Re: [syzbot] [bridge?] KCSAN: data-race in br_fdb_update / br_fdb_update (8)

On Wed, Jan 7, 2026 at 12:03 PM syzbot
<[email protected]> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    f0b9d8eb98df Merge tag 'nfsd-6.19-3' of git://git.kernel.o..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1304c922580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
> dashboard link: https://syzkaller.appspot.com/bug?extid=bfab43087ad57222ce96
> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/f91c35600c27/disk-f0b9d8eb.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/9edb3553b7a5/vmlinux-f0b9d8eb.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/4d762ee145b8/bzImage-f0b9d8eb.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email protected]
>
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
> ==================================================================
> BUG: KCSAN: data-race in br_fdb_update / br_fdb_update
>
> read to 0xffff88811a0655c0 of 8 bytes by interrupt on cpu 1:
>  br_fdb_update+0x106/0x460 net/bridge/br_fdb.c:1005
>  br_handle_frame_finish+0x340/0xfc0 net/bridge/br_input.c:144
>  br_nf_hook_thresh+0x1eb/0x220 net/bridge/br_netfilter_hooks.c:-1
>  br_nf_pre_routing_finish_ipv6+0x4d1/0x570 net/bridge/br_netfilter_ipv6.c:-1
>  NF_HOOK include/linux/netfilter.h:318 [inline]
>  br_nf_pre_routing_ipv6+0x1fa/0x2b0 net/bridge/br_netfilter_ipv6.c:184
>  br_nf_pre_routing+0x52b/0xbd0 net/bridge/br_netfilter_hooks.c:508
>  nf_hook_entry_hookfn include/linux/netfilter.h:158 [inline]
>  nf_hook_bridge_pre net/bridge/br_input.c:291 [inline]
>  br_handle_frame+0x4f0/0x9e0 net/bridge/br_input.c:442
>  __netif_receive_skb_core+0x5df/0x1920 net/core/dev.c:6026
>  __netif_receive_skb_one_core net/core/dev.c:6137 [inline]
>  __netif_receive_skb+0x59/0x270 net/core/dev.c:6252
>  process_backlog+0x228/0x420 net/core/dev.c:6604
>  __napi_poll+0x5f/0x300 net/core/dev.c:7668
>  napi_poll net/core/dev.c:7731 [inline]
>  net_rx_action+0x425/0x8c0 net/core/dev.c:7883
>  handle_softirqs+0xba/0x290 kernel/softirq.c:622
>  do_softirq+0x45/0x60 kernel/softirq.c:523
>  __local_bh_enable_ip+0x70/0x80 kernel/softirq.c:450
>  local_bh_enable include/linux/bottom_half.h:33 [inline]
>  __alloc_skb+0x476/0x4b0 net/core/skbuff.c:674
>  alloc_skb include/linux/skbuff.h:1383 [inline]
>  wg_socket_send_buffer_to_peer+0x35/0x120 drivers/net/wireguard/socket.c:192
>  wg_packet_send_handshake_initiation drivers/net/wireguard/send.c:40 [inline]
>  wg_packet_handshake_send_worker+0x10d/0x160 drivers/net/wireguard/send.c:51
>  process_one_work kernel/workqueue.c:3257 [inline]
>  process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
>  worker_thread+0x582/0x770 kernel/workqueue.c:3421
>  kthread+0x489/0x510 kernel/kthread.c:463
>  ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
>
> write to 0xffff88811a0655c0 of 8 bytes by interrupt on cpu 0:
>  br_fdb_update+0x13e/0x460 net/bridge/br_fdb.c:1006
>  br_handle_frame_finish+0x340/0xfc0 net/bridge/br_input.c:144
>  br_nf_hook_thresh+0x1eb/0x220 net/bridge/br_netfilter_hooks.c:-1
>  br_nf_pre_routing_finish_ipv6+0x4d1/0x570 net/bridge/br_netfilter_ipv6.c:-1
>  NF_HOOK include/linux/netfilter.h:318 [inline]
>  br_nf_pre_routing_ipv6+0x1fa/0x2b0 net/bridge/br_netfilter_ipv6.c:184
