Question: What is CVE-2025-21677?
Answer: CVE-2025-21677 is a vulnerability classified as nan with a CVSS score of nan. It falls under nan and affects the system as follows: In the Linux kernel, the following vulnerability has been resolved:

pfcp: Destroy device along with udp socket's netns dismantle.

pfcp_newlink() links the device to a list in dev_net(dev) instead
of net, where a udp tunnel socket is created.

Even when net is removed, the device stays alive on dev_net(dev).
Then, removing net triggers the splat below. [0]

In this example, pfcp0 is created in ns2, but the udp socket is
created in ns1.

  ip netns add ns1
  ip netns add ns2
  ip -n ns1 link add netns ns2 name pfcp0 type pfcp
  ip netns del ns1

Let's link the device to the socket's netns instead.

Now, pfcp_net_exit() needs another netdev iteration to remove
all pfcp devices in the netns.

pfcp_dev_list is not used under RCU, so the list API is converted
to the non-RCU variant.

pfcp_net_exit() can be converted to .exit_batch_rtnl() in net-next.

[0]:
ref_tracker: net notrefcnt@00000000128b34dc has 1/1 users at
     sk_alloc (./include/net/net_namespace.h:345 net/core/sock.c:2236)
     inet_create (net/ipv4/af_inet.c:326 net/ipv4/af_inet.c:252)
     __sock_create (net/socket.c:1558)
     udp_sock_create4 (net/ipv4/udp_tunnel_core.c:18)
     pfcp_create_sock (drivers/net/pfcp.c:168)
     pfcp_newlink (drivers/net/pfcp.c:182 drivers/net/pfcp.c:197)
     rtnl_newlink (net/core/rtnetlink.c:3786 net/core/rtnetlink.c:3897 net/core/rtnetlink.c:4012)
     rtnetlink_rcv_msg (net/core/rtnetlink.c:6922)
     netlink_rcv_skb (net/netlink/af_netlink.c:2542)
     netlink_unicast (net/netlink/af_netlink.c:1321 net/netlink/af_netlink.c:1347)
     netlink_sendmsg (net/netlink/af_netlink.c:1891)
     ____sys_sendmsg (net/socket.c:711 net/socket.c:726 net/socket.c:2583)
     ___sys_sendmsg (net/socket.c:2639)
     __sys_sendmsg (net/socket.c:2669)
     do_syscall_64 (arch/x86/entry/common.c:52 arch/x86/entry/common.c:83)
     entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)

WARNING: CPU: 1 PID: 11 at lib/ref_tracker.c:179 ref_tracker_dir_exit (lib/ref_tracker.c:179)
Modules linked in:
CPU: 1 UID: 0 PID: 11 Comm: kworker/u16:0 Not tainted 6.13.0-rc5-00147-g4c1224501e9d #5
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Workqueue: netns cleanup_net
RIP: 0010:ref_tracker_dir_exit (lib/ref_tracker.c:179)
Code: 00 00 00 fc ff df 4d 8b 26 49 bd 00 01 00 00 00 00 ad de 4c 39 f5 0f 85 df 00 00 00 48 8b 74 24 08 48 89 df e8 a5 cc 12 02 90 <0f> 0b 90 48 8d 6b 44 be 04 00 00 00 48 89 ef e8 80 de 67 ff 48 89
RSP: 0018:ff11000007f3fb60 EFLAGS: 00010286
RAX: 00000000000020ef RBX: ff1100000d6481e0 RCX: 1ffffffff0e40d82
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8423ee3c
RBP: ff1100000d648230 R08: 0000000000000001 R09: fffffbfff0e395af
R10: 0000000000000001 R11: 0000000000000000 R12: ff1100000d648230
R13: dead000000000100 R14: ff1100000d648230 R15: dffffc0000000000
FS:  0000000000000000(0000) GS:ff1100006ce80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005620e1363990 CR3: 000000000eeb2002 CR4: 0000000000771ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe07f0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
 <TASK>
 ? __warn (kernel/panic.c:748)
 ? ref_tracker_dir_exit (lib/ref_tracker.c:179)
 ? report_bug (lib/bug.c:201 lib/bug.c:219)
 ? handle_bug (arch/x86/kernel/traps.c:285)
 ? exc_invalid_op (arch/x86/kernel/traps.c:309 (discriminator 1))
 ? asm_exc_invalid_op (./arch/x86/include/asm/idtentry.h:621)
 ? _raw_spin_unlock_irqrestore (./arch/x86/include/asm/irqflags.h:42 ./arch/x86/include/asm/irqflags.h:97 ./arch/x86/include/asm/irqflags.h:155 ./include/linux/spinlock_api_smp.h:151 kernel/locking/spinlock.c:194)
 ? ref_tracker_dir_exit (lib/ref_tracker.c:179)
 ? __pfx_ref_tracker_dir_exit (lib/ref_tracker.c:158)
 ? kfree (mm/slub.c:4613 mm/slub.c:4761)
 net_free (net/core/net_namespace.c:476 net/core/net_namespace.c:467)
 cleanup_net (net/cor
---truncated---