Re: [syzbot] [io-uring] KCSAN: data-race in copy_mm / percpu_counter_destroy_many

[Jens Axboe <axboe@kernel.dk>]

On 5/12/25 12:36 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    3ce9925823c7 Merge tag 'mm-hotfixes-stable-2025-05-10-14-2..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=14ff74d4580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6154604431d9aaf9
> dashboard link: https://syzkaller.appspot.com/bug?extid=8be9bf36c3cf574426c8
> compiler:       Debian clang version 20.1.2 (++20250402124445+58df0ef89dd6-1~exp1~20250402004600.97), Debian LLD 20.1.2
> 
> Unfortunately, I don't have any reproducer for this issue yet.
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/afdc6302fc05/disk-3ce99258.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/fc7f98d3c420/vmlinux-3ce99258.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/ea7ca2da2258/bzImage-3ce99258.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+8be9bf36c3cf574426c8@syzkaller.appspotmail.com
> 
> ==================================================================
> BUG: KCSAN: data-race in copy_mm / percpu_counter_destroy_many
> 
> write to 0xffff8881045e19d8 of 8 bytes by task 2123 on cpu 0:
>  __list_del include/linux/list.h:195 [inline]
>  __list_del_entry include/linux/list.h:218 [inline]
>  list_del include/linux/list.h:229 [inline]
>  percpu_counter_destroy_many+0xc7/0x2b0 lib/percpu_counter.c:244
>  __mmdrop+0x22e/0x350 kernel/fork.c:947
>  mmdrop include/linux/sched/mm.h:55 [inline]
>  io_ring_ctx_free+0x31e/0x360 io_uring/io_uring.c:2740
>  io_ring_exit_work+0x529/0x560 io_uring/io_uring.c:2962
>  process_one_work kernel/workqueue.c:3238 [inline]
>  process_scheduled_works+0x4cb/0x9d0 kernel/workqueue.c:3319
>  worker_thread+0x582/0x770 kernel/workqueue.c:3400
>  kthread+0x486/0x510 kernel/kthread.c:464
>  ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:153
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> 
> read to 0xffff8881045e1600 of 1344 bytes by task 5051 on cpu 1:
>  dup_mm kernel/fork.c:1728 [inline]
>  copy_mm+0xfb/0x1310 kernel/fork.c:1786
>  copy_process+0xcf1/0x1f90 kernel/fork.c:2429
>  kernel_clone+0x16c/0x5b0 kernel/fork.c:2844
>  __do_sys_clone kernel/fork.c:2987 [inline]
>  __se_sys_clone kernel/fork.c:2971 [inline]
>  __x64_sys_clone+0xe6/0x120 kernel/fork.c:2971
>  x64_sys_call+0x2c59/0x2fb0 arch/x86/include/generated/asm/syscalls_64.h:57
>  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>  do_syscall_64+0xd0/0x1a0 arch/x86/entry/syscall_64.c:94
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 UID: 0 PID: 5051 Comm: syz.1.494 Not tainted 6.15.0-rc5-syzkaller-00300-g3ce9925823c7 #0 PREEMPT(voluntary) 

This doesn't look like an io_uring issue, it's successive setup and teardown
of a percpu counter. Adding some relevant folks.

#syz set subsystems: kernel

-- 
Jens Axboe