[PATCH RFC v2 bpf-next 3/3] bpf: minimal support for programs hooked into netfilter framework

[Florian Westphal <fw@strlen.de>]

This adds minimal support for BPF_PROG_TYPE_NETFILTER bpf programs
that will be invoked via the NF_HOOK() points in the ip stack.

Invocation incurs an indirect call.  This is not a necessity: Its
possible to add 'DEFINE_BPF_DISPATCHER(nf_progs)' and handle the
program invocation with the same method already done for xdp progs.

This isn't done here to keep the size of this chunk down.

Verifier restricts verdicts to either DROP or ACCEPT.

Signed-off-by: Florian Westphal <fw@strlen.de>
---
 include/linux/bpf_types.h           |  4 ++
 include/net/netfilter/nf_hook_bpf.h |  6 +++
 kernel/bpf/btf.c                    |  5 ++
 kernel/bpf/verifier.c               |  3 ++
 net/netfilter/nf_bpf_link.c         | 78 ++++++++++++++++++++++++++++-
 5 files changed, 95 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
index d4ee3ccd3753..39a999abb0ce 100644
--- a/include/linux/bpf_types.h
+++ b/include/linux/bpf_types.h
@@ -79,6 +79,10 @@ BPF_PROG_TYPE(BPF_PROG_TYPE_LSM, lsm,
 #endif
 BPF_PROG_TYPE(BPF_PROG_TYPE_SYSCALL, bpf_syscall,
 	      void *, void *)
+#ifdef CONFIG_NETFILTER
+BPF_PROG_TYPE(BPF_PROG_TYPE_NETFILTER, netfilter,
+	      struct bpf_nf_ctx, struct bpf_nf_ctx)
+#endif
 
 BPF_MAP_TYPE(BPF_MAP_TYPE_ARRAY, array_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_PERCPU_ARRAY, percpu_array_map_ops)
diff --git a/include/net/netfilter/nf_hook_bpf.h b/include/net/netfilter/nf_hook_bpf.h
index 9d1b338e89d7..863cbbcc66f9 100644
--- a/include/net/netfilter/nf_hook_bpf.h
+++ b/include/net/netfilter/nf_hook_bpf.h
@@ -1,2 +1,8 @@
 /* SPDX-License-Identifier: GPL-2.0 */
+
+struct bpf_nf_ctx {
+	const struct nf_hook_state *state;
+	struct sk_buff *skb;
+};
+
 int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog);
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index ef2d8969ed1f..ec6eb78b9aec 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -25,6 +25,9 @@
 #include <linux/bsearch.h>
 #include <linux/kobject.h>
 #include <linux/sysfs.h>
+
+#include <net/netfilter/nf_hook_bpf.h>
+
 #include <net/sock.h>
 #include "../tools/lib/bpf/relo_core.h"
 
@@ -7726,6 +7729,8 @@ static int bpf_prog_type_to_kfunc_hook(enum bpf_prog_type prog_type)
 	case BPF_PROG_TYPE_LWT_XMIT:
 	case BPF_PROG_TYPE_LWT_SEG6LOCAL:
 		return BTF_KFUNC_HOOK_LWT;
+	case BPF_PROG_TYPE_NETFILTER:
+		return BTF_KFUNC_HOOK_SOCKET_FILTER;
 	default:
 		return BTF_KFUNC_HOOK_MAX;
 	}
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 8dcddcc00bd0..733af5a28421 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -13142,6 +13142,9 @@ static int check_return_code(struct bpf_verifier_env *env)
 		}
 		break;
 
+	case BPF_PROG_TYPE_NETFILTER:
+		range = tnum_range(NF_DROP, NF_ACCEPT);
+		break;
 	case BPF_PROG_TYPE_EXT:
 		/* freplace program can return anything as its return value
 		 * depends on the to-be-replaced kernel func or bpf program.
diff --git a/net/netfilter/nf_bpf_link.c b/net/netfilter/nf_bpf_link.c
index fa4fae5cc669..b28b7bbdc3a9 100644
--- a/net/netfilter/nf_bpf_link.c
+++ b/net/netfilter/nf_bpf_link.c
@@ -1,12 +1,19 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/bpf.h>
+#include <linux/filter.h>
 #include <linux/netfilter.h>
 
 #include <net/netfilter/nf_hook_bpf.h>
 
 static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb, const struct nf_hook_state *s)
 {
-	return NF_ACCEPT;
+	const struct bpf_prog *prog = bpf_prog;
+	struct bpf_nf_ctx ctx = {
+		.state = s,
+		.skb = skb,
+	};
+
+	return bpf_prog_run(prog, &ctx);
 }
 
 struct bpf_nf_link {
@@ -114,3 +121,72 @@ int bpf_nf_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	kfree(link);
 	return err;
 }
+
+static int bpf_prog_test_run_nf(struct bpf_prog *prog,
+				const union bpf_attr *kattr,
+				union bpf_attr __user *uattr)
+{
+	return -EOPNOTSUPP;
+}
+
+const struct bpf_prog_ops netfilter_prog_ops = {
+	.test_run		= bpf_prog_test_run_nf,
+};
+
+static bool nf_ptr_to_btf_id(struct bpf_insn_access_aux *info, const char *name)
+{
+	struct btf *btf;
+	s32 type_id;
+
+	btf = bpf_get_btf_vmlinux();
+	if (IS_ERR_OR_NULL(btf))
+		return false;
+
+	type_id = btf_find_by_name_kind(btf, name, BTF_KIND_STRUCT);
+	if (WARN_ON_ONCE(type_id < 0))
+		return false;
+
+	info->btf = btf;
+	info->btf_id = type_id;
+	info->reg_type = PTR_TO_BTF_ID | PTR_TRUSTED;
+	return true;
+}
+
+static bool nf_is_valid_access(int off, int size, enum bpf_access_type type,
+			       const struct bpf_prog *prog,
+			       struct bpf_insn_access_aux *info)
+{
+	if (off < 0 || off >= sizeof(struct bpf_nf_ctx))
+		return false;
+
+	if (type == BPF_WRITE)
+		return false;
+
+	switch (off) {
+	case bpf_ctx_range(struct bpf_nf_ctx, skb):
+		if (size != sizeof_field(struct bpf_nf_ctx, skb))
+			return false;
+
+		return nf_ptr_to_btf_id(info, "sk_buff");
+	case bpf_ctx_range(struct bpf_nf_ctx, state):
+		if (size != sizeof_field(struct bpf_nf_ctx, state))
+			return false;
+
+		return nf_ptr_to_btf_id(info, "nf_hook_state");
+	default:
+		return false;
+	}
+
+	return false;
+}
+
+static const struct bpf_func_proto *
+bpf_nf_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
+{
+	return bpf_base_func_proto(func_id);
+}
+
+const struct bpf_verifier_ops netfilter_verifier_ops = {
+	.is_valid_access	= nf_is_valid_access,
+	.get_func_proto		= bpf_nf_func_proto,
+};
-- 
2.39.2