[moderation] [bpf?] [net?] KCSAN: data-race in bq_flush_to_queue / cpu_map_kthread_run (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9b0436804460 Merge tag 'drm-fixes-2026-01-02' of https://g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=107c6a9a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=b319ff1b6a2797ca
dashboard link: https://syzkaller.appspot.com/bug?extid=ff4986f7a6345433b85a
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
CC: [and...@kernel.org a...@kernel.org b...@vger.kernel.org dan...@iogearbox.net da...@davemloft.net edd...@gmail.com hao...@google.com ha...@kernel.org john.fa...@gmail.com jo...@kernel.org kps...@kernel.org ku...@kernel.org linux-...@vger.kernel.org marti...@linux.dev net...@vger.kernel.org s...@fomichev.me so...@kernel.org yongho...@linux.dev]

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ffb87c3b9680/disk-9b043680.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/df2ae7ed26cc/vmlinux-9b043680.xz
kernel image: https://storage.googleapis.com/syzbot-assets/8da907e4df18/bzImage-9b043680.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ff4986...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in bq_flush_to_queue / cpu_map_kthread_run

write to 0xffff888140e82458 of 8 bytes by task 12108 on cpu 0:
__ptr_ring_zero_tail include/linux/ptr_ring.h:259 [inline]
__ptr_ring_discard_one include/linux/ptr_ring.h:291 [inline]
__ptr_ring_consume include/linux/ptr_ring.h:311 [inline]
__ptr_ring_consume_batched include/linux/ptr_ring.h:323 [inline]
cpu_map_kthread_run+0x529/0x1560 kernel/bpf/cpumap.c:332
kthread+0x489/0x510 kernel/kthread.c:463
ret_from_fork+0x149/0x290 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246

read to 0xffff888140e82458 of 8 bytes by task 12107 on cpu 1:
__ptr_ring_produce include/linux/ptr_ring.h:106 [inline]
bq_flush_to_queue+0x124/0x350 kernel/bpf/cpumap.c:730
bq_enqueue kernel/bpf/cpumap.c:754 [inline]
cpu_map_enqueue+0x1a6/0x1c0 kernel/bpf/cpumap.c:780
__xdp_do_redirect_frame net/core/filter.c:4472 [inline]
xdp_do_redirect_frame+0x27c/0x560 net/core/filter.c:4523
xdp_test_run_batch net/bpf/test_run.c:330 [inline]
bpf_test_run_xdp_live+0x98c/0x11d0 net/bpf/test_run.c:379
bpf_prog_test_run_xdp+0x525/0x970 net/bpf/test_run.c:1388
bpf_prog_test_run+0x204/0x340 kernel/bpf/syscall.c:4703
__sys_bpf+0x4c0/0x7c0 kernel/bpf/syscall.c:6182
__do_sys_bpf kernel/bpf/syscall.c:6274 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6272 [inline]
__x64_sys_bpf+0x41/0x50 kernel/bpf/syscall.c:6272
x64_sys_call+0x28e1/0x3000 arch/x86/include/generated/asm/syscalls_64.h:322
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xca/0x2b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffff888154924070 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 12107 Comm: syz.1.1113 Not tainted syzkaller #0 PREEMPT(voluntary)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup