Re: [syzbot] [net?] KASAN: slab-use-after-free Read in fib_rules_lookup

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Jun 16, 2026 at 07:05:24AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    72dfa4700f78 net: dsa: sja1105: fix lastused timestamp in ..

This includes commit 759923cf03b0 ("ipv4: fib: Convert
fib_net_exit_batch() to ->exit_rtnl().") that moved ip_fib_net_exit()
(and therefore fib4_rules_exit()) earlier in the netns dismantle path.

Kuniyuki, can you please take a look?

You can use this to reproduce:

#!/bin/bash

while true; do
	ip netns add ns1
	ip -n ns1 link set dev lo up
	ip -n ns1 address add 192.0.2.1/24 dev lo
	ip -n ns1 link add name dummy1 up type dummy
	ip -n ns1 address add 198.51.100.1/24 dev dummy1
	ip -n ns1 rule add ipproto tcp sport 12345 table 12345
	ip -n ns1 fou add port 5555 ipproto 47 local 192.0.2.1 peer 198.51.100.2 peer_port 54321
	ip netns del ns1
done

Thanks

> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=15794bd2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=a0842261b62cdea8
> dashboard link: https://syzkaller.appspot.com/bug?extid=965506b59a2de0b6905c
> compiler:       Debian clang version 22.1.6 (++20260514074242+fc4aad7b5db3-1~exp1~20260514074407.73), Debian LLD 22.1.6
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/d4e16f50a97c/disk-72dfa470.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/6cd4a736e796/vmlinux-72dfa470.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/548b0011c8e8/bzImage-72dfa470.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+965506b59a2de0b6905c@syzkaller.appspotmail.com
> 
> bond0 (unregistering): Released all slaves
> bond1 (unregistering): Released all slaves
> bond2 (unregistering): (slave dummy0): Releasing active interface
> bond2 (unregistering): Released all slaves
> ==================================================================
> BUG: KASAN: slab-use-after-free in fib_rules_lookup+0x15e/0xeb0 net/core/fib_rules.c:321
> Read of size 8 at addr ffff88804ec4c680 by task kworker/u8:21/12641
> 
> CPU: 0 UID: 0 PID: 12641 Comm: kworker/u8:21 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
> Workqueue: netns cleanup_net
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  print_address_description+0x55/0x1e0 mm/kasan/report.c:378
>  print_report+0x58/0x70 mm/kasan/report.c:482
>  kasan_report+0x117/0x150 mm/kasan/report.c:595
>  fib_rules_lookup+0x15e/0xeb0 net/core/fib_rules.c:321
>  __fib_lookup+0x106/0x210 net/ipv4/fib_rules.c:96
>  ip_route_output_key_hash_rcu+0x294/0x2720 net/ipv4/route.c:2811
>  ip_route_output_key_hash+0x18d/0x2a0 net/ipv4/route.c:2702
>  __ip_route_output_key include/net/route.h:169 [inline]
>  ip_route_output_flow+0x2a/0x150 net/ipv4/route.c:2929
>  ip4_datagram_release_cb+0x89d/0xbe0 net/ipv4/datagram.c:118
>  release_sock+0x206/0x260 net/core/sock.c:3861
>  inet_shutdown+0x2b1/0x390 net/ipv4/af_inet.c:950
>  udp_tunnel_sock_release+0x6d/0x80 net/ipv4/udp_tunnel_core.c:197
>  fou_release net/ipv4/fou_core.c:562 [inline]
>  fou_exit_net+0x17d/0x1f0 net/ipv4/fou_core.c:1230
>  ops_exit_list net/core/net_namespace.c:199 [inline]
>  ops_undo_list+0x43d/0x8d0 net/core/net_namespace.c:252
>  cleanup_net+0x572/0x810 net/core/net_namespace.c:702
