On Fri, Apr 03, 2026 at 09:58:27AM +0800, Jiayuan Chen wrote: > 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. > > Note that for IPv6, the nexthdr check does not walk extension headers. > This is uncommon for TCP SYN packets in practice, and keeping it simple > was agreed upon by Kuniyuki Iwashima.
sashiko has flagged a similar issue with larger scope. Please take a look. Thanks. https://sashiko.dev/#/patchset/20260403015851.148209-1-jiayuan.chen%40linux.dev
