CVE-2026-23439: udp_tunnel: fix NULL deref caused by udp_sock_create6 when CONFIG_IPV6=n

In the Linux kernel, the following vulnerability has been resolved:

udptunnel: fix NULL deref caused by udpsockcreate6 when CONFIGIPV6=n

When CONFIGIPV6 is disabled, the udpsockcreate6() function returns 0 (success) without actually creating a socket. Callers such as foucreate() then proceed to dereference the uninitialized socket pointer, resulting in a NULL pointer dereference.

The captured NULL deref crash: BUG: kernel NULL pointer dereference, address: 0000000000000018 RIP: 0010:founladddoit (net/ipv4/foucore.c:590 net/ipv4/foucore.c:764) [...] Call Trace: <TASK> genlfamilyrcvmsgdoit.constprop.0 (net/netlink/genetlink.c:1114) genlrcvmsg (net/netlink/genetlink.c:1194 net/netlink/genetlink.c:1209) [...] netlinkrcvskb (net/netlink/afnetlink.c:2550) genlrcv (net/netlink/genetlink.c:1219) netlinkunicast (net/netlink/afnetlink.c:1319 net/netlink/afnetlink.c:1344) netlinksendmsg (net/netlink/afnetlink.c:1894) socksendmsg (net/socket.c:727 (discriminator 1) net/socket.c:742 (discriminator 1)) syssendto (./include/linux/file.h:62 (discriminator 1) ./include/linux/file.h:83 (discriminator 1) net/socket.c:2183 (discriminator 1)) x64syssendto (net/socket.c:2213 (discriminator 1) net/socket.c:2209 (discriminator 1) net/socket.c:2209 (discriminator 1)) dosyscall64 (arch/x86/entry/syscall64.c:63 (discriminator 1) arch/x86/entry/syscall64.c:94 (discriminator 1)) entrySYSCALL64afterhwframe (net/arch/x86/entry/entry64.S:130)

This patch makes udpsockcreate6 return -EPFNOSUPPORT instead, so callers correctly take their error paths. There is only one caller of the vulnerable function and only privileged users can trigger it.