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

From: Jiayuan Chen <[email-masked]>
Date: Thu, 26 Mar 2026 09:54:36 +0800

On 3/26/26 8:04 AM, Kuniyuki Iwashima wrote:
> On Mon, Mar 23, 2026 at 3:55 AM Jiayuan Chen <[email-masked]> wrote:
>> From: Jiayuan Chen <[email-masked]>
>>
>> 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 <[email-masked]>
>> Signed-off-by: Jiayuan Chen <[email-masked]>
> Reviewed-by: Kuniyuki Iwashima <[email-masked]>
>
> 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 ?
>
Hi Kuniyuki,

Thanks for the review!

While investigating this feature for gateway optimization,
we reviewed the implementation and discovered these bugs.

We haven't deployed this yet, but it's on our roadmap.