>  process_one_work kernel/workqueue.c:3314 [inline]
>  process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3397
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3478
>  kthread+0x389/0x470 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>
> 
> Allocated by task 19121:
>  kasan_save_stack mm/kasan/common.c:57 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
>  poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
>  __kasan_kmalloc+0x93/0xb0 mm/kasan/common.c:415
>  kasan_kmalloc include/linux/kasan.h:263 [inline]
>  __do_kmalloc_node mm/slub.c:5296 [inline]
>  __kmalloc_node_track_caller_noprof+0x4d7/0x7b0 mm/slub.c:5408
>  kmemdup_noprof+0x2b/0x70 mm/util.c:138
>  kmemdup_noprof include/linux/fortify-string.h:763 [inline]
>  fib_rules_register+0x2f/0x400 net/core/fib_rules.c:170
>  fib4_rules_init+0x21/0x160 net/ipv4/fib_rules.c:508
>  ip_fib_net_init net/ipv4/fib_frontend.c:1578 [inline]
>  fib_net_init+0x17a/0x3e0 net/ipv4/fib_frontend.c:1628
>  ops_init+0x35d/0x5d0 net/core/net_namespace.c:137
>  setup_net+0x118/0x350 net/core/net_namespace.c:446
>  copy_net_ns+0x4f9/0x720 net/core/net_namespace.c:579
>  create_new_namespaces+0x3f0/0x6b0 kernel/nsproxy.c:132
>  unshare_nsproxy_namespaces+0x149/0x190 kernel/nsproxy.c:234
>  ksys_unshare+0x57d/0xa00 kernel/fork.c:3242
>  __do_sys_unshare kernel/fork.c:3316 [inline]
>  __se_sys_unshare kernel/fork.c:3314 [inline]
>  __x64_sys_unshare+0x38/0x50 kernel/fork.c:3314
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Freed by task 12641:
>  kasan_save_stack mm/kasan/common.c:57 [inline]
>  kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
>  kasan_save_free_info+0x40/0x50 mm/kasan/generic.c:584
>  poison_slab_object mm/kasan/common.c:253 [inline]
>  __kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
>  kasan_slab_free include/linux/kasan.h:235 [inline]
>  slab_free_hook mm/slub.c:2689 [inline]
>  __rcu_free_sheaf_prepare+0x12d/0x2a0 mm/slub.c:2940
>  rcu_free_sheaf+0x31/0x200 mm/slub.c:5850
>  rcu_do_batch kernel/rcu/tree.c:2617 [inline]
>  rcu_core+0x78b/0x10a0 kernel/rcu/tree.c:2869
>  handle_softirqs+0x225/0x840 kernel/softirq.c:622
>  do_softirq+0x76/0xd0 kernel/softirq.c:523
>  __local_bh_enable_ip+0xf8/0x130 kernel/softirq.c:450
>  unregister_netdevice_many_notify+0x1874/0x2150 net/core/dev.c:12445
>  ops_exit_rtnl_list net/core/net_namespace.c:187 [inline]
>  ops_undo_list+0x391/0x8d0 net/core/net_namespace.c:248
>  cleanup_net+0x572/0x810 net/core/net_namespace.c:702
>  process_one_work kernel/workqueue.c:3314 [inline]
>  process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3397
>  worker_thread+0xa47/0xfb0 kernel/workqueue.c:3478
>  kthread+0x389/0x470 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
> 
> The buggy address belongs to the object at ffff88804ec4c600
>  which belongs to the cache kmalloc-192 of size 192
> The buggy address is located 128 bytes inside of
>  freed 192-byte region [ffff88804ec4c600, ffff88804ec4c6c0)
> 
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x4ec4c
> flags: 0xfff00000000000(node=0|zone=1|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 00fff00000000000 ffff88813fe163c0 dead000000000100 dead000000000122
> raw: 0000000000000000 0000000800100010 00000000f5000000 0000000000000000
