Subject: Re: [syzbot] [net?] general protection fault in arp_create (4)

From: Ido Schimmel <[email-masked]>
Date: Tue, 19 May 2026 09:40:17 +0300

+ batman maintainers

On Sun, May 17, 2026 at 09:54:28AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    70eda68668d1 Merge tag 'hid-for-linus-2026051401' of git:/..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=15f56e36580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=4caf64b1ee83dac0
> dashboard link: https://syzkaller.appspot.com/bug?extid=9fdcc9f05a98a540b816
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/6f3c10169d74/disk-70eda686.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/67977be60189/vmlinux-70eda686.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/cf642084f321/bzImage-70eda686.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email-masked]
> 
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000016: 0000 [#1] SMP KASAN NOPTI
> KASAN: null-ptr-deref in range [0x00000000000000b0-0x00000000000000b7]
> CPU: 1 UID: 0 PID: 37 Comm: kworker/u8:2 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
> Workqueue: bat_events batadv_bla_periodic_work
> RIP: 0010:arp_create+0xb1/0x980 net/ipv4/arp.c:554
> Code: 89 4c 24 78 4a 89 04 21 e8 8c f7 a0 f7 44 89 b4 24 a0 00 00 00 89 9c 24 b0 00 00 00 49 8d bf b4 00 00 00 48 89 f8 48 c1 e8 03 <42> 0f b6 04 20 84 c0 0f 85 15 06 00 00 41 0f b7 9f b4 00 00 00 49
> RSP: 0018:ffffc90000ad7720 EFLAGS: 00010213
> RAX: 0000000000000016 RBX: 0000000000000000 RCX: ffff8880202edc40
> RDX: 0000000000000000 RSI: 0000000000000806 RDI: 00000000000000b4
> RBP: ffffc90000ad7840 R08: 0000000000000000 R09: 0000000000000000
> R10: dffffc0000000000 R11: fffff5200015af20 R12: dffffc0000000000
> R13: ffff88806501d0a0 R14: 0000000000000000 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88812538a000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000032000 CR3: 0000000022ee7000 CR4: 0000000000350ef0
> Call Trace:
>  <TASK>
>  batadv_bla_send_claim+0x183/0xeb0 net/batman-adv/bridge_loop_avoidance.c:361
>  batadv_bla_send_loopdetect net/batman-adv/bridge_loop_avoidance.c:1402 [inline]
>  batadv_bla_periodic_work+0x654/0xae0 net/batman-adv/bridge_loop_avoidance.c:1492

batadv_bla_send_claim() is calling arp_create() with
primary_if->mesh_iface being NULL

>  process_one_work kernel/workqueue.c:3314 [inline]
>  process_scheduled_works+0xb5d/0x1860 kernel/workqueue.c:3397
>  worker_thread+0xa53/0xfc0 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>
> Modules linked in:
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:arp_create+0xb1/0x980 net/ipv4/arp.c:554
> Code: 89 4c 24 78 4a 89 04 21 e8 8c f7 a0 f7 44 89 b4 24 a0 00 00 00 89 9c 24 b0 00 00 00 49 8d bf b4 00 00 00 48 89 f8 48 c1 e8 03 <42> 0f b6 04 20 84 c0 0f 85 15 06 00 00 41 0f b7 9f b4 00 00 00 49
> RSP: 0018:ffffc90000ad7720 EFLAGS: 00010213
> RAX: 0000000000000016 RBX: 0000000000000000 RCX: ffff8880202edc40
> RDX: 0000000000000000 RSI: 0000000000000806 RDI: 00000000000000b4
> RBP: ffffc90000ad7840 R08: 0000000000000000 R09: 0000000000000000
> R10: dffffc0000000000 R11: fffff5200015af20 R12: dffffc0000000000
> R13: ffff88806501d0a0 R14: 0000000000000000 R15: 0000000000000000
> FS:  0000000000000000(0000) GS:ffff88812538a000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000200000032000 CR3: 0000000022ee7000 CR4: 0000000000350ef0
> ----------------
> Code disassembly (best guess):
>    0:	89 4c 24 78          	mov    %ecx,0x78(%rsp)
>    4:	4a 89 04 21          	mov    %rax,(%rcx,%r12,1)
>    8:	e8 8c f7 a0 f7       	call   0xf7a0f799
>    d:	44 89 b4 24 a0 00 00 	mov    %r14d,0xa0(%rsp)
>   14:	00
>   15:	89 9c 24 b0 00 00 00 	mov    %ebx,0xb0(%rsp)
>   1c:	49 8d bf b4 00 00 00 	lea    0xb4(%r15),%rdi
>   23:	48 89 f8             	mov    %rdi,%rax
>   26:	48 c1 e8 03          	shr    $0x3,%rax
> * 2a:	42 0f b6 04 20       	movzbl (%rax,%r12,1),%eax <-- trapping instruction
>   2f:	84 c0                	test   %al,%al
>   31:	0f 85 15 06 00 00    	jne    0x64c
>   37:	41 0f b7 9f b4 00 00 	movzwl 0xb4(%r15),%ebx
>   3e:	00
>   3f:	49                   	rex.WB
> 
> 
> ---
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