Re: [PATCH bpf v4 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

Kuniyuki Iwashima
2026-03-31T01:50:36Z

On Mon, Mar 30, 2026 at 1:08 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
> 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 TC ingress where a BPF program calls
> bpf_sk_assign_tcp_reqsk() on it and checks that the kfunc returns an
> error. A UDP server recv() is used as synchronization to ensure the
> BPF program has finished processing before checking the result.
>
> 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
> setup_tc:PASS:qdisc add dev lo clsact 0 nsec
> setup_tc:PASS:filter add dev lo ingress 0 nsec
> run_protocol_check:PASS:start tcp_server 0 nsec
> run_protocol_check:PASS:start udp_server 0 nsec
> run_protocol_check:PASS:connect udp_client 0 nsec
> run_protocol_check:PASS:send udp 0 nsec
> run_protocol_check:PASS:recv udp 0 nsec
> run_protocol_check:PASS:udp_intercepted 0 nsec
> run_protocol_check:PASS:assign_ret 0 nsec
> #471/1 tcp_custom_syncookie_protocol_check/IPv4 TCP:OK
> run_protocol_check:PASS:start tcp_server 0 nsec
> run_protocol_check:PASS:start udp_server 0 nsec
> run_protocol_check:PASS:connect udp_client 0 nsec
> run_protocol_check:PASS:send udp 0 nsec
> run_protocol_check:PASS:recv udp 0 nsec
> run_protocol_check:PASS:udp_intercepted 0 nsec
> run_protocol_check:PASS:assign_ret 0 nsec
> #471/2 tcp_custom_syncookie_protocol_check/IPv6 TCP:OK
> #471 tcp_custom_syncookie_protocol_check:OK
> Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
> .../bpf/prog_tests/tcp_custom_syncookie.c | 91 ++++++++++++++-
> .../bpf/progs/test_tcp_custom_syncookie.c | 109 ++++++++++++++++++
> 2 files changed, 196 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..6e29402c4c59 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 +145,89 @@ 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 TC ingress where a BPF program calls
> + * bpf_sk_assign_tcp_reqsk() on it. The kfunc should return an error
> + * because the skb carries UDP, not TCP.
> + *
> + * TCP and UDP servers share the same port. The BPF program intercepts
> + * the UDP packet, looks up the TCP listener via the dest port, and
> + * attempts to assign a TCP reqsk to the UDP skb.
> + */
> +static void run_protocol_check(struct test_tcp_custom_syncookie *skel,
> + int family, const char *addr)
> +{
> + int tcp_server = -1, udp_server = -1, udp_client = -1;

nit: no need to init.

> + char buf[32] = "test";

should be buf[] = "test" since you send data of sizeof(buf) below.

> + int port, ret;
> +
> + tcp_server = start_server(family, SOCK_STREAM, addr, 0, 0);
> + if (!ASSERT_NEQ(tcp_server, -1, "start tcp_server"))
> + return;
> +
> + port = ntohs(get_socket_local_port(tcp_server));
> +
> + /* UDP server on same port for synchronization and port sharing */
> + udp_server = start_server(family, SOCK_DGRAM, addr, port, 0);
> + if (!ASSERT_NEQ(udp_server, -1, "start udp_server"))
> + goto close_tcp;
> +
> + skel->bss->udp_intercepted = false;
> + skel->bss->assign_ret = 0;
> +
> + udp_client = connect_to_fd(udp_server, 0);
> + if (!ASSERT_NEQ(udp_client, -1, "connect udp_client"))
> + goto close_udp_server;
>
> + ret = send(udp_client, buf, sizeof(buf), 0);
> + if (!ASSERT_EQ(ret, sizeof(buf), "send udp"))
> + goto close_udp_client;

memset(buf, 0, sizeof(buf)) here and

> +
> + /* recv() ensures TC ingress BPF has processed the skb */
> + ret = recv(udp_server, buf, sizeof(buf), 0);
> + if (!ASSERT_EQ(ret, sizeof(buf), "recv udp"))

check ASSERT_STREQ() here ?

