CVE-2026-23120: l2tp: avoid one data-race in l2tp_tunnel_del_work()

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

l2tp: avoid one data-race in l2tptunneldelwork()

We should read sk->sksocket only when dealing with kernel sockets.

syzbot reported the following data-race:

BUG: KCSAN: data-race in l2tptunneldelwork / skcommonrelease

write to 0xffff88811c182b20 of 8 bytes by task 5365 on cpu 0: sksetsocket include/net/sock.h:2092 [inline] sockorphan include/net/sock.h:2118 [inline] skcommonrelease+0xae/0x230 net/core/sock.c:4003 udplibclose+0x15/0x20 include/net/udp.h:325 inetrelease+0xce/0xf0 net/ipv4/afinet.c:437 sockrelease net/socket.c:662 [inline] sockclose+0x6b/0x150 net/socket.c:1455 fput+0x29b/0x650 fs/filetable.c:468 fput+0x1c/0x30 fs/filetable.c:496 taskworkrun+0x131/0x1a0 kernel/taskwork.c:233 resumeusermodework include/linux/resumeusermode.h:50 [inline] exittousermodeloop kernel/entry/common.c:44 [inline] exittousermodeloop+0x1fe/0x740 kernel/entry/common.c:75 exittousermodeprepare include/linux/irq-entry-common.h:226 [inline] syscallexittousermodeprepare include/linux/irq-entry-common.h:256 [inline] syscallexittousermodework include/linux/entry-common.h:159 [inline] syscallexittousermode include/linux/entry-common.h:194 [inline] dosyscall64+0x1e1/0x2b0 arch/x86/entry/syscall64.c:100 entrySYSCALL64afterhwframe+0x77/0x7f

read to 0xffff88811c182b20 of 8 bytes by task 827 on cpu 1: l2tptunneldelwork+0x2f/0x1a0 net/l2tp/l2tpcore.c:1418 processonework kernel/workqueue.c:3257 [inline] processscheduledworks+0x4ce/0x9d0 kernel/workqueue.c:3340 workerthread+0x582/0x770 kernel/workqueue.c:3421 kthread+0x489/0x510 kernel/kthread.c:463 retfromfork+0x149/0x290 arch/x86/kernel/process.c:158 retfromforkasm+0x1a/0x30 arch/x86/entry/entry64.S:246

value changed: 0xffff88811b818000 -> 0x0000000000000000