[PATCH bpf v1 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

Jiayuan Chen
2026-03-23T10:56:04Z

From: Jiayuan Chen <jiayuan.chen@shopee.com>

Add test_tcp_custom_syncookie_protocol_check to verify that
bpf_sk_assign_tcp_reqsk() rejects non-TCP skbs. The test sends a UDP
packet through a BPF program that calls bpf_sk_assign_tcp_reqsk() on it
and checks that the kfunc returns an error.

Without the fix in bpf_sk_assign_tcp_reqsk(), the kfunc succeeds and
attaches a TCP reqsk to the UDP skb, which causes a null pointer
dereference panic when the kernel processes it through the UDP receive
path.

Test result:

./test_progs -a tcp_custom_syncookie_protocol_check -v
setup_netns:PASS:create netns 0 nsec
setup_netns:PASS:ip 0 nsec
write_sysctl:PASS:open sysctl 0 nsec
write_sysctl:PASS:write sysctl 0 nsec
setup_netns:PASS:write_sysctl 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:open_and_load 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:start tcp_server 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:getsockname 0 nsec
setup_tc:PASS:qdisc add dev lo clsact 0 nsec
setup_tc:PASS:filter add dev lo ingress 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:udp socket 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:sendto udp 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:udp_intercepted 0 nsec
test_tcp_custom_syncookie_protocol_check:PASS:assign_ret 0 nsec
#471 tcp_custom_syncookie_protocol_check:OK
Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
.../bpf/prog_tests/tcp_custom_syncookie.c | 81 ++++++++++++++++++-
.../bpf/progs/test_tcp_custom_syncookie.c | 79 ++++++++++++++++++
2 files changed, 156 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
index eaf441dc7e79..e622c5befa70 100644
--- a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
+++ b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
@@ -5,6 +5,7 @@
#include <sched.h>
#include <stdlib.h>
#include <net/if.h>
+#include <netinet/in.h>

#include "test_progs.h"
#include "cgroup_helpers.h"
@@ -47,11 +48,10 @@ static int setup_netns(void)
return -1;
}

-static int setup_tc(struct test_tcp_custom_syncookie *skel)
+static int setup_tc(int prog_fd)
{
LIBBPF_OPTS(bpf_tc_hook, qdisc_lo, .attach_point = BPF_TC_INGRESS);
- LIBBPF_OPTS(bpf_tc_opts, tc_attach,
- .prog_fd = bpf_program__fd(skel->progs.tcp_custom_syncookie));
+ LIBBPF_OPTS(bpf_tc_opts, tc_attach, .prog_fd = prog_fd);

qdisc_lo.ifindex = if_nametoindex("lo");
if (!ASSERT_OK(bpf_tc_hook_create(&qdisc_lo), "qdisc add dev lo clsact"))
@@ -127,7 +127,7 @@ void test_tcp_custom_syncookie(void)
if (!ASSERT_OK_PTR(skel, "open_and_load"))
return;

- if (setup_tc(skel))
+ if (setup_tc(bpf_program__fd(skel->progs.tcp_custom_syncookie)))
goto destroy_skel;

for (i = 0; i < ARRAY_SIZE(test_cases); i++) {
@@ -145,6 +145,79 @@ void test_tcp_custom_syncookie(void)

destroy_skel:
system("tc qdisc del dev lo clsact");
+ test_tcp_custom_syncookie__destroy(skel);
+}
+
+/* Test: bpf_sk_assign_tcp_reqsk() should reject non-TCP skb.
+ *
+ * Send a UDP packet through a BPF program that calls
+ * bpf_sk_assign_tcp_reqsk() on it. The kfunc should return -EINVAL
+ * because the skb carries UDP, not TCP.
+ *
+ * Currently the kfunc lacks L4 protocol check, so assign_ret == 0
+ * indicates the bug is present.
+ */
+void test_tcp_custom_syncookie_protocol_check(void)
+{
+ struct test_tcp_custom_syncookie *skel;
+ struct sockaddr_in tcp_addr, udp_addr;
+ socklen_t addr_len = sizeof(tcp_addr);
+ int tcp_server = -1, udp_client = -1;
+ char buf[32] = "test";
+ int ret;
+
+ if (setup_netns())
+ return;
+
+ skel = test_tcp_custom_syncookie__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "open_and_load"))
+ return;