> + goto close_udp_client;
> +
> + ASSERT_EQ(skel->bss->udp_intercepted, true, "udp_intercepted");
> +
> + ASSERT_EQ(skel->bss->assign_ret, -EINVAL, "assign_ret");
> +
> +close_udp_client:
> + close(udp_client);
> +close_udp_server:
> + close(udp_server);
> +close_tcp:
> + close(tcp_server);
> +}
> +
> +void test_tcp_custom_syncookie_protocol_check(void)
> +{
> + struct test_tcp_custom_syncookie *skel;
> + int i;
> +
> + if (setup_netns())
> + return;
> +
> + skel = test_tcp_custom_syncookie__open_and_load();
> + if (!ASSERT_OK_PTR(skel, "open_and_load"))
> + return;
> +
> + if (setup_tc(bpf_program__fd(skel->progs.tcp_custom_syncookie_badproto)))
> + goto destroy_skel;
> +
> + for (i = 0; i < ARRAY_SIZE(test_cases); i++) {
> + if (!test__start_subtest(test_cases[i].name))
> + continue;
> +
> + run_protocol_check(skel, test_cases[i].family,
> + test_cases[i].addr);
> + }
> +
> +destroy_skel:
> + system("tc qdisc del dev lo clsact");
> 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..bd3fad3dd503 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 +588,113 @@ 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 because the skb is not TCP.
> + *
> + * TCP and UDP servers share the same port. The BPF program intercepts
> + * UDP packets, looks up the TCP listener on the same port, and tries
> + * to assign a TCP reqsk to the UDP skb.
> + */
> +int assign_ret = 0;
> +bool udp_intercepted = false;

No need to init var on bss w/ 0 here.
(init in run_protocol_check() is necessary)

> +
> +static int badproto_lookup_assign(struct __sk_buff *skb, struct udphdr *udp,
> + struct bpf_sock_tuple *tuple, u32 tuple_size)
> +{
> + struct bpf_tcp_req_attrs attrs = {};
> + struct bpf_sock *skc;
> + struct sock *sk;
> +
> + skc = bpf_skc_lookup_tcp(skb, tuple, tuple_size, -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;
> +
> + assign_ret = bpf_sk_assign_tcp_reqsk(skb, sk, &attrs, sizeof(attrs));
> +
> + bpf_sk_release(skc);
> + return TC_ACT_OK;
> +}
> +
> +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 ethhdr *eth;
> + struct iphdr *iph;
> + struct ipv6hdr *ip6h;
> + struct udphdr *udp;
> +
> + eth = (struct ethhdr *)data;
> + if (eth + 1 > data_end)
> + return TC_ACT_OK;
> +
> + switch (bpf_ntohs(eth->h_proto)) {
> + case ETH_P_IP:
> + 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;
> +
> + udp_intercepted = true;
> +
> + tuple.ipv4.saddr = iph->saddr;
> + tuple.ipv4.daddr = iph->daddr;
> + tuple.ipv4.sport = udp->source;
> + tuple.ipv4.dport = udp->dest;
> +
> + return badproto_lookup_assign(skb, udp, &tuple,
> + sizeof(tuple.ipv4));
> + case ETH_P_IPV6:
> + ip6h = (struct ipv6hdr *)(eth + 1);
> + if (ip6h + 1 > data_end)
> + return TC_ACT_OK;
> +
> + if (ip6h->nexthdr != IPPROTO_UDP)
> + return TC_ACT_OK;
> +
> + udp = (struct udphdr *)(ip6h + 1);
> + if (udp + 1 > data_end)
> + return TC_ACT_OK;
> +
> + udp_intercepted = true;
> +
> + __builtin_memcpy(tuple.ipv6.saddr, &ip6h->saddr,
> + sizeof(tuple.ipv6.saddr));
> + __builtin_memcpy(tuple.ipv6.daddr, &ip6h->daddr,
> + sizeof(tuple.ipv6.daddr));
> + tuple.ipv6.sport = udp->source;
> + tuple.ipv6.dport = udp->dest;
> +
> + return badproto_lookup_assign(skb, udp, &tuple,
> + sizeof(tuple.ipv6));
> + default:
> + return TC_ACT_OK;
> + }
> +}
> +
> char _license[] SEC("license") = "GPL";
> --
> 2.43.0
>