CVE-2026-74289: ipv4: fib: Don't dump dying fib_info in fib_leaf_notify().

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

ipv4: fib: Don't dump dying fibinfo in fibleafnotify().

syzbot reported use-after-free in nsimfib4prepareevent(). [0]

The problem is that the following functions call fibinfohold() / refcountinc() while dumping fibinfo under RCU, which is unsafe.

mlxswsprouterfib4event() rockerrouterfibevent() nsimfib4prepareevent()

refcountincnotzero() must be used, but it would be too late there.

Let's guarantee the lifetime of fibinfo in fibleafnotify().

Note that IPv6 does not need the corresponding change since fib6tabledump() holds fib6table.tb6lock.

[0]: refcountt: addition on 0; use-after-free. WARNING: lib/refcount.c:25 at refcountwarnsaturate+0x9f/0x110 lib/refcount.c:25, CPU#0: kworker/u8:15/3420 Modules linked in: CPU: 0 UID: 0 PID: 3420 Comm: kworker/u8:15 Not tainted syzkaller #0 PREEMPT{RT,(full)} Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026 Workqueue: netns cleanupnet RIP: 0010:refcountwarnsaturate+0x9f/0x110 lib/refcount.c:25 Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 1b f1 22 fd 48 8d 3d 84 cb f1 0a 67 48 0f b9 3a eb 4a e8 08 f1 22 fd 48 8d 3d 81 cb f1 0a <67> 48 0f b9 3a eb 37 e8 f5 f0 22 fd 48 8d 3d 7e cb f1 0a 67 48 0f RSP: 0018:ffffc9000f2c7270 EFLAGS: 00010293 RAX: ffffffff84a18858 RBX: 0000000000000002 RCX: ffff888032ff9ec0 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8f9353e0 RBP: 0000000000000000 R08: ffff888032ff9ec0 R09: 0000000000000005 R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880570cc000 R13: dffffc0000000000 R14: ffff88802b40563c R15: ffff8880570cc000 FS: 0000000000000000(0000) GS:ffff888126173000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fb1f4d5d000 CR3: 000000006072a000 CR4: 00000000003526f0 Call Trace: <TASK> refcountadd include/linux/refcount.h:-1 [inline] refcountinc include/linux/refcount.h:366 [inline] refcountinc include/linux/refcount.h:383 [inline] fibinfohold include/net/ipfib.h:629 [inline] nsimfib4prepareevent drivers/net/netdevsim/fib.c:930 [inline] nsimfibeventschedulework drivers/net/netdevsim/fib.c:1000 [inline] nsimfibeventnb+0x1055/0x1240 drivers/net/netdevsim/fib.c:1043 callfibnotifier+0x45/0x80 net/core/fibnotifier.c:25 callfibentrynotifier net/ipv4/fibtrie.c:90 [inline] fibleafnotify net/ipv4/fibtrie.c:2176 [inline] fibtablenotify net/ipv4/fibtrie.c:2194 [inline] fibnotify+0x36b/0x5e0 net/ipv4/fibtrie.c:2217 fibnetdump net/core/fibnotifier.c:70 [inline] registerfibnotifier+0x184/0x360 net/core/fibnotifier.c:108 nsimfibcreate+0x85d/0x9f0 drivers/net/netdevsim/fib.c:1596 nsimdevreloadcreate drivers/net/netdevsim/dev.c:1604 [inline] nsimdevreloadup+0x374/0x7c0 drivers/net/netdevsim/dev.c:1058 devlinkreload+0x501/0x8d0 net/devlink/dev.c:475 devlinkpernetpreexit+0x1ff/0x420 net/devlink/core.c:558 opspreexitlist net/core/netnamespace.c:161 [inline] opsundolist+0x187/0x940 net/core/netnamespace.c:234 cleanupnet+0x56e/0x800 net/core/netnamespace.c:702 processonework kernel/workqueue.c:3314 [inline] processscheduledworks+0xb5d/0x1860 kernel/workqueue.c:3397 workerthread+0xa53/0xfc0 kernel/workqueue.c:3478 kthread+0x388/0x470 kernel/kthread.c:436 retfromfork+0x514/0xb70 arch/x86/kernel/process.c:158 retfromforkasm+0x1a/0x30 arch/x86/entry/entry64.S:245 </TASK>