From: Jiayuan Chen 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: 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 Signed-off-by: Jiayuan Chen --- 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