+ /* Create a TCP listener so the BPF can find a LISTEN socket */
+ tcp_server = start_server(AF_INET, SOCK_STREAM, "127.0.0.1", 0, 0);
+ if (!ASSERT_NEQ(tcp_server, -1, "start tcp_server"))
+ goto destroy_skel;
+
+ ret = getsockname(tcp_server, (struct sockaddr *)&tcp_addr, &addr_len);
+ if (!ASSERT_OK(ret, "getsockname"))
+ goto close_tcp;
+
+ skel->bss->tcp_listener_port = ntohs(tcp_addr.sin_port);
+ skel->bss->udp_test_port = 9999;
+
+ ret = bpf_program__fd(skel->progs.tcp_custom_syncookie_badproto);
+ if (setup_tc(ret))
+ goto close_tcp;
+
+ udp_client = socket(AF_INET, SOCK_DGRAM, 0);
+ if (!ASSERT_NEQ(udp_client, -1, "udp socket"))
+ goto cleanup_tc;
+
+ memset(&udp_addr, 0, sizeof(udp_addr));
+ udp_addr.sin_family = AF_INET;
+ udp_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+ udp_addr.sin_port = htons(9999);
+
+ ret = sendto(udp_client, buf, sizeof(buf), 0,
+ (struct sockaddr *)&udp_addr, sizeof(udp_addr));
+ ASSERT_EQ(ret, sizeof(buf), "sendto udp");
+
+ /* Wait for TC ingress BPF to process the skb. */
+ kern_sync_rcu();
+
+ ASSERT_EQ(skel->bss->udp_intercepted, true, "udp_intercepted");
+
+ /* assign_ret == 0 means kfunc accepted UDP skb (bug).
+ * assign_ret < 0 means kfunc correctly rejected it (fixed).
+ */
+ ASSERT_NEQ(skel->data->assign_ret, 0, "assign_ret");
+
+cleanup_tc:
+ system("tc qdisc del dev lo clsact");
+ if (udp_client >= 0)
+ close(udp_client);
+close_tcp:
+ close(tcp_server);
+destroy_skel:
test_tcp_custom_syncookie__destroy(skel);
}
diff --git a/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c b/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
index 7d5293de1952..386705b6c9f2 100644
--- a/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
+++ b/tools/testing/selftests/bpf/progs/test_tcp_custom_syncookie.c
@@ -588,4 +588,83 @@ int tcp_custom_syncookie(struct __sk_buff *skb)
return tcp_handle_ack(&ctx);
}

+/* Test: call bpf_sk_assign_tcp_reqsk() on a UDP skb.
+ * The kfunc should reject it, but currently it doesn't check L4 protocol.
+ */
+__u16 tcp_listener_port = 0;
+__u16 udp_test_port = 0;
+int assign_ret = -1;
+bool udp_intercepted = false;
+
+SEC("tc")
+int tcp_custom_syncookie_badproto(struct __sk_buff *skb)
+{
+ void *data = (void *)(long)skb->data;
+ void *data_end = (void *)(long)skb->data_end;
+ struct bpf_sock_tuple tuple = {};
+ struct bpf_tcp_req_attrs attrs = {};
+ struct ethhdr *eth;
+ struct iphdr *iph;
+ struct udphdr *udp;
+ struct bpf_sock *skc;
+ struct sock *sk;
+
+ eth = (struct ethhdr *)data;
+ if (eth + 1 > data_end)
+ return TC_ACT_OK;
+
+ if (bpf_ntohs(eth->h_proto) != ETH_P_IP)
+ return TC_ACT_OK;
+
+ iph = (struct iphdr *)(eth + 1);
+ if (iph + 1 > data_end)
+ return TC_ACT_OK;
+
+ if (iph->protocol != IPPROTO_UDP)
+ return TC_ACT_OK;
+
+ udp = (struct udphdr *)(iph + 1);
+ if (udp + 1 > data_end)
+ return TC_ACT_OK;
+
+ if (bpf_ntohs(udp->dest) != udp_test_port)
+ return TC_ACT_OK;
+
+ udp_intercepted = true;
+
+ tuple.ipv4.saddr = iph->saddr;
+ tuple.ipv4.daddr = iph->daddr;
+ tuple.ipv4.sport = udp->source;
+ tuple.ipv4.dport = bpf_htons(tcp_listener_port);
+
+ skc = bpf_skc_lookup_tcp(skb, &tuple, sizeof(tuple.ipv4), -1, 0);
+ if (!skc)
+ return TC_ACT_OK;
+
+ if (skc->state != TCP_LISTEN) {
+ bpf_sk_release(skc);
+ return TC_ACT_OK;
+ }
+
+ sk = (struct sock *)bpf_skc_to_tcp_sock(skc);
+ if (!sk) {
+ bpf_sk_release(skc);
+ return TC_ACT_OK;
+ }
+
+ attrs.mss = 1460;
+ attrs.wscale_ok = 1;
+ attrs.snd_wscale = 7;
+ attrs.rcv_wscale = 7;
+ attrs.sack_ok = 1;
+
+ /* Call bpf_sk_assign_tcp_reqsk on a UDP skb. */
+ assign_ret = bpf_sk_assign_tcp_reqsk(skb, sk, &attrs, sizeof(attrs));
+
+ bpf_sk_release(skc);
+
+ /* Let the packet continue into the kernel */
+ return TC_ACT_OK;
+}
+
char _license[] SEC("license") = "GPL";
--
2.43.0