Re: [syzbot] [can?] KCSAN: data-race in can_send / can_send (5)

[Oliver Hartkopp]

Hello Marc,

On 09.03.25 11:46, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    0f52fd4f67c6 Merge tag 'bcachefs-2025-03-06' of git://evil..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=12d12a54580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=523b0e2f15224775
> dashboard link: https://syzkaller.appspot.com/bug?extid=78ce4489b812515d5e4d
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/eb0d7b540c67/disk-0f52fd4f.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/51c261332ad9/vmlinux-0f52fd4f.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/38914a4790c8/bzImage-0f52fd4f.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+78ce4489b812515d5e4d@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> ==================================================================
> BUG: KCSAN: data-race in can_send / can_send
>
> read-write to 0xffff888117566290 of 8 bytes by interrupt on cpu 0:
>   can_send+0x5a2/0x6d0 net/can/af_can.c:290
>   bcm_can_tx+0x314/0x420 net/can/bcm.c:314
>   bcm_tx_timeout_handler+0xea/0x280
>   __run_hrtimer kernel/time/hrtimer.c:1801 [inline]
>   __hrtimer_run_queues+0x20d/0x5e0 kernel/time/hrtimer.c:1865
>   hrtimer_run_softirq+0xe4/0x2c0 kernel/time/hrtimer.c:1882
>   handle_softirqs+0xbf/0x280 kernel/softirq.c:561
>   run_ksoftirqd+0x1c/0x30 kernel/softirq.c:950
>   smpboot_thread_fn+0x31c/0x4c0 kernel/smpboot.c:164
>   kthread+0x4ae/0x520 kernel/kthread.c:464
>   ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:148
>   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>
> read-write to 0xffff888117566290 of 8 bytes by interrupt on cpu 1:
>   can_send+0x5a2/0x6d0 net/can/af_can.c:290
>   bcm_can_tx+0x314/0x420 net/can/bcm.c:314
>   bcm_tx_timeout_handler+0xea/0x280
>   __run_hrtimer kernel/time/hrtimer.c:1801 [inline]
>   __hrtimer_run_queues+0x20d/0x5e0 kernel/time/hrtimer.c:1865
>   hrtimer_run_softirq+0xe4/0x2c0 kernel/time/hrtimer.c:1882
>   handle_softirqs+0xbf/0x280 kernel/softirq.c:561
>   run_ksoftirqd+0x1c/0x30 kernel/softirq.c:950
>   smpboot_thread_fn+0x31c/0x4c0 kernel/smpboot.c:164
>   kthread+0x4ae/0x520 kernel/kthread.c:464
>   ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:148
>   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
>
> value changed: 0x0000000000002b9d -> 0x0000000000002b9e

Increased by '1' ...

I assume this problem is caused by increasing the per-netdevice statistic in

https://elixir.bootlin.com/linux/v6.13.6/source/net/can/af_can.c#L289

pkg_stats->tx_frames++;
pkg_stats->tx_frames_delta++;

We update the statistics for the device and in this specific case the 
hrtimer fired on two CPUs resulting in a can_send() to the same netdevice.

Do you agree with this quick analysis?

Isn't there some lock-less per-cpu safe statistic handling within netdev 
we might pick for our use-case?

Best regards,
Oliver

> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 UID: 0 PID: 23 Comm: ksoftirqd/1 Tainted: G        W          6.14.0-rc5-syzkaller-00109-g0f52fd4f67c6 #0
> Tainted: [W]=WARN
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
> ==================================================================
>
>
> ---
> 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 syzkaller@xxxxxxxxxxxxxxxx.
>
> 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