[PATCH bpf v8 0/2] bpf: tcp: Fix null-ptr-deref in arbitrary SYN Cookie

[PATCH bpf v8 0/2] bpf: tcp: Fix null-ptr-deref in arbitrary SYN Cookie

[Jiayuan Chen]

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

Validating the protocol in the helper is not enough: a BPF program can
bypass an ip_hdr(skb)->protocol check via TOCTOU by rewriting the header
around the call, and bpf_sk_assign() has the same problem since it can
assign any socket type to any skb. So validate the protocol where the
assigned socket is consumed instead.

Patch 1: Validate the L4 protocol in skb_steal_sock(). Each caller passes
the protocol it handles (TCP or UDP), and a prefetched socket whose
protocol does not match is rejected, regardless of how it was assigned.
Patch 2: Add a selftest that calls bpf_sk_assign_tcp_reqsk() on a UDP skb
and verifies the stack no longer crashes.

---
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/
v5: https://lore.kernel.org/bpf/20260401110511.73355-1-jiayuan.chen@linux.dev/
v6: https://lore.kernel.org/all/20260403015851.148209-1-jiayuan.chen@linux.dev/

Changes in v6 & v7: 
- resend and keep selftest.

Changes in v5:
- use skb_header_pointer instead of pskb_may_pull.

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):
  net: Validate protocol in skb_steal_sock() for BPF-assigned sockets
  selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

 include/net/inet6_hashtables.h                |   7 +-
 include/net/inet_hashtables.h                 |   7 +-
 include/net/request_sock.h                    |  16 ++-
 net/ipv4/udp.c                                |   2 +-
 net/ipv6/udp.c                                |   2 +-
 .../bpf/prog_tests/tcp_custom_syncookie.c     |  87 ++++++++++++++-
 .../bpf/progs/test_tcp_custom_syncookie.c     | 102 ++++++++++++++++++
 7 files changed, 210 insertions(+), 13 deletions(-)

-- 
2.43.0

[PATCH bpf v8 1/2] net: Validate protocol in skb_steal_sock() for BPF-assigned sockets

[Jiayuan Chen]

bpf_sk_assign_tcp_reqsk() can assign a TCP reqsk to a non-TCP skb,
causing a panic when the skb enters the wrong L4 receive path [1].
An initial attempt tried to fix this in the BPF helper by checking
iph->protocol, but Sashiko [2] revealed that BPF programs can bypass
this check via a TOCTOU attack by modifying iph->protocol around the
call:

    iph->protocol = IPPROTO_TCP;
    bpf_sk_assign_tcp_reqsk(udp_skb, tcp_sk);
    iph->protocol = IPPROTO_UDP;

Furthermore, bpf_sk_assign() has had the same class of vulnerability
since its introduction — it can assign any socket type to any skb
without protocol validation.  Since the BPF helper check alone cannot
prevent a malicious BPF program from crashing the kernel, add protocol
validation in skb_steal_sock() to reject mismatched sockets regardless
of how they were assigned.

The check is applied to all prefetched sockets.  Early demux paths
already only assign matching protocols (e.g., UDP early demux only
assigns UDP sockets to UDP skbs), so they pass the check naturally and
the extra branch is negligible.

Pass the expected protocol from callers rather than extracting it from
the IP header.  For IPv6, walking extension headers to find the L4
protocol is complex and unnecessary since each caller already knows
the protocol it handles.

[1] https://lore.kernel.org/bpf/20260403015851.148209-1-jiayuan.chen@linux.dev/
[2] https://sashiko.dev/#/patchset/20260403015851.148209-1-jiayuan.chen%40linux.dev

Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
Fixes: e472f88891ab ("bpf: tcp: Support arbitrary SYN Cookie.")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 include/net/inet6_hashtables.h |  7 ++++---
 include/net/inet_hashtables.h  |  7 ++++---
 include/net/request_sock.h     | 16 +++++++++++++++-
 net/ipv4/udp.c                 |  2 +-
 net/ipv6/udp.c                 |  2 +-
 5 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
index 2cc5d416bbb5a..218498373a9c1 100644
--- a/include/net/inet6_hashtables.h
+++ b/include/net/inet6_hashtables.h
@@ -106,12 +106,13 @@ static inline
 struct sock *inet6_steal_sock(struct net *net, struct sk_buff *skb, int doff,
 			      const struct in6_addr *saddr, const __be16 sport,
 			      const struct in6_addr *daddr, const __be16 dport,
-			      bool *refcounted, inet6_ehashfn_t *ehashfn)
+			      bool *refcounted, inet6_ehashfn_t *ehashfn,
+			      int protocol)
 {
 	struct sock *sk, *reuse_sk;
 	bool prefetched;
 
-	sk = skb_steal_sock(skb, refcounted, &prefetched);
+	sk = skb_steal_sock(skb, refcounted, &prefetched, protocol);
 	if (!sk)
 		return NULL;
 
@@ -153,7 +154,7 @@ static inline struct sock *__inet6_lookup_skb(struct sk_buff *skb, int doff,
 	struct sock *sk;
 
 	sk = inet6_steal_sock(net, skb, doff, &ip6h->saddr, sport, &ip6h->daddr, dport,
-			      refcounted, inet6_ehashfn);
+			      refcounted, inet6_ehashfn, IPPROTO_TCP);
 	if (IS_ERR(sk))
 		return NULL;
 	if (sk)
diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
index 6e2fe186d0dcb..a2a044f93cc4b 100644
--- a/include/net/inet_hashtables.h
+++ b/include/net/inet_hashtables.h
@@ -446,12 +446,13 @@ static inline
 struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff,
 			     const __be32 saddr, const __be16 sport,
 			     const __be32 daddr, const __be16 dport,
-			     bool *refcounted, inet_ehashfn_t *ehashfn)
+			     bool *refcounted, inet_ehashfn_t *ehashfn,
+			     int protocol)
 {
 	struct sock *sk, *reuse_sk;
 	bool prefetched;
 
-	sk = skb_steal_sock(skb, refcounted, &prefetched);
+	sk = skb_steal_sock(skb, refcounted, &prefetched, protocol);
 	if (!sk)
 		return NULL;
 
@@ -494,7 +495,7 @@ static inline struct sock *__inet_lookup_skb(struct sk_buff *skb,
 	struct sock *sk;
 
 	sk = inet_steal_sock(net, skb, doff, iph->saddr, sport, iph->daddr, dport,
-			     refcounted, inet_ehashfn);
+			     refcounted, inet_ehashfn, IPPROTO_TCP);
 	if (IS_ERR(sk))
 		return NULL;
 	if (sk)
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index 5a9c826a7092d..c2b8c6350b624 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -89,9 +89,11 @@ static inline struct sock *req_to_sk(struct request_sock *req)
  * @skb: sk_buff to steal the socket from
  * @refcounted: is set to true if the socket is reference-counted
  * @prefetched: is set to true if the socket was assigned from bpf
+ * @protocol: expected L4 protocol
  */
 static inline struct sock *skb_steal_sock(struct sk_buff *skb,
-					  bool *refcounted, bool *prefetched)
+					  bool *refcounted, bool *prefetched,
+					  int protocol)
 {
 	struct sock *sk = skb->sk;
 
@@ -103,6 +105,18 @@ static inline struct sock *skb_steal_sock(struct sk_buff *skb,
 
 	*prefetched = skb_sk_is_prefetched(skb);
 	if (*prefetched) {
+		/* A non-full socket here is either a reqsk or a
+		 * timewait sock, both only contain sock_common and
+		 * lack sk_protocol. Since both can only be TCP,
+		 * use IPPROTO_TCP as the protocol.
+		 */
+		if (unlikely(((sk_fullsock(sk) ? sk->sk_protocol : IPPROTO_TCP) != protocol))) {
+			skb_orphan(skb);
+			*prefetched = false;
+			*refcounted = false;
+			return NULL;
+		}
+
 #if IS_ENABLED(CONFIG_SYN_COOKIES)
 		if (sk->sk_state == TCP_NEW_SYN_RECV && inet_reqsk(sk)->syncookie) {
 			struct request_sock *req = inet_reqsk(sk);
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 0ac2bf4f8759b..ceb4d29a64aca 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2618,7 +2618,7 @@ int udp_rcv(struct sk_buff *skb)
 		goto csum_error;
 
 	sk = inet_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
-			     &refcounted, udp_ehashfn);
+			     &refcounted, udp_ehashfn, IPPROTO_UDP);
 	if (IS_ERR(sk))
 		goto no_sk;
 
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 15e032194eccc..d9c12cce5acef 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1106,7 +1106,7 @@ INDIRECT_CALLABLE_SCOPE int udpv6_rcv(struct sk_buff *skb)
 
 	/* Check if the socket is already available, e.g. due to early demux */
 	sk = inet6_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
-			      &refcounted, udp6_ehashfn);
+			      &refcounted, udp6_ehashfn, IPPROTO_UDP);
 	if (IS_ERR(sk))
 		goto no_sk;
 
-- 
2.43.0

Re: [PATCH bpf v8 1/2] net: Validate protocol in skb_steal_sock() for BPF-assigned sockets

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1536 bytes --]

> diff --git a/include/net/request_sock.h b/include/net/request_sock.h
> index 5a9c826a7092..c2b8c6350b62 100644
> --- a/include/net/request_sock.h
> +++ b/include/net/request_sock.h

[ ... ]

> @@ -103,6 +105,18 @@ static inline struct sock *skb_steal_sock(struct sk_buff *skb,
>
>  	*prefetched = skb_sk_is_prefetched(skb);
>  	if (*prefetched) {
> +		/* A non-full socket here is either a reqsk or a
> +		 * timewait sock, both only contain sock_common and
> +		 * lack sk_protocol. Since both can only be TCP,
> +		 * use IPPROTO_TCP as the protocol.
> +		 */
> +		if (unlikely(((sk_fullsock(sk) ? sk->sk_protocol : IPPROTO_TCP) != protocol))) {
                         ^^^

This isn't a bug, but there are extra parentheses around the expression
inside unlikely(). Kuniyuki Iwashima requested adding unlikely() in v7
review, but the v8 code has unlikely(((expression))) instead of
unlikely(expression).

Reference: https://lore.kernel.org/bpf/CAAVpQUA6z4bZVReNDMToRVL1V7xmYg4Tq2YMnWyU8AYXWoeB5A@mail.gmail.com/

> +			skb_orphan(skb);
> +			*prefetched = false;
> +			*refcounted = false;
> +			return NULL;
> +		}
> +
>  #if IS_ENABLED(CONFIG_SYN_COOKIES)
>  		if (sk->sk_state == TCP_NEW_SYN_RECV && inet_reqsk(sk)->syncookie) {
>  			struct request_sock *req = inet_reqsk(sk);

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27139886597

Re: [PATCH bpf v8 1/2] net: Validate protocol in skb_steal_sock() for BPF-assigned sockets

[Kuniyuki Iwashima]

On Mon, Jun 8, 2026 at 5:59 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
> bpf_sk_assign_tcp_reqsk() can assign a TCP reqsk to a non-TCP skb,
> causing a panic when the skb enters the wrong L4 receive path [1].
> An initial attempt tried to fix this in the BPF helper by checking
> iph->protocol, but Sashiko [2] revealed that BPF programs can bypass
> this check via a TOCTOU attack by modifying iph->protocol around the
> call:
>
>     iph->protocol = IPPROTO_TCP;
>     bpf_sk_assign_tcp_reqsk(udp_skb, tcp_sk);
>     iph->protocol = IPPROTO_UDP;
>
> Furthermore, bpf_sk_assign() has had the same class of vulnerability
> since its introduction — it can assign any socket type to any skb
> without protocol validation.  Since the BPF helper check alone cannot
> prevent a malicious BPF program from crashing the kernel, add protocol

I'm curious about the BPF maintainers' stance on this kind of "bug"
where admin tries to shoot oneself in the foot.

I saw Alexei said this recently, and I guess it applies here as well ?
https://lore.kernel.org/bpf/CAADnVQLh7VEKAtckzz=XOVPT8ovpDQshvPPCWHDQu2OWQx27_w@mail.gmail.com/

---8<---
Not every "bug" needs a fix.
If a malicious bpf user wants to crash the kernel they will
find a way to do so. Especially with agents.
We cannot realistically close all of the holes.
Right now the priority is to fix the issues that normal
users can hit and not bots.
---8<---


> validation in skb_steal_sock() to reject mismatched sockets regardless
> of how they were assigned.
>
> The check is applied to all prefetched sockets.  Early demux paths
> already only assign matching protocols (e.g., UDP early demux only
> assigns UDP sockets to UDP skbs), so they pass the check naturally and
> the extra branch is negligible.
>
> Pass the expected protocol from callers rather than extracting it from
> the IP header.  For IPv6, walking extension headers to find the L4
> protocol is complex and unnecessary since each caller already knows
> the protocol it handles.
>
> [1] https://lore.kernel.org/bpf/20260403015851.148209-1-jiayuan.chen@linux.dev/
> [2] https://sashiko.dev/#/patchset/20260403015851.148209-1-jiayuan.chen%40linux.dev
>
> Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
> Fixes: e472f88891ab ("bpf: tcp: Support arbitrary SYN Cookie.")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  include/net/inet6_hashtables.h |  7 ++++---
>  include/net/inet_hashtables.h  |  7 ++++---
>  include/net/request_sock.h     | 16 +++++++++++++++-
>  net/ipv4/udp.c                 |  2 +-
>  net/ipv6/udp.c                 |  2 +-
>  5 files changed, 25 insertions(+), 9 deletions(-)
>
> diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
> index 2cc5d416bbb5a..218498373a9c1 100644
> --- a/include/net/inet6_hashtables.h
> +++ b/include/net/inet6_hashtables.h
> @@ -106,12 +106,13 @@ static inline
>  struct sock *inet6_steal_sock(struct net *net, struct sk_buff *skb, int doff,
>                               const struct in6_addr *saddr, const __be16 sport,
>                               const struct in6_addr *daddr, const __be16 dport,
> -                             bool *refcounted, inet6_ehashfn_t *ehashfn)
> +                             bool *refcounted, inet6_ehashfn_t *ehashfn,
> +                             int protocol)
>  {
>         struct sock *sk, *reuse_sk;
>         bool prefetched;
>
> -       sk = skb_steal_sock(skb, refcounted, &prefetched);
> +       sk = skb_steal_sock(skb, refcounted, &prefetched, protocol);
>         if (!sk)
>                 return NULL;
>
> @@ -153,7 +154,7 @@ static inline struct sock *__inet6_lookup_skb(struct sk_buff *skb, int doff,
>         struct sock *sk;
>
>         sk = inet6_steal_sock(net, skb, doff, &ip6h->saddr, sport, &ip6h->daddr, dport,
> -                             refcounted, inet6_ehashfn);
> +                             refcounted, inet6_ehashfn, IPPROTO_TCP);
>         if (IS_ERR(sk))
>                 return NULL;
>         if (sk)
> diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
> index 6e2fe186d0dcb..a2a044f93cc4b 100644
> --- a/include/net/inet_hashtables.h
> +++ b/include/net/inet_hashtables.h
> @@ -446,12 +446,13 @@ static inline
>  struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff,
>                              const __be32 saddr, const __be16 sport,
>                              const __be32 daddr, const __be16 dport,
> -                            bool *refcounted, inet_ehashfn_t *ehashfn)
> +                            bool *refcounted, inet_ehashfn_t *ehashfn,
> +                            int protocol)
>  {
>         struct sock *sk, *reuse_sk;
>         bool prefetched;
>
> -       sk = skb_steal_sock(skb, refcounted, &prefetched);
> +       sk = skb_steal_sock(skb, refcounted, &prefetched, protocol);
>         if (!sk)
>                 return NULL;
>
> @@ -494,7 +495,7 @@ static inline struct sock *__inet_lookup_skb(struct sk_buff *skb,
>         struct sock *sk;
>
>         sk = inet_steal_sock(net, skb, doff, iph->saddr, sport, iph->daddr, dport,
> -                            refcounted, inet_ehashfn);
> +                            refcounted, inet_ehashfn, IPPROTO_TCP);
>         if (IS_ERR(sk))
>                 return NULL;
>         if (sk)
> diff --git a/include/net/request_sock.h b/include/net/request_sock.h
> index 5a9c826a7092d..c2b8c6350b624 100644
> --- a/include/net/request_sock.h
> +++ b/include/net/request_sock.h
> @@ -89,9 +89,11 @@ static inline struct sock *req_to_sk(struct request_sock *req)
>   * @skb: sk_buff to steal the socket from
>   * @refcounted: is set to true if the socket is reference-counted
>   * @prefetched: is set to true if the socket was assigned from bpf
> + * @protocol: expected L4 protocol
>   */
>  static inline struct sock *skb_steal_sock(struct sk_buff *skb,
> -                                         bool *refcounted, bool *prefetched)
> +                                         bool *refcounted, bool *prefetched,
> +                                         int protocol)
>  {
>         struct sock *sk = skb->sk;
>
> @@ -103,6 +105,18 @@ static inline struct sock *skb_steal_sock(struct sk_buff *skb,
>
>         *prefetched = skb_sk_is_prefetched(skb);
>         if (*prefetched) {
> +               /* A non-full socket here is either a reqsk or a
> +                * timewait sock, both only contain sock_common and
> +                * lack sk_protocol. Since both can only be TCP,
> +                * use IPPROTO_TCP as the protocol.
> +                */
> +               if (unlikely(((sk_fullsock(sk) ? sk->sk_protocol : IPPROTO_TCP) != protocol))) {
> +                       skb_orphan(skb);
> +                       *prefetched = false;
> +                       *refcounted = false;
> +                       return NULL;
> +               }
> +
>  #if IS_ENABLED(CONFIG_SYN_COOKIES)
>                 if (sk->sk_state == TCP_NEW_SYN_RECV && inet_reqsk(sk)->syncookie) {
>                         struct request_sock *req = inet_reqsk(sk);
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 0ac2bf4f8759b..ceb4d29a64aca 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -2618,7 +2618,7 @@ int udp_rcv(struct sk_buff *skb)
>                 goto csum_error;
>
>         sk = inet_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
> -                            &refcounted, udp_ehashfn);
> +                            &refcounted, udp_ehashfn, IPPROTO_UDP);
>         if (IS_ERR(sk))
>                 goto no_sk;
>
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 15e032194eccc..d9c12cce5acef 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1106,7 +1106,7 @@ INDIRECT_CALLABLE_SCOPE int udpv6_rcv(struct sk_buff *skb)
>
>         /* Check if the socket is already available, e.g. due to early demux */
>         sk = inet6_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
> -                             &refcounted, udp6_ehashfn);
> +                             &refcounted, udp6_ehashfn, IPPROTO_UDP);
>         if (IS_ERR(sk))
>                 goto no_sk;
>
> --
> 2.43.0
>

