bpf: Add support for writing to nf_conn:mark

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.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/ebca06dea366e3e7e861c12f375a548cc4c61108.1662568410.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 66 insertions, 1 deletions

diff --git a/net/netfilter/nf_conntrack_bpf.c b/net/netfilter/nf_conntrack_bpf.c
index 1cd87b28c9b0..77eb8e959f61 100644
--- a/net/netfilter/nf_conntrack_bpf.c
+++ b/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);
 }
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index da65c6e8eeeb..0195f60fc43b 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/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;
 }