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

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Kuniyuki Iwashima <kuniyu@google.com>
Date: Tue, 16 Jun 2026 10:06:55 -0700
> On Tue, Jun 16, 2026 at 8:55 AM Eric Dumazet <edumazet@google.com> wrote:
> >
> > On Tue, Jun 16, 2026 at 8:31 AM Ido Schimmel <idosch@nvidia.com> wrote:
> > >
> > > 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
> > >
> >
> > Oh right.
> >
> > While looking at this syzbot report I also found an old issue.
> >
> > https://lore.kernel.org/netdev/20260616141317.407791-1-edumazet@google.com/T/#u
> >
> > I guess adding some delays in enqueue_to_backlog() could trigger a
> > similar bug even if we revert Kuniyuki's patch.
> 
> I'll look into it, thank you both !

I'll move fib4_rules_exit() to ->exit().

fib_unmerge() requires RTNL, but it is not needed in ->delete()
in the first place since it's already called in ->configure().

---8<---
diff --git a/net/ipv4/fib_frontend.c b/net/ipv4/fib_frontend.c
index c7d1f31650d7..42212970d735 100644
--- a/net/ipv4/fib_frontend.c
+++ b/net/ipv4/fib_frontend.c
@@ -1612,10 +1612,6 @@ static void ip_fib_net_exit(struct net *net)
 			fib_free_table(tb);
 		}
 	}
-
-#ifdef CONFIG_IP_MULTIPLE_TABLES
-	fib4_rules_exit(net);
-#endif
 }
 
 static int __net_init fib_net_init(struct net *net)
@@ -1652,6 +1648,9 @@ static int __net_init fib_net_init(struct net *net)
 	ip_fib_net_exit(net);
 	rtnl_net_unlock(net);
 
+#ifdef CONFIG_IP_MULTIPLE_TABLES
+	fib4_rules_exit(net);
+#endif
 	kfree(net->ipv4.fib_table_hash);
 	fib4_notifier_exit(net);
 	goto out;
@@ -1671,6 +1670,9 @@ static void __net_exit fib_net_exit_rtnl(struct net *net,
 
 static void __net_exit fib_net_exit(struct net *net)
 {
+#ifdef CONFIG_IP_MULTIPLE_TABLES
+	fib4_rules_exit(net);
+#endif
 	kfree(net->ipv4.fib_table_hash);
 	fib4_notifier_exit(net);
 	fib4_semantics_exit(net);
diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
index 51f0193092f0..0bf6204468c5 100644
--- a/net/ipv4/fib_rules.c
+++ b/net/ipv4/fib_rules.c
@@ -352,12 +352,6 @@ static int fib4_rule_configure(struct fib_rule *rule, struct sk_buff *skb,
 static int fib4_rule_delete(struct fib_rule *rule)
 {
 	struct net *net = rule->fr_net;
-	int err;
-
-	/* split local/main if they are not already split */
-	err = fib_unmerge(net);
-	if (err)
-		goto errout;
 
 #ifdef CONFIG_IP_ROUTE_CLASSID
 	if (((struct fib4_rule *)rule)->tclassid)
@@ -368,8 +362,8 @@ static int fib4_rule_delete(struct fib_rule *rule)
 	if (net->ipv4.fib_rules_require_fldissect &&
 	    fib_rule_requires_fldissect(rule))
 		net->ipv4.fib_rules_require_fldissect--;
-errout:
-	return err;
+
+	return 0;
 }
 
 static int fib4_rule_compare(struct fib_rule *rule, struct fib_rule_hdr *frh,
---8<---



> 
> >
> >
> >
> >
> > > 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
> > > > ==================================================================
> > > >
> > > >
> > > > ---
> > > > This report is generated by a bot. It may contain errors.
> > > > See https://goo.gl/tpsmEJ for more information about syzbot.
> > > > syzbot engineers can be reached at syzkaller@googlegroups.com.
> > > >
> > > > syzbot will keep track of this issue. See:
> > > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> > > >
> > > > If the report is already addressed, let syzbot know by replying with:
> > > > #syz fix: exact-commit-title
> > > >
> > > > If you want to overwrite report's subsystems, reply with:
> > > > #syz set subsystems: new-subsystem
> > > > (See the list of subsystem names on the web dashboard)
> > > >
> > > > If the report is a duplicate of another one, reply with:
> > > > #syz dup: exact-subject-of-another-report
> > > >
> > > > If you want to undo deduplication, reply with:
> > > > #syz undup
>