Message-ID: <e2acc82f-8928-4ba2-876e-2a669c07cd70@blackwall.org>
Date: Mon, 3 Nov 2025 14:51:04 +0200
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: syzbot <syzbot+dd280197f0f7ab3917be@...kaller.appspotmail.com>,
 bridge@...ts.linux.dev, davem@...emloft.net, edumazet@...gle.com,
 horms@...nel.org, idosch@...dia.com, kuba@...nel.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org, pabeni@...hat.com,
 syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [bridge?] KASAN: slab-use-after-free Read in
 br_switchdev_fdb_populate

On 11/3/25 13:20, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    fd57572253bc Merge tag 'sched_ext-for-6.18-rc3-fixes' of g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13f957e2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=609c87dcb0628493
> dashboard link: https://syzkaller.appspot.com/bug?extid=dd280197f0f7ab3917be
> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/a4541b741dd8/disk-fd575722.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/eae4ddf523dd/vmlinux-fd575722.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/28e03b35c3fc/bzImage-fd575722.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+dd280197f0f7ab3917be@...kaller.appspotmail.com
> 
> ==================================================================
> BUG: KASAN: slab-use-after-free in br_switchdev_fdb_populate+0x390/0x3a0 net/bridge/br_switchdev.c:141
> Read of size 8 at addr ffff888025122808 by task kworker/0:2/977
> 

hmm I have an idea about this one, it's possibly related to
MST enabled && vlan filtering disabled. I'll try to repro and
post a fix if I'm correct.

Cheers,
 Nik

> CPU: 0 UID: 0 PID: 977 Comm: kworker/0:2 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
> Workqueue: events_long br_fdb_cleanup
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:94 [inline]
>  dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xcd/0x630 mm/kasan/report.c:482
>  kasan_report+0xe0/0x110 mm/kasan/report.c:595
>  br_switchdev_fdb_populate+0x390/0x3a0 net/bridge/br_switchdev.c:141
>  br_switchdev_fdb_notify+0x1df/0x2c0 net/bridge/br_switchdev.c:165
>  fdb_notify+0x16c/0x1a0 net/bridge/br_fdb.c:186
>  fdb_delete+0x76e/0x12a0 net/bridge/br_fdb.c:324
>  br_fdb_cleanup+0x43e/0x600 net/bridge/br_fdb.c:574
>  process_one_work+0x9cf/0x1b70 kernel/workqueue.c:3263
>  process_scheduled_works kernel/workqueue.c:3346 [inline]
>  worker_thread+0x6c8/0xf10 kernel/workqueue.c:3427
>  kthread+0x3c5/0x780 kernel/kthread.c:463
>  ret_from_fork+0x675/0x7d0 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
> 
> Allocated by task 5810:
>  kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
>  kasan_save_track+0x14/0x30 mm/kasan/common.c:77
>  poison_kmalloc_redzone mm/kasan/common.c:400 [inline]
>  __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:417
>  kmalloc_noprof include/linux/slab.h:957 [inline]
>  kzalloc_noprof include/linux/slab.h:1094 [inline]
>  new_nbp net/bridge/br_if.c:431 [inline]
>  br_add_if+0x41d/0x1b70 net/bridge/br_if.c:599
>  do_set_master+0x40f/0x730 net/core/rtnetlink.c:2956
>  do_setlink.constprop.0+0xbd8/0x4380 net/core/rtnetlink.c:3158
>  rtnl_changelink net/core/rtnetlink.c:3769 [inline]
>  __rtnl_newlink net/core/rtnetlink.c:3928 [inline]
>  rtnl_newlink+0x1446/0x2000 net/core/rtnetlink.c:4065
>  rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6951
>  netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
>  netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
>  netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1346
>  netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1896
>  sock_sendmsg_nosec net/socket.c:727 [inline]
>  __sock_sendmsg net/socket.c:742 [inline]
>  __sys_sendto+0x4a3/0x520 net/socket.c:2244
>  __do_sys_sendto net/socket.c:2251 [inline]
>  __se_sys_sendto net/socket.c:2247 [inline]
>  __x64_sys_sendto+0xe0/0x1c0 net/socket.c:2247