>  br_nf_pre_routing+0x52b/0xbd0 net/bridge/br_netfilter_hooks.c:508
>  nf_hook_entry_hookfn include/linux/netfilter.h:158 [inline]
>  nf_hook_bridge_pre net/bridge/br_input.c:291 [inline]
>  br_handle_frame+0x4f0/0x9e0 net/bridge/br_input.c:442
>  __netif_receive_skb_core+0x5df/0x1920 net/core/dev.c:6026
>  __netif_receive_skb_one_core net/core/dev.c:6137 [inline]
>  __netif_receive_skb+0x59/0x270 net/core/dev.c:6252
>  process_backlog+0x228/0x420 net/core/dev.c:6604
>  __napi_poll+0x5f/0x300 net/core/dev.c:7668
>  napi_poll net/core/dev.c:7731 [inline]
>  net_rx_action+0x425/0x8c0 net/core/dev.c:7883
>  handle_softirqs+0xba/0x290 kernel/softirq.c:622
>  do_softirq+0x45/0x60 kernel/softirq.c:523
>  __local_bh_enable_ip+0x70/0x80 kernel/softirq.c:450
>  local_bh_enable include/linux/bottom_half.h:33 [inline]
>  fpregs_unlock arch/x86/include/asm/fpu/api.h:77 [inline]
>  kernel_fpu_end+0x6c/0x80 arch/x86/kernel/fpu/core.c:480
>  blake2s_compress+0x67/0x1740 lib/crypto/x86/blake2s.h:42
>  blake2s_update+0xa3/0x160 lib/crypto/blake2s.c:119
>  hmac+0x141/0x270 drivers/net/wireguard/noise.c:324
>  kdf+0x10b/0x1d0 drivers/net/wireguard/noise.c:375
>  mix_dh drivers/net/wireguard/noise.c:413 [inline]
>  wg_noise_handshake_create_initiation+0x1ac/0x520 drivers/net/wireguard/noise.c:550
>  wg_packet_send_handshake_initiation drivers/net/wireguard/send.c:34 [inline]
>  wg_packet_handshake_send_worker+0xb2/0x160 drivers/net/wireguard/send.c:51
>  process_one_work kernel/workqueue.c:3257 [inline]
>  process_scheduled_works+0x4ce/0x9d0 kernel/workqueue.c:3340
>  worker_thread+0x582/0x770 kernel/workqueue.c:3421
>  kthread+0x489/0x510 kernel/kthread.c:463
>  ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
>
> value changed: 0x0000000100026abc -> 0x0000000100026abd
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 UID: 0 PID: 8678 Comm: kworker/u8:42 Not tainted syzkaller #0 PREEMPT(voluntary)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
> Workqueue: wg-kex-wg0 wg_packet_handshake_send_worker
> ==================================================================
> net_ratelimit: 6540 callbacks suppressed
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:aa, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:aa, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:96:6e:14:75:db:9d, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:96:6e:14:75:db:9d, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
> net_ratelimit: 7050 callbacks suppressed
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:aa, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:aa, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:96:6e:14:75:db:9d, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:96:6e:14:75:db:9d, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on veth0_to_bridge with own address as source address (addr:aa:aa:aa:aa:aa:0c, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
> bridge0: received packet on bridge_slave_0 with own address as source address (addr:aa:aa:aa:aa:aa:1b, vlan:0)
>
>
> ---
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I am taking care of this issue, I will add the syzbot tags when sending V3 of

https://lore.kernel.org/netdev/CANn89iLaMpL1Kz=t13b0eGZ+m5dBxUpXx8oPKD1V-VwBAkzbJA@mail.gmail.com/T/#m19446ad4b132da817bda52a98a77a815034ed020

Thanks !