Merge branch 'Support direct writes to nf_conn:mark'

Daniel Xu says:

====================

Support direct writes to nf_conn:mark from TC and XDP prog types. This
is useful when applications want to store per-connection metadata. This
is also particularly useful for applications that run both bpf and
iptables/nftables because the latter can trivially access this metadata.

One example use case would be if a bpf prog is responsible for advanced
packet classification and iptables/nftables is later used for routing
due to pre-existing/legacy code.

Past discussion:
- v4: https://lore.kernel.org/bpf/cover.1661192455.git.dxu@dxuuu.xyz/
- v3: https://lore.kernel.org/bpf/cover.1660951028.git.dxu@dxuuu.xyz/
- v2: https://lore.kernel.org/bpf/CAP01T74Sgn354dXGiFWFryu4vg+o8b9s9La1d9zEbC4LGvH4qg@mail.gmail.com/T/
- v1: https://lore.kernel.org/bpf/cover.1660592020.git.dxu@dxuuu.xyz/

Changes since v4:
- Use exported function pointer + mutex to handle CONFIG_NF_CONNTRACK=m
  case

Changes since v3:
- Use a mutex to protect module load/unload critical section

Changes since v2:
- Remove use of NOT_INIT for btf_struct_access write path
- Disallow nf_conn writing when nf_conntrack module not loaded
- Support writing to nf_conn___init:mark

Changes since v1:
- Add unimplemented stub for when !CONFIG_BPF_SYSCALL
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -2211,6 +2211,15 @@ static inline struct bpf_prog *bpf_prog_by_id(u32 id)
 	return ERR_PTR(-ENOTSUPP);
 }
 
+static inline int btf_struct_access(struct bpf_verifier_log *log,
+				    const struct btf *btf,
+				    const struct btf_type *t, int off, int size,
+				    enum bpf_access_type atype,
+				    u32 *next_btf_id, enum bpf_type_flag *flag)
+{
+	return -EACCES;
+}
+
 static inline const struct bpf_func_proto *
 bpf_base_func_proto(enum bpf_func_id func_id)
 {

include/net/netfilter/nf_conntrack_bpf.h

@@ -3,13 +3,22 @@
 #ifndef _NF_CONNTRACK_BPF_H
 #define _NF_CONNTRACK_BPF_H
 
+#include <linux/bpf.h>
 #include <linux/btf.h>
 #include <linux/kconfig.h>
+#include <linux/mutex.h>
 
 #if (IS_BUILTIN(CONFIG_NF_CONNTRACK) && IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) || \
     (IS_MODULE(CONFIG_NF_CONNTRACK) && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES))
 
 extern int register_nf_conntrack_bpf(void);
+extern void cleanup_nf_conntrack_bpf(void);
+
+extern struct mutex nf_conn_btf_access_lock;
+extern int (*nfct_bsa)(struct bpf_verifier_log *log, const struct btf *btf,
+		       const struct btf_type *t, int off, int size,
+		       enum bpf_access_type atype, u32 *next_btf_id,
+		       enum bpf_type_flag *flag);
 
 #else
 
@@ -18,6 +27,20 @@ static inline int register_nf_conntrack_bpf(void)
 	return 0;
 }
 
+static inline void cleanup_nf_conntrack_bpf(void)
+{
+}
+
+static inline int nf_conntrack_btf_struct_access(struct bpf_verifier_log *log,
+						 const struct btf *btf,
+						 const struct btf_type *t, int off,
+						 int size, enum bpf_access_type atype,
+						 u32 *next_btf_id,
+						 enum bpf_type_flag *flag)
+{
+	return -EACCES;
+}
+
 #endif
 
 #endif /* _NF_CONNTRACK_BPF_H */

kernel/bpf/btf.c

@@ -818,6 +818,7 @@ const struct btf_type *btf_type_by_id(const struct btf *btf, u32 type_id)
 		return NULL;
 	return btf->types[type_id];
 }
+EXPORT_SYMBOL_GPL(btf_type_by_id);
 