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Freed by task 15061:
>  kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
>  kasan_save_track+0x14/0x30 mm/kasan/common.c:77
>  __kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:587
>  kasan_save_free_info mm/kasan/kasan.h:406 [inline]
>  poison_slab_object mm/kasan/common.c:252 [inline]
>  __kasan_slab_free+0x5f/0x80 mm/kasan/common.c:284
>  kasan_slab_free include/linux/kasan.h:234 [inline]
>  slab_free_hook mm/slub.c:2539 [inline]
>  slab_free mm/slub.c:6630 [inline]
>  kfree+0x2b8/0x6d0 mm/slub.c:6837
>  kobject_cleanup lib/kobject.c:689 [inline]
>  kobject_release lib/kobject.c:720 [inline]
>  kref_put include/linux/kref.h:65 [inline]
>  kobject_put+0x1e7/0x5a0 lib/kobject.c:737
>  rcu_do_batch kernel/rcu/tree.c:2605 [inline]
>  rcu_core+0x79c/0x1530 kernel/rcu/tree.c:2861
>  handle_softirqs+0x219/0x8e0 kernel/softirq.c:622
>  __do_softirq kernel/softirq.c:656 [inline]
>  invoke_softirq kernel/softirq.c:496 [inline]
>  __irq_exit_rcu+0x109/0x170 kernel/softirq.c:723
>  irq_exit_rcu+0x9/0x30 kernel/softirq.c:739
>  instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1052 [inline]
>  sysvec_apic_timer_interrupt+0xa4/0xc0 arch/x86/kernel/apic/apic.c:1052
>  asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
> 
> Last potentially related work creation:
>  kasan_save_stack+0x33/0x60 mm/kasan/common.c:56
>  kasan_record_aux_stack+0xa7/0xc0 mm/kasan/generic.c:559
>  __call_rcu_common.constprop.0+0xa5/0xa10 kernel/rcu/tree.c:3123
>  br_del_if+0xe0/0x270 net/bridge/br_if.c:740
>  do_set_master+0x172/0x730 net/core/rtnetlink.c:2940
>  do_setlink.constprop.0+0xbd8/0x4380 net/core/rtnetlink.c:3158
>  rtnl_group_changelink net/core/rtnetlink.c:3783 [inline]
>  __rtnl_newlink net/core/rtnetlink.c:3937 [inline]
>  rtnl_newlink+0x18e0/0x2000 net/core/rtnetlink.c:4065
>  rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6951
>  netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
>  netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
>  netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1346
>  netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1896
>  sock_sendmsg_nosec net/socket.c:727 [inline]
>  __sock_sendmsg net/socket.c:742 [inline]
>  ____sys_sendmsg+0xa98/0xc70 net/socket.c:2630
>  ___sys_sendmsg+0x134/0x1d0 net/socket.c:2684
>  __sys_sendmsg+0x16d/0x220 net/socket.c:2716
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> The buggy address belongs to the object at ffff888025122800
>  which belongs to the cache kmalloc-1k of size 1024
> The buggy address is located 8 bytes inside of
>  freed 1024-byte region [ffff888025122800, ffff888025122c00)
> 
> The buggy address belongs to the physical page:
> page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff888025125800 pfn:0x25120
> head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
> flags: 0xfff00000000240(workingset|head|node=0|zone=1|lastcpupid=0x7ff)
> page_type: f5(slab)
> raw: 00fff00000000240 ffff88813ffa6dc0 ffffea0001ee1810 ffffea0000c8c210
> raw: ffff888025125800 000000000010000e 00000000f5000000 0000000000000000
> head: 00fff00000000240 ffff88813ffa6dc0 ffffea0001ee1810 ffffea0000c8c210
> head: ffff888025125800 000000000010000e 00000000f5000000 0000000000000000
> head: 00fff00000000003 ffffea0000944801 00000000ffffffff 00000000ffffffff
> head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
> page dumped because: kasan: bad access detected
> page_owner tracks the page as allocated
> page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5807, tgid 5807 (syz-executor), ts 72613372336, free_ts 72596370994
>  set_page_owner include/linux/page_owner.h:32 [inline]
>  post_alloc_hook+0x1c0/0x230 mm/page_alloc.c:1850
>  prep_new_page mm/page_alloc.c:1858 [inline]
>  get_page_from_freelist+0x10a3/0x3a30 mm/page_alloc.c:3884
>  __alloc_frozen_pages_noprof+0x25f/0x2470 mm/page_alloc.c:5183
