bpf: add cg_skb_is_valid_access for BPF_PROG_TYPE_CGROUP_SKB

BPF programs of BPF_PROG_TYPE_CGROUP_SKB need to access headers in the
skb. This patch enables direct access of skb for these programs.

Two helper functions bpf_compute_and_save_data_end() and
bpf_restore_data_end() are introduced. There are used in
__cgroup_bpf_run_filter_skb(), to compute proper data_end for the
BPF program, and restore original data afterwards.

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: liu-song-6

include/linux/filter.h

@@ -548,6 +548,27 @@ static inline void bpf_compute_data_pointers(struct sk_buff *skb)
 	cb->data_end  = skb->data + skb_headlen(skb);
 }
 
+/* Similar to bpf_compute_data_pointers(), except that save orginal
+ * data in cb->data and cb->meta_data for restore.
+ */
+static inline void bpf_compute_and_save_data_end(
+	struct sk_buff *skb, void **saved_data_end)
+{
+	struct bpf_skb_data_end *cb = (struct bpf_skb_data_end *)skb->cb;
+
+	*saved_data_end = cb->data_end;
+	cb->data_end  = skb->data + skb_headlen(skb);
+}
+
+/* Restore data saved by bpf_compute_data_pointers(). */
+static inline void bpf_restore_data_end(
+	struct sk_buff *skb, void *saved_data_end)
+{
+	struct bpf_skb_data_end *cb = (struct bpf_skb_data_end *)skb->cb;
+
+	cb->data_end = saved_data_end;
+}
+
 static inline u8 *bpf_skb_cb(struct sk_buff *skb)
 {
 	/* eBPF programs may read/write skb->cb[] area to transfer meta

kernel/bpf/cgroup.c

@@ -553,6 +553,7 @@ int __cgroup_bpf_run_filter_skb(struct sock *sk,
 {
 	unsigned int offset = skb->data - skb_network_header(skb);
 	struct sock *save_sk;
+	void *saved_data_end;
 	struct cgroup *cgrp;
 	int ret;
 
@@ -566,8 +567,13 @@ int __cgroup_bpf_run_filter_skb(struct sock *sk,
 	save_sk = skb->sk;
 	skb->sk = sk;
 	__skb_push(skb, offset);
+
+	/* compute pointers for the bpf prog */
+	bpf_compute_and_save_data_end(skb, &saved_data_end);
+
 	ret = BPF_PROG_RUN_ARRAY(cgrp->bpf.effective[type], skb,
 				 bpf_prog_run_save_cb);
+	bpf_restore_data_end(skb, saved_data_end);
 	__skb_pull(skb, offset);
 	skb->sk = save_sk;
 	return ret == 1 ? 0 : -EPERM;

net/core/filter.c

@@ -5352,6 +5352,40 @@ static bool sk_filter_is_valid_access(int off, int size,
 	return bpf_skb_is_valid_access(off, size, type, prog, info);
 }
 
+static bool cg_skb_is_valid_access(int off, int size,
+				   enum bpf_access_type type,
+				   const struct bpf_prog *prog,
+				   struct bpf_insn_access_aux *info)
+{
+	switch (off) {
+	case bpf_ctx_range(struct __sk_buff, tc_classid):
+	case bpf_ctx_range(struct __sk_buff, data_meta):
+	case bpf_ctx_range(struct __sk_buff, flow_keys):
+		return false;
+	}
+	if (type == BPF_WRITE) {
+		switch (off) {
+		case bpf_ctx_range(struct __sk_buff, mark):
+		case bpf_ctx_range(struct __sk_buff, priority):
+		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
+			break;
+		default:
+			return false;
+		}
+	}
+
+	switch (off) {
+	case bpf_ctx_range(struct __sk_buff, data):
+		info->reg_type = PTR_TO_PACKET;
+		break;
+	case bpf_ctx_range(struct __sk_buff, data_end):
+		info->reg_type = PTR_TO_PACKET_END;
+		break;
+	}
+
+	return bpf_skb_is_valid_access(off, size, type, prog, info);
+}
+
 static bool lwt_is_valid_access(int off, int size,
 				enum bpf_access_type type,
 				const struct bpf_prog *prog,
@@ -7044,7 +7078,7 @@ const struct bpf_prog_ops xdp_prog_ops = {
 
 const struct bpf_verifier_ops cg_skb_verifier_ops = {
 	.get_func_proto		= cg_skb_func_proto,
-	.is_valid_access	= sk_filter_is_valid_access,
+	.is_valid_access	= cg_skb_is_valid_access,
 	.convert_ctx_access	= bpf_convert_ctx_access,
 };