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

[Jiayuan Chen]

On 3/26/26 8:04 AM, Kuniyuki Iwashima wrote:

On Mon, Mar 23, 2026 at 3:55 AM Jiayuan Chen <jiayuan.c...@linux.dev> wrote:

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

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.c...@linux.dev>
Signed-off-by: Jiayuan Chen <jiayuan.c...@shopee.com>

Reviewed-by: Kuniyuki Iwashima <kun...@google.com>

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.