WARNING in sk_mc_loop

Status: upstream: reported on 2026/08/04 15:16
Subsystems: net
[Documentation on labels]
Reported-by: syzbot+22c3218a6fa219e47321@syzkaller.appspotmail.com
Fix commit: b8a39a09ae4e net: remove WARN_ON_ONCE() from sk_mc_loop()
Patched on: [ci-qemu-gce-upstream-auto ci-qemu-upstream ci-qemu-upstream-386 ci-qemu2-arm32 ci-qemu2-arm64 ci-qemu2-arm64-compat ci-qemu2-arm64-mte ci-snapshot-upstream-root ci-upstream-bpf-kasan-gce ci-upstream-bpf-next-kasan-gce ci-upstream-gce-leak ci-upstream-kasan-badwrites-root ci-upstream-kasan-gce ci-upstream-kasan-gce-386 ci-upstream-kasan-gce-root ci-upstream-kasan-gce-selinux-root ci-upstream-kasan-gce-smack-root ci-upstream-linux-next-kasan-gce-root ci-upstream-net-kasan-gce ci-upstream-net-this-kasan-gce ci2-upstream-fs ci2-upstream-kcsan-gce], missing on: [ci-qemu-native-arm64-kvm ci-qemu2-riscv64 ci-upstream-gce-arm64 ci-upstream-kmsan-gce-386-root ci-upstream-kmsan-gce-root ci-upstream-rust-kasan-gce ci2-upstream-usb]
First crash: 4d21h, last: 3d08h
✨ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision
8ffc4b18 assessment-security 💥 WARNING in sk_mc_loop 2026/08/05 00:08 2026/08/05 00:08 2026/08/05 00:08 d4abbeac
Discussions (2)
Title Replies (including bot) Last reply
[PATCH net] net: remove WARN_ON_ONCE() from sk_mc_loop() 2 (2) 2026/08/06 11:20
[syzbot] [net?] WARNING in sk_mc_loop 0 (1) 2026/08/04 15:16

Sample crash report:
------------[ cut here ]------------
1
WARNING: net/core/sock.c:782 at sk_mc_loop+0x126/0x160 net/core/sock.c:782, CPU#1: syz.7.4910/24202
Modules linked in:
CPU: 1 UID: 0 PID: 24202 Comm: syz.7.4910 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:sk_mc_loop+0x126/0x160 net/core/sock.c:782
Code: 08 48 89 df e8 db 03 9c f8 48 8b 1b 81 e3 00 00 10 00 31 ff 48 89 de e8 48 71 2e f8 48 89 d8 c1 e8 14 eb 0b e8 5b 6c 2e f8 90 <0f> 0b 90 b0 01 5b 41 5e 41 5f 5d c3 cc cc cc cc cc 44 89 f1 80 e1
RSP: 0018:ffffc9000628ebc8 EFLAGS: 00010287
RAX: ffffffff8998a7a5 RBX: ffff8880519be000 RCX: 0000000000080000
RDX: ffffc9001bbf9000 RSI: 0000000000039aab RDI: 0000000000039aac
RBP: 0000000000000000 R08: ffff888043ae0000 R09: 0000000000000002
R10: 000000000000000a R11: 0000000000000002 R12: ffff888056d05280
R13: 0000000000000010 R14: 0000000000000011 R15: dffffc0000000000
FS:  00007fcdd9ca86c0(0000) GS:ffff88812504d000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00005555786afa38 CR3: 0000000078294000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 ip6_finish_output2+0x62b/0x13c0 net/ipv6/ip6_output.c:84
 NF_HOOK_COND include/linux/netfilter.h:307 [inline]
 ip6_output+0x337/0x540 net/ipv6/ip6_output.c:246
 ip6_mr_output+0x53e/0x1170 net/ipv6/ip6mr.c:2458
 dst_output include/net/dst.h:470 [inline]
 vrf_ip6_local_out drivers/net/vrf.c:406 [inline]
 vrf_process_v6_outbound drivers/net/vrf.c:454 [inline]
 is_ip_tx_frame drivers/net/vrf.c:556 [inline]
 vrf_xmit+0x1a60/0x1b40 drivers/net/vrf.c:568
 __netdev_start_xmit include/linux/netdevice.h:5400 [inline]
 netdev_start_xmit include/linux/netdevice.h:5409 [inline]
 xmit_one net/core/dev.c:3889 [inline]
 dev_hard_start_xmit+0x2cd/0x830 net/core/dev.c:3905
 sch_direct_xmit+0x257/0x4c0 net/sched/sch_generic.c:372
 qdisc_restart net/sched/sch_generic.c:437 [inline]
 __qdisc_run+0xb11/0x1620 net/sched/sch_generic.c:445
 qdisc_run include/net/pkt_sched.h:120 [inline]
 __dev_xmit_skb net/core/dev.c:4297 [inline]
 __dev_queue_xmit+0x1c05/0x3820 net/core/dev.c:4836
 packet_snd net/packet/af_packet.c:3095 [inline]
 packet_sendmsg+0x3d95/0x5040 net/packet/af_packet.c:3127
 sock_sendmsg_nosec+0x13a/0x180 net/socket.c:775
 __sock_sendmsg net/socket.c:790 [inline]
 ____sys_sendmsg+0x54e/0x850 net/socket.c:2684
 ___sys_sendmsg+0x2a5/0x360 net/socket.c:2738
 __sys_sendmsg net/socket.c:2770 [inline]
 __do_sys_sendmsg net/socket.c:2775 [inline]
 __se_sys_sendmsg net/socket.c:2773 [inline]
 __x64_sys_sendmsg+0x1b1/0x290 net/socket.c:2773
 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
RIP: 0033:0x7fcdd8d9e019
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fcdd9ca8028 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fcdd9026270 RCX: 00007fcdd8d9e019
RDX: 0000000000000000 RSI: 00002000000000c0 RDI: 0000000000000008
RBP: 00007fcdd8e3500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fcdd9026308 R14: 00007fcdd9026270 R15: 00007ffcce71c548
 </TASK>

Crashes (3):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/08/05 02:53 net cf6f8b29befb d4abbeac .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-net-this-kasan-gce WARNING in sk_mc_loop
2026/08/04 04:47 net d1ad8fb2ac6a a68cb31b .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-net-this-kasan-gce WARNING in sk_mc_loop
2026/08/03 14:41 net-next 69963a0678a3 e611ffe1 .config console log report info [disk image] [vmlinux] [kernel image] ci-upstream-net-kasan-gce WARNING in sk_mc_loop
* Struck through repros no longer work on HEAD.