 /*
  * Regular int is not a bit field and it must be either

kernel/bpf/verifier.c

@@ -370,6 +370,7 @@ __printf(2, 3) void bpf_log(struct bpf_verifier_log *log,
 	bpf_verifier_vlog(log, fmt, args);
 	va_end(args);
 }
+EXPORT_SYMBOL_GPL(bpf_log);
 
 static const char *ltrim(const char *s)
 {
@@ -13406,9 +13407,6 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 				insn->code = BPF_LDX | BPF_PROBE_MEM |
 					BPF_SIZE((insn)->code);
 				env->prog->aux->num_exentries++;
-			} else if (resolve_prog_type(env->prog) != BPF_PROG_TYPE_STRUCT_OPS) {
-				verbose(env, "Writes through BTF pointers are not allowed\n");
-				return -EINVAL;
 			}
 			continue;
 		default:

net/core/filter.c

@@ -18,6 +18,7 @@
  */
 
 #include <linux/atomic.h>
+#include <linux/bpf_verifier.h>
 #include <linux/module.h>
 #include <linux/types.h>
 #include <linux/mm.h>
@@ -8604,6 +8605,36 @@ static bool tc_cls_act_is_valid_access(int off, int size,
 	return bpf_skb_is_valid_access(off, size, type, prog, info);
 }
 
+DEFINE_MUTEX(nf_conn_btf_access_lock);
+EXPORT_SYMBOL_GPL(nf_conn_btf_access_lock);
+
+int (*nfct_bsa)(struct bpf_verifier_log *log, const struct btf *btf,
+		const struct btf_type *t, int off, int size,
+		enum bpf_access_type atype, u32 *next_btf_id,
+		enum bpf_type_flag *flag);
+EXPORT_SYMBOL_GPL(nfct_bsa);
+
+static int tc_cls_act_btf_struct_access(struct bpf_verifier_log *log,
+					const struct btf *btf,
+					const struct btf_type *t, int off,
+					int size, enum bpf_access_type atype,
+					u32 *next_btf_id,
+					enum bpf_type_flag *flag)
+{
+	int ret = -EACCES;
+
+	if (atype == BPF_READ)
+		return btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+					 flag);
+
+	mutex_lock(&nf_conn_btf_access_lock);
+	if (nfct_bsa)
+		ret = nfct_bsa(log, btf, t, off, size, atype, next_btf_id, flag);
+	mutex_unlock(&nf_conn_btf_access_lock);
+
+	return ret;
+}
+
 static bool __is_valid_xdp_access(int off, int size)
 {
 	if (off < 0 || off >= sizeof(struct xdp_md))
@@ -8663,6 +8694,27 @@ void bpf_warn_invalid_xdp_action(struct net_device *dev, struct bpf_prog *prog,
 }
 EXPORT_SYMBOL_GPL(bpf_warn_invalid_xdp_action);
 
+static int xdp_btf_struct_access(struct bpf_verifier_log *log,
+				 const struct btf *btf,
+				 const struct btf_type *t, int off,
+				 int size, enum bpf_access_type atype,
+				 u32 *next_btf_id,
+				 enum bpf_type_flag *flag)
+{
+	int ret = -EACCES;
+
+	if (atype == BPF_READ)
+		return btf_struct_access(log, btf, t, off, size, atype, next_btf_id,
+					 flag);
+
+	mutex_lock(&nf_conn_btf_access_lock);
+	if (nfct_bsa)
+		ret = nfct_bsa(log, btf, t, off, size, atype, next_btf_id, flag);
+	mutex_unlock(&nf_conn_btf_access_lock);
+
+	return ret;
+}
+
 static bool sock_addr_is_valid_access(int off, int size,
 				      enum bpf_access_type type,
 				      const struct bpf_prog *prog,
@@ -10557,6 +10609,7 @@ const struct bpf_verifier_ops tc_cls_act_verifier_ops = {
 	.convert_ctx_access	= tc_cls_act_convert_ctx_access,
 	.gen_prologue		= tc_cls_act_prologue,
 	.gen_ld_abs		= bpf_gen_ld_abs,
+	.btf_struct_access	= tc_cls_act_btf_struct_access,
 };
 
 const struct bpf_prog_ops tc_cls_act_prog_ops = {
@@ -10568,6 +10621,7 @@ const struct bpf_verifier_ops xdp_verifier_ops = {
 	.is_valid_access	= xdp_is_valid_access,
 	.convert_ctx_access	= xdp_convert_ctx_access,
 	.gen_prologue		= bpf_noop_prologue,
+	.btf_struct_access	= xdp_btf_struct_access,
 };
 
 const struct bpf_prog_ops xdp_prog_ops = {

net/ipv4/bpf_tcp_ca.c

@@ -124,7 +124,7 @@ static int bpf_tcp_ca_btf_struct_access(struct bpf_verifier_log *log,
 		return -EACCES;
 	}
 
