bpf: bpf_out_neigh_v4: Fix nd_tbl NULL dereference when IPv6 is disabled

When booting with the 'ipv6.disable=1' parameter, the nd_tbl is never
initialized because inet6_init() exits before ndisc_init() is called which
initializes it. If bpf_redirect_neigh() is called from tc with an explicit
nexthop of nh_family == AF_INET6, bpf_out_neigh_v4() takes the AF_INET6
branch and calls ip_neigh_gw6(), which relies on ipv6_stub->nd_tbl.

 BUG: kernel NULL pointer dereference, address: 0000000000000248
 Oops: Oops: 0000 [#1] SMP NOPTI
 RIP: 0010:skb_do_redirect+0xb93/0xf00
 Call Trace:
  <TASK>
  ? srso_alias_return_thunk+0x5/0xfbef5
  ? __tcf_classify.constprop.0+0x83/0x160
  ? srso_alias_return_thunk+0x5/0xfbef5
  ? tcf_classify+0x2b/0x50
  ? srso_alias_return_thunk+0x5/0xfbef5
  ? tc_run+0xb8/0x120
  ? srso_alias_return_thunk+0x5/0xfbef5
  __dev_queue_xmit+0x6fa/0x1000
  ? srso_alias_return_thunk+0x5/0xfbef5
  ? srso_alias_return_thunk+0x5/0xfbef5
  ? alloc_skb_with_frags+0x58/0x200
  packet_sendmsg+0x10da/0x1700
  ? srso_alias_return_thunk+0x5/0xfbef5
  __sys_sendto+0x1f3/0x220
  __x64_sys_sendto+0x24/0x30
  do_syscall_64+0x101/0xf80
  ? exc_page_fault+0x6e/0x170
  ? srso_alias_return_thunk+0x5/0xfbef5
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  </TASK>

Fix this by adding an early check in the AF_INET6 branch of
bpf_out_neigh_v4(). If IPv6 is disabled, unlock RCU and drop the packet.

Suggested-by: Fernando Fernandez Mancera <fmancera@suse.de>
Fixes: ba452c9 ("bpf: Fix bpf_redirect_neigh helper api to support supplying nexthop")
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/20260307-net-nd_tbl_fixes-v4-3-e2677e85628c@suse.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: rbmarliere

net/core/filter.c

@@ -2335,6 +2335,10 @@ static int bpf_out_neigh_v4(struct net *net, struct sk_buff *skb,
 
 		neigh = ip_neigh_for_gw(rt, skb, &is_v6gw);
 	} else if (nh->nh_family == AF_INET6) {
+		if (unlikely(!ipv6_mod_enabled())) {
+			rcu_read_unlock();
+			goto out_drop;
+		}
 		neigh = ip_neigh_gw6(dev, &nh->ipv6_nh);
 		is_v6gw = true;
 	} else if (nh->nh_family == AF_INET) {