[PATCH bpf-next 4/6] selftests/bpf: Add ksock LSM recursion test

[Mahe Tardy <mahe.tardy@gmail.com>]

The bpf_ksock_send() kfunc triggers the security_socket_sendmsg() LSM
hook via kernel_sendmsg(), thus attaching an LSM program triggering that
kfunc on the same hook would provoke recursion. This test exercises that
path and make sure that recursion is guarded and prevented.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
---
 .../testing/selftests/bpf/prog_tests/ksock.c  | 84 +++++++++++++++++--
 .../testing/selftests/bpf/progs/ksock_basic.c | 33 +-------
 .../selftests/bpf/progs/ksock_common.h        | 37 ++++++++
 .../selftests/bpf/progs/ksock_recursion.c     | 69 +++++++++++++++
 4 files changed, 186 insertions(+), 37 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/ksock_recursion.c

diff --git a/tools/testing/selftests/bpf/prog_tests/ksock.c b/tools/testing/selftests/bpf/prog_tests/ksock.c
index 085ddb59067e..8b14fd98d410 100644
--- a/tools/testing/selftests/bpf/prog_tests/ksock.c
+++ b/tools/testing/selftests/bpf/prog_tests/ksock.c
@@ -6,8 +6,10 @@
 #include "test_progs.h"
 #include "network_helpers.h"
 #include "ksock_basic.skel.h"
+#include "ksock_recursion.skel.h"

 #define NS_TEST "ksock_basic_ns"
+#define NS_LSM_RECURSION_TEST "ksock_lsm_recursion_ns"
 #define LOOPBACK_IP "127.0.0.1"
 #define RECV_PORT 7777
 #define RECV_TIMEOUT_SEC 5
@@ -98,15 +100,16 @@ static void ksock_assert_recv(struct ksock_test_env *env, const char *data,
 	ASSERT_MEMEQ(env->buf, data, data_sz, "payload match");
 }

-static bool ksock_setup_ctx(struct ksock_basic *skel)
+static bool ksock_setup_ctx(struct bpf_program *prog, __be32 *ipv4_remote,
+			    __u16 *remote_port)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, opts);
 	int err, pfd;

-	skel->bss->ipv4_remote = inet_addr(LOOPBACK_IP);
-	skel->bss->remote_port = RECV_PORT;
+	*ipv4_remote = inet_addr(LOOPBACK_IP);
+	*remote_port = RECV_PORT;

-	pfd = bpf_program__fd(skel->progs.ksock_setup);
+	pfd = bpf_program__fd(prog);
 	if (!ASSERT_OK_FD(pfd, "ksock_setup fd"))
 		return false;

@@ -140,7 +143,9 @@ void test_ksock_basic(void)
 		goto fail;

 	/* Step 1: Run the setup SYSCALL prog to create ksock */
-	if (!ksock_setup_ctx(skel))
+	if (!ksock_setup_ctx(skel->progs.ksock_setup,
+			     &skel->bss->ipv4_remote,
+			     &skel->bss->remote_port))
 		goto fail;

 	/* Step 2: Run the send SYSCALL prog */
@@ -163,3 +168,72 @@ void test_ksock_basic(void)
 	ksock_test_env_cleanup(&env);
 	ksock_basic__destroy(skel);
 }
+
+void test_ksock_lsm_recursion(void)
+{
+	struct ksock_test_env env;
+	struct ksock_recursion *skel;
+	char trigger = 'x';
+	int tfd = -1;
+	int err;
+	ssize_t n;
+
+	skel = ksock_recursion__open();
+	if (!ASSERT_OK_PTR(skel, "skel open"))
+		return;
+
+	err = ksock_recursion__load(skel);
+	if (!ASSERT_OK(err, "skel load")) {
+		ksock_recursion__destroy(skel);
+		return;
+	}
+
+	if (!ksock_test_env_setup(&env, NS_LSM_RECURSION_TEST))
+		goto fail;
+
+	/* Step 1: Run the setup SYSCALL prog to create the ksock */
+	if (!ksock_setup_ctx(skel->progs.ksock_setup,
+			     &skel->bss->ipv4_remote,
+			     &skel->bss->remote_port))
+		goto fail;
+
+	/* Step 2: Attach LSM prog and trigger socket_sendmsg from userspace */
+	skel->links.ksock_socket_sendmsg =
+		bpf_program__attach_lsm(skel->progs.ksock_socket_sendmsg);
+	if (!ASSERT_OK_PTR(skel->links.ksock_socket_sendmsg,
+			   "attach socket_sendmsg lsm"))
+		goto fail;
+
+	tfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
+	if (!ASSERT_OK_FD(tfd, "trigger socket"))
+		goto fail;
+
+	skel->bss->target_pid = getpid();
+	skel->bss->trigger_send = 1;
+	n = sendto(tfd, &trigger, sizeof(trigger), 0,
+		   (struct sockaddr *)&env.addr, sizeof(env.addr));
+	skel->bss->target_pid = 0;
+	skel->bss->trigger_send = 0;
+	if (!ASSERT_EQ(n, sizeof(trigger), "trigger sendto"))
+		goto fail;
+
+	/* Step 3: The nested bpf_ksock_send() must hit the recursion guard */
+	if (!ASSERT_EQ(skel->bss->rec_count, 2,
+		       "socket_sendmsg recursion count"))
+		goto fail;
+	if (!ASSERT_EQ(skel->data->rec_kfunc_rets[0], -EBUSY,
+		       "recursive send status"))
+		goto fail;
+	if (!ASSERT_EQ(skel->data->rec_kfunc_rets[1],
+		       sizeof(skel->data->send_data), "outer send bytes"))
+		goto fail;
+
+	ksock_assert_recv(&env, skel->data->send_data,
+			  sizeof(skel->data->send_data));
+
+fail:
+	if (tfd >= 0)
+		close(tfd);
+	ksock_test_env_cleanup(&env);
+	ksock_recursion__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/ksock_basic.c b/tools/testing/selftests/bpf/progs/ksock_basic.c
index e8131932d97b..189b0fc15770 100644
--- a/tools/testing/selftests/bpf/progs/ksock_basic.c
+++ b/tools/testing/selftests/bpf/progs/ksock_basic.c
@@ -8,41 +8,10 @@
 #include "bpf_tracing_net.h"
 #include "ksock_common.h"