>  alloc_pages_mpol+0x1fb/0x550 mm/mempolicy.c:2416
>  alloc_slab_page mm/slub.c:3055 [inline]
>  allocate_slab mm/slub.c:3228 [inline]
>  new_slab+0x24a/0x360 mm/slub.c:3282
>  ___slab_alloc+0xdae/0x1a60 mm/slub.c:4651
>  __slab_alloc.constprop.0+0x63/0x110 mm/slub.c:4770
>  __slab_alloc_node mm/slub.c:4846 [inline]
>  slab_alloc_node mm/slub.c:5268 [inline]
>  __do_kmalloc_node mm/slub.c:5641 [inline]
>  __kmalloc_noprof+0x501/0x880 mm/slub.c:5654
>  kmalloc_noprof include/linux/slab.h:961 [inline]
>  kzalloc_noprof include/linux/slab.h:1094 [inline]
>  __alloc_workqueue+0x112/0x1810 kernel/workqueue.c:5715
>  alloc_workqueue_noprof+0xd2/0x200 kernel/workqueue.c:5818
>  wg_newlink+0x24d/0x780 drivers/net/wireguard/device.c:341
>  rtnl_newlink_create net/core/rtnetlink.c:3833 [inline]
>  __rtnl_newlink net/core/rtnetlink.c:3950 [inline]
>  rtnl_newlink+0xc45/0x2000 net/core/rtnetlink.c:4065
>  rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6951
>  netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
>  netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
>  netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1346
>  netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1896
> page last free pid 5809 tgid 5809 stack trace:
>  reset_page_owner include/linux/page_owner.h:25 [inline]
>  free_pages_prepare mm/page_alloc.c:1394 [inline]
>  __free_frozen_pages+0x7df/0x1160 mm/page_alloc.c:2906
>  qlink_free mm/kasan/quarantine.c:163 [inline]
>  qlist_free_all+0x4d/0x120 mm/kasan/quarantine.c:179
>  kasan_quarantine_reduce+0x195/0x1e0 mm/kasan/quarantine.c:286
>  __kasan_slab_alloc+0x69/0x90 mm/kasan/common.c:352
>  kasan_slab_alloc include/linux/kasan.h:252 [inline]
>  slab_post_alloc_hook mm/slub.c:4970 [inline]
>  slab_alloc_node mm/slub.c:5280 [inline]
>  __kmalloc_cache_noprof+0x274/0x780 mm/slub.c:5758
>  kmalloc_noprof include/linux/slab.h:957 [inline]
>  kzalloc_noprof include/linux/slab.h:1094 [inline]
>  kset_create lib/kobject.c:965 [inline]
>  kset_create_and_add+0x4d/0x190 lib/kobject.c:1008
>  register_queue_kobjects net/core/net-sysfs.c:2106 [inline]
>  netdev_register_kobject+0x1ef/0x3d0 net/core/net-sysfs.c:2362
>  register_netdevice+0x13dc/0x2270 net/core/dev.c:11294
>  ipcaif_newlink+0x73/0x2c0 net/caif/chnl_net.c:452
>  rtnl_newlink_create net/core/rtnetlink.c:3833 [inline]
>  __rtnl_newlink net/core/rtnetlink.c:3950 [inline]
>  rtnl_newlink+0xc45/0x2000 net/core/rtnetlink.c:4065
>  rtnetlink_rcv_msg+0x95e/0xe90 net/core/rtnetlink.c:6951
>  netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2552
>  netlink_unicast_kernel net/netlink/af_netlink.c:1320 [inline]
>  netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1346
>  netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1896
>  sock_sendmsg_nosec net/socket.c:727 [inline]
>  __sock_sendmsg net/socket.c:742 [inline]
>  __sys_sendto+0x4a3/0x520 net/socket.c:2244
>  __do_sys_sendto net/socket.c:2251 [inline]
>  __se_sys_sendto net/socket.c:2247 [inline]
>  __x64_sys_sendto+0xe0/0x1c0 net/socket.c:2247
> 
> Memory state around the buggy address:
>  ffff888025122700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff888025122780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>> ffff888025122800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                       ^
>  ffff888025122880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff888025122900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> 
> 
> ---
> 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@...glegroups.com.
> 
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> 
> If the report is already addressed, let syzbot know by replying with:
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> 
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> If the report is a duplicate of another one, reply with:
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