arg1_type	= ARG_PTR_TO_SOCKET,
+	.arg2_type	= ARG_ANYTHING,
+};
+
 static const struct bpf_func_proto *
 bpf_base_func_proto(enum bpf_func_id func_id)
 {
@@ -4181,6 +4275,10 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	case BPF_FUNC_skb_get_xfrm_state:
 		return &bpf_skb_get_xfrm_state_proto;
 #endif
+	case BPF_FUNC_sk_lookup:
+		return &bpf_sk_lookup_proto;
+	case BPF_FUNC_sk_release:
+		return &bpf_sk_release_proto;
 	default:
 		return bpf_base_func_proto(func_id);
 	}
@@ -4292,6 +4390,10 @@ sk_skb_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_socket_uid_proto;
 	case BPF_FUNC_sk_redirect_map:
 		return &bpf_sk_redirect_map_proto;
+	case BPF_FUNC_sk_lookup:
+		return &bpf_sk_lookup_proto;
+	case BPF_FUNC_sk_release:
+		return &bpf_sk_release_proto;
 	default:
 		return bpf_base_func_proto(func_id);
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index fff51c187d1e..29f38838dbca 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -117,6 +117,7 @@ enum bpf_map_type {
 	BPF_MAP_TYPE_DEVMAP,
 	BPF_MAP_TYPE_SOCKMAP,
 	BPF_MAP_TYPE_CPUMAP,
+	BPF_MAP_TYPE_XSKMAP,
 };
 
 enum bpf_prog_type {
@@ -1827,6 +1828,25 @@ union bpf_attr {
  * 	Return
  * 		0 on success, or a negative error in case of failure.
  *
+ * struct bpf_sock_ops *bpf_sk_lookup(ctx, tuple, tuple_size, netns, flags)
+ * 	Decription
+ * 		Look for socket matching 'tuple'. The return value must be checked,
+ * 		and if non-NULL, released via bpf_sk_release().
+ * 		@ctx: pointer to ctx
+ * 		@tuple: pointer to struct bpf_sock_tuple
+ * 		@tuple_size: size of the tuple
+ * 		@flags: flags value
+ * 	Return
+ * 		pointer to socket ops on success, or
+ * 		NULL in case of failure
+ *
+ *  int bpf_sk_release(sock, flags)
+ * 	Description
+ * 		Release the reference held by 'sock'.
+ * 		@sock: Pointer reference to release. Must be found via bpf_sk_lookup().
+ * 		@flags: flags value
+ * 	Return
+ * 		0 on success, or a negative error in case of failure.
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -1897,7 +1917,9 @@ union bpf_attr {
 	FN(xdp_adjust_tail),		\
 	FN(skb_get_xfrm_state),		\
 	FN(get_stack),			\
-	FN(skb_load_bytes_relative),
+	FN(skb_load_bytes_relative),	\
+	FN(sk_lookup),			\
+	FN(sk_release),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -2059,6 +2081,22 @@ struct bpf_sock {
 				 */
 };
 
+struct bpf_sock_tuple {
+	union {
+		__be32 ipv6[4];
+		__be32 ipv4;
+	} saddr;
+	union {
+		__be32 ipv6[4];
+		__be32 ipv4;
+	} daddr;
+	__be16 sport;
+	__be16 dport;
+	__u32 dst_if;
+	__u8 family;
+	__u8 proto;
+};
+
 #define XDP_PACKET_HEADROOM 256
 
 /* User return codes for XDP prog type.
diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h
index 265f8e0e8ada..4dc311ea0c16 100644
--- a/tools/testing/selftests/bpf/bpf_helpers.h
+++ b/tools/testing/selftests/bpf/bpf_helpers.h
@@ -103,6 +103,13 @@ static int (*bpf_skb_get_xfrm_state)(void *ctx, int index, void *state,
 	(void *) BPF_FUNC_skb_get_xfrm_state;
 static int (*bpf_get_stack)(void *ctx, void *buf, int size, int flags) =
 	(void *) BPF_FUNC_get_stack;
+static struct bpf_sock *(*bpf_sk_lookup)(void *ctx,
+					 struct bpf_sock_tuple *tuple,
+					 int size, unsigned int netns_id,
+					 unsigned long long flags) =
+	(void *) BPF_FUNC_sk_lookup;
+static int (*bpf_sk_release)(struct bpf_sock *sk, unsigned long long flags) =
+	(void *) BPF_FUNC_sk_release;
 
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions
-- 
2.14.1