* [PATCH bpf v3 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()
2026-03-27 13:38 [PATCH bpf v3 0/2] bpf: tcp: Fix null-ptr-deref in arbitrary SYN Cookie Jiayuan Chen
@ 2026-03-27 13:38 ` Jiayuan Chen
2026-03-27 13:38 ` [PATCH bpf v3 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk() Jiayuan Chen
1 sibling, 0 replies; 4+ messages in thread
From: Jiayuan Chen @ 2026-03-27 13:38 UTC (permalink / raw)
To: bpf
Cc: Jiayuan Chen, Jiayuan Chen, Kuniyuki Iwashima, Alexei Starovoitov,
Daniel Borkmann, Andrii Nakryiko, Martin KaFai Lau,
Eduard Zingerman, Song Liu, Yonghong Song, John Fastabend,
KP Singh, Stanislav Fomichev, Hao Luo, Jiri Olsa, David S. Miller,
Eric Dumazet, Jakub Kicinski, Paolo Abeni, Simon Horman,
Shuah Khan, netdev, linux-kernel, linux-kselftest
From: Jiayuan Chen <jiayuan.chen@shopee.com>
bpf_sk_assign_tcp_reqsk() only validates skb->protocol (L3) but does not
check the L4 protocol in the IP header. A BPF program can call this kfunc
on a UDP skb with a valid TCP listener socket, which will succeed and
attach a TCP reqsk to the UDP skb.
When the UDP skb enters the UDP receive path, skb_steal_sock() returns
the TCP listener from the reqsk. The UDP code then passes this TCP socket
to udp_unicast_rcv_skb() -> __udp_enqueue_schedule_skb(), which casts
it to udp_sock and accesses UDP-specific fields at invalid offsets,
causing a null pointer dereference and kernel panic:
BUG: KASAN: null-ptr-deref in __udp_enqueue_schedule_skb+0x19d/0x1df0
Read of size 4 at addr 0000000000000008 by task test_progs/537
CPU: 1 UID: 0 PID: 537 Comm: test_progs Not tainted 7.0.0-rc4+ #46 PREEMPT
Call Trace:
<IRQ>
dump_stack_lvl (lib/dump_stack.c:123)
print_report (mm/kasan/report.c:487)
kasan_report (mm/kasan/report.c:597)
__kasan_check_read (mm/kasan/shadow.c:32)
__udp_enqueue_schedule_skb (net/ipv4/udp.c:1719)
udp_queue_rcv_one_skb (net/ipv4/udp.c:2370 net/ipv4/udp.c:2500)
udp_queue_rcv_skb (net/ipv4/udp.c:2532)
udp_unicast_rcv_skb (net/ipv4/udp.c:2684)
__udp4_lib_rcv (net/ipv4/udp.c:2742)
udp_rcv (net/ipv4/udp.c:2937)
ip_protocol_deliver_rcu (net/ipv4/ip_input.c:209)
ip_local_deliver_finish (./include/linux/rcupdate.h:879 net/ipv4/ip_input.c:242)
ip_local_deliver (net/ipv4/ip_input.c:265)
__netif_receive_skb_one_core (net/core/dev.c:6164 (discriminator 4))
__netif_receive_skb (net/core/dev.c:6280)
Fix this by checking the IP header's protocol field in
bpf_sk_assign_tcp_reqsk() and rejecting non-TCP skbs with -EINVAL.
Fixes: e472f88891ab ("bpf: tcp: Support arbitrary SYN Cookie.")
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
net/core/filter.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/net/core/filter.c b/net/core/filter.c
index 78b548158fb0..fb975bcce804 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12248,11 +12248,17 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
switch (skb->protocol) {
case htons(ETH_P_IP):
+ if (ip_hdr(skb)->protocol != IPPROTO_TCP)
+ return -EINVAL;
+
ops = &tcp_request_sock_ops;
min_mss = 536;
break;
#if IS_BUILTIN(CONFIG_IPV6)
case htons(ETH_P_IPV6):
+ if (ipv6_hdr(skb)->nexthdr != IPPROTO_TCP)
+ return -EINVAL;
+
ops = &tcp6_request_sock_ops;
min_mss = IPV6_MIN_MTU - 60;
break;
--
2.43.0
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH bpf v3 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()
2026-03-27 13:38 [PATCH bpf v3 0/2] bpf: tcp: Fix null-ptr-deref in arbitrary SYN Cookie Jiayuan Chen
2026-03-27 13:38 ` [PATCH bpf v3 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk() Jiayuan Chen
@ 2026-03-27 13:38 ` Jiayuan Chen
2026-03-27 19:53 ` Martin KaFai Lau
1 sibling, 1 reply; 4+ messages in thread
From: Jiayuan Chen @ 2026-03-27 13:38 UTC (permalink / raw)
To: bpf
Cc: Jiayuan Chen, Jiayuan Chen, Martin KaFai Lau, Daniel Borkmann,
John Fastabend, Stanislav Fomichev, Alexei Starovoitov,
Andrii Nakryiko, Eduard Zingerman, Song Liu, Yonghong Song,
KP Singh, Hao Luo, Jiri Olsa, David S. Miller, Eric Dumazet,
Jakub Kicinski, Paolo Abeni, Simon Horman, Shuah Khan,
Kuniyuki Iwashima, netdev, linux-kernel, linux-kselftest
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 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
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
.../bpf/prog_tests/tcp_custom_syncookie.c | 94 ++++++++++++++-
.../bpf/progs/test_tcp_custom_syncookie.c | 109 ++++++++++++++++++
2 files changed, 199 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..e46031d0786b 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 +145,92 @@ 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;
+ char buf[32] = "test";
+ 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->data->assign_ret = -1;
+
+ 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;
+
+ /* 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"))
+ goto close_udp_client;
+
+ 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");
+
+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..c4834abea9c5 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 = -1;
+bool udp_intercepted = false;
+
+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
^ permalink raw reply related [flat|nested] 4+ messages in thread