[PATCH net] bpf: lwt: clear stale IP control block after pushing an encap header

bpf_lwt_push_ip_encap() prepends an encapsulation header and calls
skb_reset_network_header(), but leaves the IP control block describing the
inner header. Its offsets are relative to the network header, so they now
point into the encapsulated packet.

On an ingress lwt-bpf encap route a forwarded packet carrying a Record
Route option then reaches __ip_options_echo() with a stale opt->rr. It
takes the option length from the packet and copies up to 255 bytes into the
fixed-size option storage on the __icmp_send() stack. ip_forward_options()
writes at a packet-controlled offset for the same reason.

Clear the stale state once the outer header is in place, as the tunnel
encapsulators do and as seg6_local.c was taught to do in commit
f967455fb2a5 ("seg6: reset IP6CB after IPv6 decapsulation") and commit
44930446dde4 ("ipv6: seg6: clear IPv4 control block on IPIP
decapsulation").

  BUG: KASAN: stack-out-of-bounds in __ip_options_echo (net/ipv4/ip_options.c:96)
  Write of size 255 at addr ffff8880bee08930 by task exploit/150

  Call Trace:
   <IRQ>
   __asan_memcpy (mm/kasan/shadow.c:106)
   __ip_options_echo (net/ipv4/ip_options.c:96)
   __icmp_send (net/ipv4/icmp.c:949)
   ip_forward (./include/net/icmp.h:43 net/ipv4/ip_forward.c:176)
   lwtunnel_input (net/core/lwtunnel.c:465)
   ip_rcv (net/ipv4/ip_input.c:612)
   __netif_receive_skb_one_core (net/core/dev.c:6264)
   process_backlog (net/core/dev.c:6728)
   __napi_poll (net/core/dev.c:7787)
   net_rx_action (net/core/dev.c:8007)
   handle_softirqs (kernel/softirq.c:645)
   do_softirq.part.0 (kernel/softirq.c:546)
   </IRQ>
   __local_bh_enable_ip (kernel/softirq.c:473)
   __dev_queue_xmit (net/core/dev.c:4961)
   packet_sendmsg (net/packet/af_packet.c:3143)
   __sys_sendto (net/socket.c:2281)
   __x64_sys_sendto (net/socket.c:2288)
   do_syscall_64 (arch/x86/entry/syscall_64.c:84)
   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)

Cc: stable@vger.kernel.org
Fixes: 52f278774e79 ("bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap")
Reported-by: <redacted>
Assisted-by: LLM
Signed-off-by: Xiang Mei <redacted>
---
 net/core/lwt_bpf.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index da49364ec63d..17be476b5717 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c

@@ -649,6 +649,18 @@ int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
 	if (ingress)
 		skb_postpush_rcsum(skb, iph, len);
 	skb_reset_network_header(skb);
+	if (ipv4) {
+		memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));
+	} else {
+		bool l3slave = ipv6_l3mdev_skb(IP6CB(skb)->flags);
+		int iif = IP6CB(skb)->iif;
+
+		memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
+		IP6CB(skb)->iif = iif;
+		IP6CB(skb)->nhoff = offsetof(struct ipv6hdr, nexthdr);
+		if (l3slave)
+			IP6CB(skb)->flags |= IP6SKB_L3SLAVE;
+	}
 	if (is_udp_tunnel) {
 		size_t iph_sz = ipv4 ? iph->ihl * 4 : sizeof(struct ipv6hdr);
 

-- 
2.43.0