[syzbot] [net?] KCSAN: data-race in __udp4_lib_mcast_deliver / udp_lib_unhash (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c23719abc330 Merge tag 'x86-urgent-2026-03-08' of git://gi..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=141b0806580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6b32b22d8934c136
dashboard link: https://syzkaller.appspot.com/bug?extid=04905b8b3523856476c0
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/c0ec3784100d/disk-c23719ab.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ff5ee751c9be/vmlinux-c23719ab.xz
kernel image: https://storage.googleapis.com/syzbot-assets/1dda25fb6996/bzImage-c23719ab.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+04905b...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in __udp4_lib_mcast_deliver / udp_lib_unhash

read-write to 0xffff88810194a9c8 of 4 bytes by task 15107 on cpu 0:
udp_lib_unhash+0x1f4/0x3e0 net/ipv4/udp.c:2261
sk_common_release+0x7a/0x230 net/core/sock.c:3990
udp_lib_close+0x15/0x20 include/net/udp.h:324
inet_release+0xcd/0xf0 net/ipv4/af_inet.c:437
__sock_release net/socket.c:662 [inline]
sock_close+0x6b/0x150 net/socket.c:1455
__fput+0x29b/0x650 fs/file_table.c:469
____fput+0x1c/0x30 fs/file_table.c:497
task_work_run+0x130/0x1a0 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
exit_to_user_mode_loop+0x1f4/0x6f0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:226 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:256 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:325 [inline]
do_syscall_64+0x249/0x370 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88810194a9c8 of 4 bytes by task 15111 on cpu 1:
__udp4_lib_mcast_deliver+0x9a/0x790 net/ipv4/udp.c:2564
__udp4_lib_rcv+0x972/0x1250 net/ipv4/udp.c:2749
udp_rcv+0x4f/0x60 net/ipv4/udp.c:2936
ip_protocol_deliver_rcu+0x3f7/0x790 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_sublist_rcv_finish net/ipv4/ip_input.c:584 [inline]
ip_list_rcv_finish net/ipv4/ip_input.c:628 [inline]
ip_sublist_rcv+0x415/0x6a0 net/ipv4/ip_input.c:644
ip_list_rcv+0x261/0x290 net/ipv4/ip_input.c:678
__netif_receive_skb_list_ptype net/core/dev.c:6207 [inline]
__netif_receive_skb_list_core+0x4dc/0x500 net/core/dev.c:6254
__netif_receive_skb_list net/core/dev.c:6306 [inline]
netif_receive_skb_list_internal+0x47d/0x5f0 net/core/dev.c:6397
netif_receive_skb_list+0x35/0x220 net/core/dev.c:6449
xdp_recv_frames net/bpf/test_run.c:269 [inline]
xdp_test_run_batch net/bpf/test_run.c:350 [inline]
bpf_test_run_xdp_live+0x104c/0x1360 net/bpf/test_run.c:379
bpf_prog_test_run_xdp+0x57b/0xa10 net/bpf/test_run.c:1397
bpf_prog_test_run+0x204/0x340 kernel/bpf/syscall.c:4722
__sys_bpf+0x52e/0x7e0 kernel/bpf/syscall.c:6246
__do_sys_bpf kernel/bpf/syscall.c:6341 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6339 [inline]
__x64_sys_bpf+0x41/0x50 kernel/bpf/syscall.c:6339
x64_sys_call+0x10cb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:322
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: 0x00000001 -> 0x00000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 15111 Comm: syz.6.4060 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================


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