Linux: net: Remove conflicting altnames for dying netns in __dev_change_net_namespace().

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

net: Remove conflicting altnames for dying netns in __dev_change_net_namespace().

syzbot reported the warning in cfg80211_pernet_exit(). [0]

The repro does the following:

1. create two device in root netns and non-root netns
2. assign the same altname for the two devices
3. remove the non-root netns

Since commit 7663d522099e ("net: check for altname conflicts when changing netdev's netns"), cfg80211_switch_netns() and cfg802154_switch_netns() fail if init_net has a device with the conflicting altname.

default_device_exit_net() had the same issue and commit d09486a04f5d ("net: fix removing a namespace with conflicting altnames") fixed it.

cfg80211_pernet_exit() and cfg802154_pernet_exit() need the same fix.

Let's generalise the fix by removing conflicting altnames for dying netns in __dev_change_net_namespace().

[0]: cfg80211_switch_netns(rdev, &init_net) WARNING: net/wireless/core.c:1871 at cfg80211_pernet_exit+0xd5/0x120 net/wireless/core.c:1871, CPU#1: kworker/u8:9/1160 Modules linked in: CPU: 1 UID: 0 PID: 1160 Comm: kworker/u8:9 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026 Workqueue: netns cleanup_net RIP: 0010:cfg80211_pernet_exit+0xd5/0x120 net/wireless/core.c:1871 Code: e8 03 42 80 3c 20 00 74 08 4c 89 f7 e8 b4 ef 0e f7 4d 8b 36 49 81 fe 20 10 4a 90 74 12 e8 03 3d 9f f6 eb 85 e8 fc 3c 9f f6 90 <0f> 0b 90 eb cc e8 f1 3c 9f f6 eb 05 e8 ea 3c 9f f6 5b 41 5c 41 5e RSP: 0018:ffffc900057a78f0 EFLAGS: 00010293 RAX: ffffffff8b287154 RBX: ffff88807ba72780 RCX: ffff8880213e8000 RDX: 0000000000000000 RSI: 00000000ffffffef RDI: 0000000000000000 RBP: 00000000ffffffef R08: ffffffff9024cc67 R09: 0000000000000000 R10: fffff52000af4eb0 R11: fffffbfff204998d R12: dffffc0000000000 R13: ffffffff904a1080 R14: ffff888144ed0008 R15: ffff888144ed0e20 FS: 0000000000000000(0000) GS:ffff888124de6000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00005642de0a8a70 CR3: 000000007a40c000 CR4: 00000000003526f0 Call Trace: <TASK> ops_exit_list net/core/net_namespace.c:200 [inline] ops_undo_list+0x43d/0x8d0 net/core/net_namespace.c:253 cleanup_net+0x572/0x810 net/core/net_namespace.c:706 process_one_work kernel/workqueue.c:3387 [inline] process_scheduled_works+0xc3d/0x1630 kernel/workqueue.c:3470 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3551 kthread+0x38b/0x480 kernel/kthread.c:436 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245 </TASK>