[syzbot] [net?] KCSAN: data-race in igmp_rcv / igmp_rcv (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 2d1373e4246d Merge tag 'for-7.0-rc4-tag' of git://git.kern..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1184a8da580000
kernel config: https://syzkaller.appspot.com/x/.config?x=3a78dd265deac3a9
dashboard link: https://syzkaller.appspot.com/bug?extid=ae9a171f239b14485310
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/b2aee7aa6da1/disk-2d1373e4.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/6cbfa1602880/vmlinux-2d1373e4.xz
kernel image: https://storage.googleapis.com/syzbot-assets/52fe8db03e03/bzImage-2d1373e4.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ae9a17...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in igmp_rcv / igmp_rcv

read to 0xffff88810a66f058 of 8 bytes by task 5213 on cpu 1:
igmp_heard_query net/ipv4/igmp.c:1025 [inline]
igmp_rcv+0x10af/0x1200 net/ipv4/igmp.c:1103
ip_protocol_deliver_rcu+0x421/0x790 net/ipv4/ip_input.c:207
ip_local_deliver_finish+0x1fc/0x2f0 net/ipv4/ip_input.c:241
NF_HOOK include/linux/netfilter.h:318 [inline]
ip_local_deliver+0xe8/0x1e0 net/ipv4/ip_input.c:262
dst_input include/net/dst.h:480 [inline]
ip_rcv_finish+0x188/0x1a0 net/ipv4/ip_input.c:453
NF_HOOK include/linux/netfilter.h:318 [inline]
ip_rcv+0x62/0x160 net/ipv4/ip_input.c:573
__netif_receive_skb_one_core net/core/dev.c:6164 [inline]
__netif_receive_skb net/core/dev.c:6277 [inline]
netif_receive_skb_internal net/core/dev.c:6363 [inline]
netif_receive_skb+0x137/0x530 net/core/dev.c:6422
tun_rx_batched+0x106/0x440 drivers/net/tun.c:1485
tun_get_user+0x2011/0x27c0 drivers/net/tun.c:1953
tun_chr_write_iter+0x15e/0x210 drivers/net/tun.c:1999
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x5a6/0x9f0 fs/read_write.c:688
ksys_write+0xdc/0x1a0 fs/read_write.c:740
__do_sys_write fs/read_write.c:751 [inline]
__se_sys_write fs/read_write.c:748 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:748
x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

write to 0xffff88810a66f058 of 8 bytes by task 5212 on cpu 0:
igmp_heard_query net/ipv4/igmp.c:1026 [inline]
igmp_rcv+0x10eb/0x1200 net/ipv4/igmp.c:1103
ip_protocol_deliver_rcu+0x421/0x790 net/ipv4/ip_input.c:207
ip_local_deliver_finish+0x1fc/0x2f0 net/ipv4/ip_input.c:241
NF_HOOK include/linux/netfilter.h:318 [inline]
ip_local_deliver+0xe8/0x1e0 net/ipv4/ip_input.c:262
dst_input include/net/dst.h:480 [inline]
ip_rcv_finish+0x188/0x1a0 net/ipv4/ip_input.c:453
NF_HOOK include/linux/netfilter.h:318 [inline]
ip_rcv+0x62/0x160 net/ipv4/ip_input.c:573
__netif_receive_skb_one_core net/core/dev.c:6164 [inline]
__netif_receive_skb net/core/dev.c:6277 [inline]
netif_receive_skb_internal net/core/dev.c:6363 [inline]
netif_receive_skb+0x137/0x530 net/core/dev.c:6422
tun_rx_batched+0x106/0x440 drivers/net/tun.c:1485
tun_get_user+0x2011/0x27c0 drivers/net/tun.c:1953
tun_chr_write_iter+0x15e/0x210 drivers/net/tun.c:1999
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x5a6/0x9f0 fs/read_write.c:688
ksys_write+0xdc/0x1a0 fs/read_write.c:740
__do_sys_write fs/read_write.c:751 [inline]
__se_sys_write fs/read_write.c:748 [inline]
__x64_sys_write+0x40/0x50 fs/read_write.c:748
x64_sys_call+0x27e1/0x3020 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000000000003e8 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 5212 Comm: syz.1.463 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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