bpf: add bpf_skb_change_type helper

This work adds a helper for changing skb->pkt_type in a controlled way.
We only allow a subset of possible values and can extend that in future
should other use cases come up. Doing this as a helper has the advantage
that errors can be handeled gracefully and thus helper kept extensible.

It's a write counterpart to pkt_type member we can already read from
struct __sk_buff context. Major use case is to change incoming skbs to
PACKET_HOST in a programmatic way instead of having to recirculate via
redirect(..., BPF_F_INGRESS), for example.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 33 insertions, 0 deletions

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 66cd738a937a..be6ac1291680 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -327,6 +327,15 @@ enum bpf_func_id {
          */
         BPF_FUNC_skb_change_proto,
 
+        /**
+         * bpf_skb_change_type(skb, type)
+         * Change packet type of skb.
+         * @skb: pointer to skb
+         * @type: new skb->pkt_type type
+         * Return: 0 on success or negative error
+         */
+        BPF_FUNC_skb_change_type,
+
         __BPF_FUNC_MAX_ID,
 };
 
diff --git a/net/core/filter.c b/net/core/filter.c
index d983e765787a..76f9a4938be4 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1979,6 +1979,28 @@ static const struct bpf_func_proto bpf_skb_change_proto_proto = {
         .arg3_type      = ARG_ANYTHING,
 };
 
+static u64 bpf_skb_change_type(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+        struct sk_buff *skb = (struct sk_buff *) (long) r1;
+        u32 pkt_type = r2;
+
+        /* We only allow a restricted subset to be changed for now. */
+        if (unlikely(skb->pkt_type > PACKET_OTHERHOST ||
+                     pkt_type > PACKET_OTHERHOST))
+                return -EINVAL;
+
+        skb->pkt_type = pkt_type;
+        return 0;
+}
+
+static const struct bpf_func_proto bpf_skb_change_type_proto = {
+        .func           = bpf_skb_change_type,
+        .gpl_only       = false,
+        .ret_type       = RET_INTEGER,
+        .arg1_type      = ARG_PTR_TO_CTX,
+        .arg2_type      = ARG_ANYTHING,
+};
+
 bool bpf_helper_changes_skb_data(void *func)
 {
         if (func == bpf_skb_vlan_push)
@@ -2278,6 +2300,8 @@ tc_cls_act_func_proto(enum bpf_func_id func_id)
                 return &bpf_skb_vlan_pop_proto;
         case BPF_FUNC_skb_change_proto:
                 return &bpf_skb_change_proto_proto;
+        case BPF_FUNC_skb_change_type:
+                return &bpf_skb_change_type_proto;
         case BPF_FUNC_skb_get_tunnel_key:
                 return &bpf_skb_get_tunnel_key_proto;
         case BPF_FUNC_skb_set_tunnel_key: