[PATCH bpf v4 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()

Jiayuan Chen
2026-03-30T08:08:13Z

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.")
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
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