[PATCH bpf v5 0/2] bpf: tcp: Fix null-ptr-deref in arbitrary SYN Cookie

[PATCH bpf v5 0/2] bpf: tcp: Fix null-ptr-deref in arbitrary SYN Cookie

[Jiayuan Chen]

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

bpf_sk_assign_tcp_reqsk() does not validate the L4 protocol of the skb,
only checking skb->protocol (L3). A BPF program that calls this kfunc on
a non-TCP skb (e.g. UDP) will succeed, attaching a TCP reqsk to the skb.

When the skb enters the UDP receive path, skb_steal_sock() returns the
TCP listener socket from the reqsk. The UDP code then casts this TCP
socket to udp_sock and accesses UDP-specific fields at invalid offsets,
causing a null pointer dereference:

  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)

Solution

Patch 1: Add L4 protocol validation in bpf_sk_assign_tcp_reqsk(). Check
ip_hdr(skb)->protocol (IPv4) and ipv6_hdr(skb)->nexthdr (IPv6) against
IPPROTO_TCP, returning -EINVAL for non-TCP skbs.
Patch 2: Add selftest that calls bpf_sk_assign_tcp_reqsk() on a UDP skb
and verifies the kfunc rejects it.


---
v1: https://lore.kernel.org/bpf/20260323105510.51990-1-jiayuan.chen@linux.dev/
v2: https://lore.kernel.org/bpf/20260326062657.88446-1-jiayuan.chen@linux.dev/
v3: https://lore.kernel.org/bpf/20260327133915.286037-1-jiayuan.chen@linux.dev/
v4: https://lore.kernel.org/bpf/20260330080746.319680-1-jiayuan.chen@linux.dev/

Changes in v5:
- Add pskb_may_pull before accessing IP/IPv6 headers in kfunc
- Use buf[] instead of buf[32], verify recv data with ASSERT_STREQ
- Remove unnecessary variable initializations in selftest and BPF

Changes in v4:
- Check if assign_ret is EINVAL instead of checking if it is 0

Changes in v3:
- Add IPv6 test coverage, reuse test_cases[] to iterate over both
  address families
- Share TCP/UDP port to simplify BPF program, remove unnecessary
  global variables
- Use connect_to_fd() + send()/recv() instead of manual sockaddr
  construction
- Suggested by Kuniyuki Iwashima

Changes in v2:
- Add Reviewed-by tag from Kuniyuki Iwashima for patch 1
- Use UDP socket recv() instead of kern_sync_rcu() for synchronization
  in selftest

Jiayuan Chen (2):
  bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()
  selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

 net/core/filter.c                             |  12 ++
 .../bpf/prog_tests/tcp_custom_syncookie.c     |  93 ++++++++++++++-
 .../bpf/progs/test_tcp_custom_syncookie.c     | 109 ++++++++++++++++++
 3 files changed, 210 insertions(+), 4 deletions(-)

-- 
2.43.0

[PATCH bpf v5 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()

[Jiayuan Chen]

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.

Note that for IPv6, the nexthdr check does not walk extension headers.
This is uncommon for TCP SYN packets in practice, and keeping it simple
was agreed upon by Kuniyuki Iwashima.

Fixes: e472f88891ab ("bpf: tcp: Support arbitrary SYN Cookie.")
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/core/filter.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index 78b548158fb0..f251744025e2 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -12248,11 +12248,23 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
 
 	switch (skb->protocol) {
 	case htons(ETH_P_IP):
+		if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+			return -EINVAL;
+
+		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 (!pskb_may_pull(skb, sizeof(struct ipv6hdr)))
+			return -EINVAL;
+
+		if (ipv6_hdr(skb)->nexthdr != IPPROTO_TCP)
+			return -EINVAL;
+
 		ops = &tcp6_request_sock_ops;
 		min_mss = IPV6_MIN_MTU - 60;
 		break;
-- 
2.43.0

[PATCH bpf v5 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

[Jiayuan Chen]

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:recv data 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:recv data 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@linux.dev>
---
 .../bpf/prog_tests/tcp_custom_syncookie.c     |  93 ++++++++++++++-
 .../bpf/progs/test_tcp_custom_syncookie.c     | 109 ++++++++++++++++++
 2 files changed, 198 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..017836581669 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 +145,91 @@ 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, udp_server, udp_client;
+	char buf[] = "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->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));
+
+	/* 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_STREQ(buf, "test", "recv data");
+	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..331435c837cc 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;
+bool udp_intercepted;
+
+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

Re: [PATCH bpf v5 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()

[Martin KaFai Lau]

On Wed, Apr 01, 2026 at 07:04:49PM +0800, Jiayuan Chen wrote:
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 78b548158fb0..f251744025e2 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -12248,11 +12248,23 @@ __bpf_kfunc int bpf_sk_assign_tcp_reqsk(struct __sk_buff *s, struct sock *sk,
>
>  	switch (skb->protocol) {
>  	case htons(ETH_P_IP):
> +		if (!pskb_may_pull(skb, sizeof(struct iphdr)))

The ai-review [1] is correct. skb_header_pointer() should be used.
pskb_may_pull() could invalidate the skb->data[_end] pointers
in the bpf prog.

[1]: https://sashiko.dev/#/patchset/20260401110511.73355-1-jiayuan.chen%40linux.dev

It needs one more spin. Thanks.

pw-bot: cr