-__be32 ipv4_remote;
-__u16 remote_port;
-
-char send_data[32] = "hello from bpf ksock";
-
 SEC("syscall")
 int ksock_setup(void *ctx)
 {
-	struct bpf_ksock_create_opts create_opts = {};
-	struct bpf_ksock_addr_opts addr_opts = {};
-	struct bpf_ksock *ks;
-	int err = 0;
-
-	create_opts.family = AF_INET;
-	create_opts.type = SOCK_DGRAM;
-	create_opts.protocol = IPPROTO_UDP;
-
-	ks = bpf_ksock_create(&create_opts, sizeof(create_opts), &err);
-	if (!ks)
-		return err;
-
-	addr_opts.family = AF_INET;
-	addr_opts.port = remote_port;
-	addr_opts.ipv4_addr = ipv4_remote;
-
-	err = bpf_ksock_connect(ks, &addr_opts, sizeof(addr_opts));
-	if (err) {
-		bpf_ksock_release(ks);
-		return err;
-	}
-
-	err = ksock_ctx_insert(ks);
-	if (err && err != -EEXIST)
-		return err;
-	return 0;
+	return do_ksock_setup();
 }

 SEC("syscall")
diff --git a/tools/testing/selftests/bpf/progs/ksock_common.h b/tools/testing/selftests/bpf/progs/ksock_common.h
index 87d92372d28d..7cf64319b901 100644
--- a/tools/testing/selftests/bpf/progs/ksock_common.h
+++ b/tools/testing/selftests/bpf/progs/ksock_common.h
@@ -70,4 +70,41 @@ static inline int ksock_ctx_insert(struct bpf_ksock *ctx)
 	return 0;
 }

+/* Globals for passing config from userspace */
+__be32 ipv4_remote;
+__u16 remote_port;
+
+char send_data[32] = "hello from bpf ksock";
+
+static inline int do_ksock_setup(void)
+{
+	struct bpf_ksock_create_opts create_opts = {};
+	struct bpf_ksock_addr_opts addr_opts = {};
+	struct bpf_ksock *ks;
+	int err = 0;
+
+	create_opts.family = AF_INET;
+	create_opts.type = SOCK_DGRAM;
+	create_opts.protocol = IPPROTO_UDP;
+
+	ks = bpf_ksock_create(&create_opts, sizeof(create_opts), &err);
+	if (!ks)
+		return err;
+
+	addr_opts.family = AF_INET;
+	addr_opts.port = remote_port;
+	addr_opts.ipv4_addr = ipv4_remote;
+
+	err = bpf_ksock_connect(ks, &addr_opts, sizeof(addr_opts));
+	if (err) {
+		bpf_ksock_release(ks);
+		return err;
+	}
+
+	err = ksock_ctx_insert(ks);
+	if (err && err != -EEXIST)
+		return err;
+	return 0;
+}
+
 #endif /* _KSOCK_COMMON_H */
diff --git a/tools/testing/selftests/bpf/progs/ksock_recursion.c b/tools/testing/selftests/bpf/progs/ksock_recursion.c
new file mode 100644
index 000000000000..52f5200943e3
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/ksock_recursion.c
@@ -0,0 +1,69 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Isovalent */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_endian.h>
+#include "bpf_tracing_net.h"
+#include "ksock_common.h"
+
+void bpf_rcu_read_lock(void) __ksym;
+void bpf_rcu_read_unlock(void) __ksym;
+
+int target_pid;
+int trigger_send;
+
+unsigned int rec_count;
+int rec_kfunc_rets[] = { -1, -1 };
+
+SEC("syscall")
+int ksock_setup(void *ctx)
+{
+	return do_ksock_setup();
+}
+
+SEC("lsm.s/socket_sendmsg")
+int BPF_PROG(ksock_socket_sendmsg, struct socket *sock, struct msghdr *msg,
+	     int size, int ret)
+{
+	struct __ksock_ctx_value *v;
+	struct bpf_ksock *ks, *tmp;
+	u32 pid = bpf_get_current_pid_tgid() >> 32;
+	int kfunc_ret;
+
+	if (ret || !trigger_send || pid != target_pid)
+		return ret;
+
+	v = ksock_ctx_value_lookup();
+	if (!v) {
+		kfunc_ret = -ENOENT;
+		goto out;
+	}
+
+	ks = NULL;
+	bpf_rcu_read_lock();
+	tmp = v->ctx;
+	if (tmp)
+		ks = bpf_ksock_acquire(tmp);
+	bpf_rcu_read_unlock();
+
+	if (!ks) {
+		kfunc_ret = -ENOENT;
+		goto out;
+	}
+
+	kfunc_ret = bpf_ksock_send(ks, send_data, sizeof(send_data));
+	bpf_ksock_release(ks);
+
+out:
+	rec_kfunc_rets[rec_count & 1] = kfunc_ret;
+	__sync_fetch_and_add(&rec_count, 1);
+
+	if (kfunc_ret != -EBUSY)
+		trigger_send = 0;
+
+	return ret;
+}
+
+char __license[] SEC("license") = "GPL";
--
2.34.1