CVE-2025-21858 geneve: Fix use-after-free in geneve_find_dev().
In the Linux kernel, the following vulnerability has been resolved:

geneve: Fix use-after-free in geneve_find_dev().

syzkaller reported a use-after-free in geneve_find_dev() [0]
without repro.

geneve_configure() links struct geneve_dev.next to
net_generic(net, geneve_net_id)->geneve_list.

The net here could differ from dev_net(dev) if IFLA_NET_NS_PID,
IFLA_NET_NS_FD, or IFLA_TARGET_NETNSID is set.

When dev_net(dev) is dismantled, geneve_exit_batch_rtnl() finally
calls unregister_netdevice_queue() for each dev in the netns,
and later the dev is freed.

However, its geneve_dev.next is still linked to the backend UDP
socket netns.

Then, use-after-free will occur when another geneve dev is created
in the netns.

Let's call geneve_dellink() instead in geneve_destroy_tunnels().

[0]:
BUG: KASAN: slab-use-after-free in geneve_find_dev drivers/net/geneve.c:1295 [inline]
BUG: KASAN: slab-use-after-free in geneve_configure+0x234/0x858 drivers/net/geneve.c:1343
Read of size 2 at addr ffff000054d6ee24 by task syz.1.4029/13441

CPU: 1 UID: 0 PID: 13441 Comm: syz.1.4029 Not tainted 6.13.0-g0ad9617c78ac #24 dc35ca22c79fb82e8e7bc5c9c9adafea898b1e3d
Hardware name: linux,dummy-virt (DT)
Call trace:
 show_stack+0x38/0x50 arch/arm64/kernel/stacktrace.c:466 (C)
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0xbc/0x108 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0x16c/0x6f0 mm/kasan/report.c:489
 kasan_report+0xc0/0x120 mm/kasan/report.c:602
 __asan_report_load2_noabort+0x20/0x30 mm/kasan/report_generic.c:379
 geneve_find_dev drivers/net/geneve.c:1295 [inline]
 geneve_configure+0x234/0x858 drivers/net/geneve.c:1343
 geneve_newlink+0xb8/0x128 drivers/net/geneve.c:1634
 rtnl_newlink_create+0x23c/0x868 net/core/rtnetlink.c:3795
 __rtnl_newlink net/core/rtnetlink.c:3906 [inline]
 rtnl_newlink+0x1054/0x1630 net/core/rtnetlink.c:4021
 rtnetlink_rcv_msg+0x61c/0x918 net/core/rtnetlink.c:6911
 netlink_rcv_skb+0x1dc/0x398 net/netlink/af_netlink.c:2543
 rtnetlink_rcv+0x34/0x50 net/core/rtnetlink.c:6938
 netlink_unicast_kernel net/netlink/af_netlink.c:1322 [inline]
 netlink_unicast+0x618/0x838 net/netlink/af_netlink.c:1348
 netlink_sendmsg+0x5fc/0x8b0 net/netlink/af_netlink.c:1892
 sock_sendmsg_nosec net/socket.c:713 [inline]
 __sock_sendmsg net/socket.c:728 [inline]
 ____sys_sendmsg+0x410/0x6f8 net/socket.c:2568
 ___sys_sendmsg+0x178/0x1d8 net/socket.c:2622
 __sys_sendmsg net/socket.c:2654 [inline]
 __do_sys_sendmsg net/socket.c:2659 [inline]
 __se_sys_sendmsg net/socket.c:2657 [inline]
 __arm64_sys_sendmsg+0x12c/0x1c8 net/socket.c:2657
 __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
 invoke_syscall+0x90/0x278 arch/arm64/kernel/syscall.c:49
 el0_svc_common+0x13c/0x250 arch/arm64/kernel/syscall.c:132
 do_el0_svc+0x54/0x70 arch/arm64/kernel/syscall.c:151
 el0_svc+0x4c/0xa8 arch/arm64/kernel/entry-common.c:744
 el0t_64_sync_handler+0x78/0x108 arch/arm64/kernel/entry-common.c:762
 el0t_64_sync+0x198/0x1a0 arch/arm64/kernel/entry.S:600

Allocated by task 13247:
 kasan_save_stack mm/kasan/common.c:47 [inline]
 kasan_save_track+0x30/0x68 mm/kasan/common.c:68
 kasan_save_alloc_info+0x44/0x58 mm/kasan/generic.c:568
 poison_kmalloc_redzone mm/kasan/common.c:377 [inline]
 __kasan_kmalloc+0x84/0xa0 mm/kasan/common.c:394
 kasan_kmalloc include/linux/kasan.h:260 [inline]
 __do_kmalloc_node mm/slub.c:4298 [inline]
 __kmalloc_node_noprof+0x2a0/0x560 mm/slub.c:4304
 __kvmalloc_node_noprof+0x9c/0x230 mm/util.c:645
 alloc_netdev_mqs+0xb8/0x11a0 net/core/dev.c:11470
 rtnl_create_link+0x2b8/0xb50 net/core/rtnetlink.c:3604
 rtnl_newlink_create+0x19c/0x868 net/core/rtnetlink.c:3780
 __rtnl_newlink net/core/rtnetlink.c:3906 [inline]
 rtnl_newlink+0x1054/0x1630 net/core/rtnetlink.c:4021
 rtnetlink_rcv_msg+0x61c/0x918 net/core/rtnetlink.c:6911
 netlink_rcv_skb+0x1dc/0x398 net/netlink/af_netlink.c:2543
 rtnetlink_rcv+0x34/0x50 net/core/rtnetlink.c:6938
 netlink_unicast_kernel net/netlink/af_n
---truncated---
CVSS v3.1 score: 7.8