bpf: add helper for getting xfrm states

This commit introduces a helper which allows fetching xfrm state
parameters by eBPF programs attached to TC.

Prototype:
bpf_skb_get_xfrm_state(skb, index, xfrm_state, size, flags)

skb: pointer to skb
index: the index in the skb xfrm_state secpath array
xfrm_state: pointer to 'struct bpf_xfrm_state'
size: size of 'struct bpf_xfrm_state'
flags: reserved for future extensions

The helper returns 0 on success. Non zero if no xfrm state at the index
is found - or non exists at all.

struct bpf_xfrm_state currently includes the SPI, peer IPv4/IPv6
address and the reqid; it can be further extended by adding elements to
its end - indicating the populated fields by the 'size' argument -
keeping backwards compatibility.

Typical usage:

struct bpf_xfrm_state x = {};
bpf_skb_get_xfrm_state(skb, 0, &x, sizeof(x), 0);
...

Signed-off-by: Eyal Birger <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

include/uapi/linux/bpf.h

@@ -774,6 +774,15 @@ union bpf_attr {
  * @xdp_md: pointer to xdp_md
  * @delta: A negative integer to be added to xdp_md.data_end
  * Return: 0 on success or negative on error
+ *
+ * int bpf_skb_get_xfrm_state(skb, index, xfrm_state, size, flags)
+ * retrieve XFRM state
+ * @skb: pointer to skb
+ * @index: index of the xfrm state in the secpath
+ * @key: pointer to 'struct bpf_xfrm_state'
+ * @size: size of 'struct bpf_xfrm_state'
+ * @flags: room for future extensions
+ * Return: 0 on success or negative error
  */
 #define __BPF_FUNC_MAPPER(FN) \
  FN(unspec), \
@@ -841,7 +850,8 @@ union bpf_attr {
  FN(msg_cork_bytes), \
  FN(msg_pull_data), \
  FN(bind), \
- FN(xdp_adjust_tail),
+ FN(xdp_adjust_tail), \
+ FN(skb_get_xfrm_state),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
@@ -947,6 +957,19 @@ struct bpf_tunnel_key {
  __u32 tunnel_label;
 };
 
+/* user accessible mirror of in-kernel xfrm_state.
+ * new fields can only be added to the end of this structure
+ */
+struct bpf_xfrm_state {
+ __u32 reqid;
+ __u32 spi; /* Stored in network byte order */
+ __u16 family;
+ union {
+ __u32 remote_ipv4; /* Stored in network byte order */
+ __u32 remote_ipv6[4]; /* Stored in network byte order */
+ };
+};
+
 /* Generic BPF return codes which all BPF program types may support.
  * The values are binary compatible with their TC_ACT_* counter-part to
  * provide backwards compatibility with existing SCHED_CLS and SCHED_ACT

net/core/filter.c

@@ -57,6 +57,7 @@
 #include <net/sock_reuseport.h>
 #include <net/busy_poll.h>
 #include <net/tcp.h>
+#include <net/xfrm.h>
 #include <linux/bpf_trace.h>
 
 /**
@@ -3743,6 +3744,49 @@ static const struct bpf_func_proto bpf_bind_proto = {
  .arg3_type = ARG_CONST_SIZE,
 };
 
+#ifdef CONFIG_XFRM
+BPF_CALL_5(bpf_skb_get_xfrm_state, struct sk_buff *, skb, u32, index,
+ struct bpf_xfrm_state *, to, u32, size, u64, flags)
+{
+ const struct sec_path *sp = skb_sec_path(skb);
+ const struct xfrm_state *x;
+
+ if (!sp || unlikely(index >= sp->len || flags))
+ goto err_clear;
+
+ x = sp->xvec[index];
+
+ if (unlikely(size != sizeof(struct bpf_xfrm_state)))
+ goto err_clear;
+
+ to->reqid = x->props.reqid;
+ to->spi = x->id.spi;
+ to->family = x->props.family;
+ if (to->family == AF_INET6) {
+ memcpy(to->remote_ipv6, x->props.saddr.a6,
+ sizeof(to->remote_ipv6));
+ } else {
+ to->remote_ipv4 = x->props.saddr.a4;
+ }
+
+ return 0;
+err_clear:
+ memset(to, 0, size);
+ return -EINVAL;
+}
+
+static const struct bpf_func_proto bpf_skb_get_xfrm_state_proto = {
+ .func = bpf_skb_get_xfrm_state,
+ .gpl_only = false,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_PTR_TO_CTX,
+ .arg2_type = ARG_ANYTHING,
+ .arg3_type = ARG_PTR_TO_UNINIT_MEM,
+ .arg4_type = ARG_CONST_SIZE,
+ .arg5_type = ARG_ANYTHING,
+};
+#endif
+
 static const struct bpf_func_proto *
 bpf_base_func_proto(enum bpf_func_id func_id)
 {
@@ -3884,6 +3928,10 @@ tc_cls_act_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
  return &bpf_get_socket_cookie_proto;
  case BPF_FUNC_get_socket_uid:
  return &bpf_get_socket_uid_proto;
+#ifdef CONFIG_XFRM
+ case BPF_FUNC_skb_get_xfrm_state:
+ return &bpf_skb_get_xfrm_state_proto;
+#endif
  default:
  return bpf_base_func_proto(func_id);
  }