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 0, migratetype Unmovable, gfp_mask 0xd2cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 13856, tgid 13853 (syz.3.2144), ts 351172300879, free_ts 351133053454
>  set_page_owner include/linux/page_owner.h:32 [inline]
>  post_alloc_hook+0x22d/0x280 mm/page_alloc.c:1853
>  prep_new_page mm/page_alloc.c:1861 [inline]
>  get_page_from_freelist+0x24ae/0x2530 mm/page_alloc.c:3941
>  __alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5221
>  alloc_slab_page mm/slub.c:3278 [inline]
>  allocate_slab+0x77/0x660 mm/slub.c:3467
>  new_slab mm/slub.c:3525 [inline]
>  refill_objects+0x336/0x3d0 mm/slub.c:7272
>  refill_sheaf mm/slub.c:2816 [inline]
>  __pcs_replace_empty_main+0x320/0x720 mm/slub.c:4652
>  alloc_from_pcs mm/slub.c:4750 [inline]
>  slab_alloc_node mm/slub.c:4884 [inline]
>  __do_kmalloc_node mm/slub.c:5295 [inline]
>  __kmalloc_noprof+0x464/0x750 mm/slub.c:5308
>  kmalloc_noprof include/linux/slab.h:954 [inline]
>  kzalloc_noprof include/linux/slab.h:1188 [inline]
>  new_dir fs/proc/proc_sysctl.c:966 [inline]
>  get_subdir fs/proc/proc_sysctl.c:1010 [inline]
>  sysctl_mkdir_p fs/proc/proc_sysctl.c:1320 [inline]
>  __register_sysctl_table+0xc02/0x1370 fs/proc/proc_sysctl.c:1395
>  neigh_sysctl_register+0x9b1/0xa90 net/core/neighbour.c:3915
>  addrconf_sysctl_register+0xb3/0x1c0 net/ipv6/addrconf.c:7396
>  ipv6_add_dev+0xd26/0x13a0 net/ipv6/addrconf.c:460
>  addrconf_notify+0x771/0x1050 net/ipv6/addrconf.c:3679
>  notifier_call_chain+0x1a5/0x3d0 kernel/notifier.c:85
>  call_netdevice_notifiers_extack net/core/dev.c:2288 [inline]
>  call_netdevice_notifiers net/core/dev.c:2302 [inline]
>  register_netdevice+0x18db/0x1f00 net/core/dev.c:11474
>  macsec_newlink+0x706/0x1200 drivers/net/macsec.c:4218
>  rtnl_newlink_create+0x310/0xb00 net/core/rtnetlink.c:3905
> page last free pid 12657 tgid 12657 stack trace:
>  reset_page_owner include/linux/page_owner.h:25 [inline]
>  __free_pages_prepare mm/page_alloc.c:1397 [inline]
>  __free_frozen_pages+0xc0d/0xd20 mm/page_alloc.c:2938
>  __tlb_remove_table_free mm/mmu_gather.c:228 [inline]
>  tlb_remove_table_rcu+0x85/0x100 mm/mmu_gather.c:291
>  rcu_do_batch kernel/rcu/tree.c:2617 [inline]
>  rcu_core+0x78b/0x10a0 kernel/rcu/tree.c:2869
>  handle_softirqs+0x225/0x840 kernel/softirq.c:622
>  __do_softirq kernel/softirq.c:656 [inline]
>  invoke_softirq kernel/softirq.c:496 [inline]
>  __irq_exit_rcu+0xca/0x220 kernel/softirq.c:735
>  irq_exit_rcu+0x9/0x30 kernel/softirq.c:752
>  instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1061 [inline]
>  sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1061
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
> 
> Memory state around the buggy address:
>  ffff88804ec4c580: 00 00 00 fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff88804ec4c600: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff88804ec4c680: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>                    ^
>  ffff88804ec4c700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>  ffff88804ec4c780: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
> ==================================================================
> 
> 
> ---
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