[PATCH bpf v8 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

[Jiayuan Chen]

Add test_tcp_custom_syncookie_protocol_check to test assigning a TCP sk to
UDP skb->sk. The test sends a UDP packet through TC ingress, where a BPF
program calls bpf_sk_assign_tcp_reqsk() on it. A UDP server's recv() is
used for synchronization to ensure the BPF program has finished processing
before checking the result.

Without the fix in bpf_sk_assign_tcp_reqsk(), this will cause 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
	#492/1   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
	#492/2   tcp_custom_syncookie_protocol_check/IPv6 TCP:OK
	#492     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     |  87 ++++++++++++++-
 .../bpf/progs/test_tcp_custom_syncookie.c     | 102 ++++++++++++++++++
 2 files changed, 185 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 00d5c32674fc9..03103b7b9c5bf 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"))
@@ -147,7 +147,7 @@ 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++) {
@@ -165,6 +165,85 @@ void test_tcp_custom_syncookie(void)
 
 destroy_skel:
 	system("tc qdisc del dev lo clsact");
+	test_tcp_custom_syncookie__destroy(skel);
+}
+
+/* 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.
+ * Although bpf_sk_assign_tcp_reqsk() assign udp skb to tcp reqsk, the
+ * network stack should not crash.
+ */
+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;
+
+	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");
+
+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 7d5293de19523..5cf99d2a12aa5 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 +588,106 @@ int tcp_custom_syncookie(struct __sk_buff *skb)
 	return tcp_handle_ack(&ctx);
 }
 
+/* Test: call bpf_sk_assign_tcp_reqsk() on a UDP skb. */
+bool udp_intercepted;
+
+static int badproto_lookup_assign(struct __sk_buff *skb,
+				  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;
+
+	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, &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, &tuple,
+					      sizeof(tuple.ipv6));
+	default:
+		return TC_ACT_OK;
+	}
+}
+
 char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf v8 2/2] selftests/bpf: Add protocol check test for bpf_sk_assign_tcp_reqsk()

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1529 bytes --]

> 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..b644acd3e903 100644
> --- a/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c
> +++ b/tools/testing/selftests/bpf/prog_tests/tcp_custom_syncookie.c

[ ... ]

> @@ -145,6 +145,85 @@ void test_tcp_custom_syncookie(void)
>
>  destroy_skel:
>  	system("tc qdisc del dev lo clsact");
> +	test_tcp_custom_syncookie__destroy(skel);
> +}
> +
> +/* TCP and UDP servers share the same port. The BPF program intercepts

The BPF subsystem style guidelines ask that the opening /* of a multi-line
comment be on its own line. Should this be:

	/*
	 * 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.
	 ...
	 */

The same nit was raised on v8 by the automated review at:

	https://lore.kernel.org/bpf/20260608130746.218891F00898@smtp.kernel.org

> + * the UDP packet, looks up the TCP listener via the dest port, and
> + * attempts to assign a TCP reqsk to the UDP skb.
> + * Although bpf_sk_assign_tcp_reqsk() assign udp skb to tcp reqsk, the
> + * network stack should not crash.
> + */


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27139886597