-	return NOT_INIT;
+	return 0;
 }
 
 BPF_CALL_2(bpf_tcp_send_ack, struct tcp_sock *, tp, u32, rcv_nxt)

net/netfilter/nf_conntrack_bpf.c

@@ -6,8 +6,10 @@
  * are exposed through to BPF programs is explicitly unstable.
  */
 
+#include <linux/bpf_verifier.h>
 #include <linux/bpf.h>
 #include <linux/btf.h>
+#include <linux/mutex.h>
 #include <linux/types.h>
 #include <linux/btf_ids.h>
 #include <linux/net_namespace.h>
@@ -184,6 +186,54 @@ static struct nf_conn *__bpf_nf_ct_lookup(struct net *net,
 	return ct;
 }
 
+BTF_ID_LIST(btf_nf_conn_ids)
+BTF_ID(struct, nf_conn)
+BTF_ID(struct, nf_conn___init)
+
+/* Check writes into `struct nf_conn` */
+static int _nf_conntrack_btf_struct_access(struct bpf_verifier_log *log,
+					   const struct btf *btf,
+					   const struct btf_type *t, int off,
+					   int size, enum bpf_access_type atype,
+					   u32 *next_btf_id,
+					   enum bpf_type_flag *flag)
+{
+	const struct btf_type *ncit;
+	const struct btf_type *nct;
+	size_t end;
+
+	ncit = btf_type_by_id(btf, btf_nf_conn_ids[1]);
+	nct = btf_type_by_id(btf, btf_nf_conn_ids[0]);
+
+	if (t != nct && t != ncit) {
+		bpf_log(log, "only read is supported\n");
+		return -EACCES;
+	}
+
+	/* `struct nf_conn` and `struct nf_conn___init` have the same layout
+	 * so we are safe to simply merge offset checks here
+	 */
+	switch (off) {
+#if defined(CONFIG_NF_CONNTRACK_MARK)
+	case offsetof(struct nf_conn, mark):
+		end = offsetofend(struct nf_conn, mark);
+		break;
+#endif
+	default:
+		bpf_log(log, "no write support to nf_conn at off %d\n", off);
+		return -EACCES;
+	}
+
+	if (off + size > end) {
+		bpf_log(log,
+			"write access at off %d with size %d beyond the member of nf_conn ended at %zu\n",
+			off, size, end);
+		return -EACCES;
+	}
+
+	return 0;
+}
+
 __diag_push();
 __diag_ignore_all("-Wmissing-prototypes",
 		  "Global functions as their definitions will be in nf_conntrack BTF");
@@ -449,5 +499,19 @@ int register_nf_conntrack_bpf(void)
 	int ret;
 
 	ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &nf_conntrack_kfunc_set);
-	return ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &nf_conntrack_kfunc_set);
+	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &nf_conntrack_kfunc_set);
+	if (!ret) {
+		mutex_lock(&nf_conn_btf_access_lock);
+		nfct_bsa = _nf_conntrack_btf_struct_access;
+		mutex_unlock(&nf_conn_btf_access_lock);
+	}
+
+	return ret;
+}
+
+void cleanup_nf_conntrack_bpf(void)
+{
+	mutex_lock(&nf_conn_btf_access_lock);
+	nfct_bsa = NULL;
+	mutex_unlock(&nf_conn_btf_access_lock);
 }

net/netfilter/nf_conntrack_core.c

@@ -2512,6 +2512,7 @@ static int kill_all(struct nf_conn *i, void *data)
 
 void nf_conntrack_cleanup_start(void)
 {
+	cleanup_nf_conntrack_bpf();
 	conntrack_gc_work.exiting = true;
 }
 

