Re: [PATCH bpf v1 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()

[Kuniyuki Iwashima]

On Mon, Mar 23, 2026 at 3:55 AM Jiayuan Chen <jiayuan.chen@xxxxxxxxx> wrote:
>
> From: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>
>
> 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@xxxxxxxxx>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxxx>

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>

Thanks, the fix looks good.
Just curious how you found this, are you trying to use this
feature for CDN or AI just found it ?


> ---
>  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
>