Re: [syzbot] [openvswitch?] WARNING in set_ipv6_addr

[Fernando Fernandez Mancera <fmancera@suse.de>]

On 9/30/26 7:02 PM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    47a144672573 Merge branch 'mptcp-misc-improvements-for-v7-4'
> git tree:       net-next
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=1342db25580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=88147dd7e4cb6a0f
> dashboard link: https://syzkaller.appspot.com/bug?extid=4cc63fcfb3845e149969
> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12c35e05580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11908035580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/974586b80355/disk-47a14467.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/05ff7f1eb7cf/vmlinux-47a14467.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/d63ff990e2c6/bzImage-47a14467.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+4cc63fcfb3845e149969@syzkaller.appspotmail.com
> 
> ovs-dp0: entered promiscuous mode
> ------------[ cut here ]------------
> !skb_transport_header_was_set(skb)
> WARNING: ./include/linux/skbuff.h:3075 at skb_transport_header include/linux/skbuff.h:3075 [inline], CPU#1: syz-executor463/5635
> WARNING: ./include/linux/skbuff.h:3075 at skb_transport_offset include/linux/skbuff.h:3250 [inline], CPU#1: syz-executor463/5635
> WARNING: ./include/linux/skbuff.h:3075 at update_ipv6_checksum net/openvswitch/actions.c:361 [inline], CPU#1: syz-executor463/5635
> WARNING: ./include/linux/skbuff.h:3075 at set_ipv6_addr+0x462/0x660 net/openvswitch/actions.c:399, CPU#1: syz-executor463/5635
> Modules linked in:
> CPU: 1 UID: 0 PID: 5635 Comm: syz-executor463 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> RIP: 0010:skb_transport_header include/linux/skbuff.h:3075 [inline]
> RIP: 0010:skb_transport_offset include/linux/skbuff.h:3250 [inline]
> RIP: 0010:update_ipv6_checksum net/openvswitch/actions.c:361 [inline]
> RIP: 0010:set_ipv6_addr+0x462/0x660 net/openvswitch/actions.c:399
> Code: f6 e8 12 d3 05 00 ba 10 00 00 00 4c 89 ef 48 89 ee 48 83 c4 20 5b 41 5c 41 5d 41 5e 41 5f 5d e9 14 89 7a f6 e8 ff 80 0a f6 90 <0f> 0b 90 48 8b 44 24 10 42 0f b6 04 30 84 c0 0f 85 c8 01 00 00 41
> RSP: 0018:ffffc90003b3ee68 EFLAGS: 00010293
> RAX: ffffffff8bbd5b01 RBX: 000000000000ffff RCX: ffff8880780bbe80
> RDX: 0000000000000000 RSI: 000000000000ffff RDI: 000000000000ffff
> RBP: 000000000000004c R08: 0000000000000001 R09: 0000000000000011
> R10: 000000000000001f R11: 0000000000000000 R12: ffff888071cdbd36
> R13: ffff88802d028396 R14: dffffc0000000000 R15: ffff888071cdbc80
> FS:  0000555579c36400(0000) GS:ffff888124dc5000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffd90696af0 CR3: 000000007065a000 CR4: 00000000003526f0
> Call Trace:
>   <TASK>
>   set_ipv6 net/openvswitch/actions.c:531 [inline]
>   execute_masked_set_action net/openvswitch/actions.c:1073 [inline]
>   do_execute_actions+0x557e/0x8600 net/openvswitch/actions.c:1366
>   ovs_execute_actions+0xde/0x520 net/openvswitch/actions.c:1592
>   ovs_packet_cmd_execute+0xb4f/0xf10 net/openvswitch/datapath.c:705
>   genl_family_rcv_msg_doit+0x233/0x340 net/netlink/genetlink.c:1114
>   genl_family_rcv_msg net/netlink/genetlink.c:1194 [inline]
>   genl_rcv_msg+0x614/0x7a0 net/netlink/genetlink.c:1209
>   netlink_rcv_skb+0x226/0x4a0 net/netlink/af_netlink.c:2572
>   genl_rcv+0x28/0x40 net/netlink/genetlink.c:1218
>   netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
>   netlink_unicast+0x7bd/0x940 net/netlink/af_netlink.c:1361
>   netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1916
>   sock_sendmsg_nosec+0x14e/0x190 net/socket.c:800
>   __sock_sendmsg net/socket.c:818 [inline]
>   ____sys_sendmsg+0x54e/0x850 net/socket.c:2715
>   ___sys_sendmsg+0x2a5/0x360 net/socket.c:2769
>   __sys_sendmsg net/socket.c:2801 [inline]
>   __do_sys_sendmsg net/socket.c:2806 [inline]
>   __se_sys_sendmsg net/socket.c:2804 [inline]
>   __x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2804
>   do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
>   do_syscall_64+0x166/0x520 arch/x86/entry/syscall_64.c:84
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fe073a4fd39
> Code: c0 79 93 eb d5 48 8d 7c 1d 00 eb 99 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 d8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffece969af8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
> RAX: ffffffffffffffda RBX: 00007ffece969c28 RCX: 00007fe073a4fd39
> RDX: 0000000000000000 RSI: 0000200000000040 RDI: 0000000000000003
> RBP: 0000000000000001 R08: 00007ffece969c28 R09: 00007ffece969c28
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffece969c18
> R13: 0000000000000002 R14: 00007fe073ae0ca0 R15: 0000000000000002
>   </TASK>
> 
> 

Hi,

I have a fix for this trace. I have verified it with the reproducer but 
I want to do a more testing before sending it, so will send it tomorrow.

In addition, I am not so sure about how likely can this be hit in a 
production environment.. as fragmentation isn't common and conntrack 
auto-reassembly should kick in.. pretty edge-case in my opinion.

Thanks,
Fernando.