tools/testing/selftests/bpf/prog_tests/bpf_nf.c

@@ -17,6 +17,7 @@ struct {
 	{ "set_status_after_insert", "kernel function bpf_ct_set_status args#0 expected pointer to STRUCT nf_conn___init but" },
 	{ "change_timeout_after_alloc", "kernel function bpf_ct_change_timeout args#0 expected pointer to STRUCT nf_conn but" },
 	{ "change_status_after_alloc", "kernel function bpf_ct_change_status args#0 expected pointer to STRUCT nf_conn but" },
+	{ "write_not_allowlisted_field", "no write support to nf_conn at off" },
 };
 
 enum {
@@ -113,6 +114,7 @@ static void test_bpf_nf_ct(int mode)
 	ASSERT_LE(skel->bss->test_delta_timeout, 10, "Test for max ct timeout update");
 	/* expected status is IPS_SEEN_REPLY */
 	ASSERT_EQ(skel->bss->test_status, 2, "Test for ct status update ");
+	ASSERT_EQ(skel->bss->test_insert_lookup_mark, 77, "Test for insert and lookup mark value");
 	ASSERT_EQ(skel->data->test_exist_lookup, 0, "Test existing connection lookup");
 	ASSERT_EQ(skel->bss->test_exist_lookup_mark, 43, "Test existing connection lookup ctmark");
 end:

tools/testing/selftests/bpf/progs/test_bpf_nf.c

@@ -23,6 +23,7 @@ int test_insert_entry = -EAFNOSUPPORT;
 int test_succ_lookup = -ENOENT;
 u32 test_delta_timeout = 0;
 u32 test_status = 0;
+u32 test_insert_lookup_mark = 0;
 __be32 saddr = 0;
 __be16 sport = 0;
 __be32 daddr = 0;
@@ -144,6 +145,7 @@ nf_ct_test(struct nf_conn *(*lookup_fn)(void *, struct bpf_sock_tuple *, u32,
 
 		bpf_ct_set_timeout(ct, 10000);
 		bpf_ct_set_status(ct, IPS_CONFIRMED);
+		ct->mark = 77;
 
 		ct_ins = bpf_ct_insert_entry(ct);
 		if (ct_ins) {
@@ -157,6 +159,7 @@ nf_ct_test(struct nf_conn *(*lookup_fn)(void *, struct bpf_sock_tuple *, u32,
 				test_delta_timeout = ct_lk->timeout - bpf_jiffies64();
 				test_delta_timeout /= CONFIG_HZ;
 				test_status = IPS_SEEN_REPLY;
+				test_insert_lookup_mark = ct_lk->mark;
 				bpf_ct_change_status(ct_lk, IPS_SEEN_REPLY);
 				bpf_ct_release(ct_lk);
 				test_succ_lookup = 0;
@@ -175,8 +178,10 @@ nf_ct_test(struct nf_conn *(*lookup_fn)(void *, struct bpf_sock_tuple *, u32,
 		       sizeof(opts_def));
 	if (ct) {
 		test_exist_lookup = 0;
-		if (ct->mark == 42)
-			test_exist_lookup_mark = 43;
+		if (ct->mark == 42) {
+			ct->mark++;
+			test_exist_lookup_mark = ct->mark;
+		}
 		bpf_ct_release(ct);
 	} else {
 		test_exist_lookup = opts_def.error;

tools/testing/selftests/bpf/progs/test_bpf_nf_fail.c

@@ -69,6 +69,20 @@ int lookup_insert(struct __sk_buff *ctx)
 	return 0;
 }
 
+SEC("?tc")
+int write_not_allowlisted_field(struct __sk_buff *ctx)
+{
+	struct bpf_ct_opts___local opts = {};
+	struct bpf_sock_tuple tup = {};
+	struct nf_conn *ct;
+
+	ct = bpf_skb_ct_lookup(ctx, &tup, sizeof(tup.ipv4), &opts, sizeof(opts));
+	if (!ct)
+		return 0;
+	ct->status = 0xF00;
+	return 0;
+}
+
 SEC("?tc")
 int set_timeout_after_insert(struct __sk_buff *ctx)
 {