Re: [PATCH v4] xen: avoid crash in disable_hotplug_cpu

[Boris Ostrovsky]

On 09/07/2018 10:31 AM, Olaf Hering wrote:
> The command 'xl vcpu-set 0 0', issued in dom0, will crash dom0:
>
> BUG: unable to handle kernel NULL pointer dereference at 00000000000002d8
> PGD 0 P4D 0
> Oops: 0000 [#1] PREEMPT SMP NOPTI
> CPU: 7 PID: 65 Comm: xenwatch Not tainted 4.19.0-rc2-1.ga9462db-default #1 openSUSE Tumbleweed (unreleased)
> Hardware name: Intel Corporation S5520UR/S5520UR, BIOS S5500.86B.01.00.0050.050620101605 05/06/2010
> RIP: e030:device_offline+0x9/0xb0
> Code: 77 24 00 e9 ce fe ff ff 48 8b 13 e9 68 ff ff ff 48 8b 13 e9 29 ff ff ff 48 8b 13 e9 ea fe ff ff 90 66 66 66 66 90 41 54 55 53 <f6> 87 d8 02 00 00 01 0f 85 88 00 00 00 48 c7 c2 20 09 60 81 31 f6
> RSP: e02b:ffffc90040f27e80 EFLAGS: 00010203
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff8801f3800000 RSI: ffffc90040f27e70 RDI: 0000000000000000
> RBP: 0000000000000000 R08: ffffffff820e47b3 R09: 0000000000000000
> R10: 0000000000007ff0 R11: 0000000000000000 R12: ffffffff822e6d30
> R13: dead000000000200 R14: dead000000000100 R15: ffffffff8158b4e0
> FS:  00007ffa595158c0(0000) GS:ffff8801f39c0000(0000) knlGS:0000000000000000
> CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000000002d8 CR3: 00000001d9602000 CR4: 0000000000002660
> Call Trace:
>  handle_vcpu_hotplug_event+0xb5/0xc0
>  xenwatch_thread+0x80/0x140
>  ? wait_woken+0x80/0x80
>  kthread+0x112/0x130
>  ? kthread_create_worker_on_cpu+0x40/0x40
>  ret_from_fork+0x3a/0x50
>
> This happens because handle_vcpu_hotplug_event is called twice. In the
> first iteration cpu_present is still true, in the second iteration
> cpu_present is false which causes get_cpu_device to return NULL.
> In case of cpu#0, cpu_online is apparently always true.
>
> Fix this crash by checking if the cpu can be hotplugged, which is false
> for a cpu that was just removed.
>
> Also check if the cpu was actually offlined by device_remove, otherwise
> leave the cpu_present state as it is.
>
> Rearrange to code to do all work with device_hotplug_lock held.
>
> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> ---
>  drivers/xen/cpu_hotplug.c | 15 ++++++++-------
>  1 file changed, 8 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/xen/cpu_hotplug.c b/drivers/xen/cpu_hotplug.c
> index d4265c8ebb22..b1357aa4bc55 100644
> --- a/drivers/xen/cpu_hotplug.c
> +++ b/drivers/xen/cpu_hotplug.c
> @@ -19,15 +19,16 @@ static void enable_hotplug_cpu(int cpu)
>  
>  static void disable_hotplug_cpu(int cpu)
>  {
> -	if (cpu_online(cpu)) {
> -		lock_device_hotplug();
> +	if (!cpu_is_hotpluggable(cpu))
> +		return;
> +	lock_device_hotplug();
> +	if (cpu_online(cpu))
>  		device_offline(get_cpu_device(cpu));
> -		unlock_device_hotplug();
> -	}
> -	if (cpu_present(cpu))
> +	if (!cpu_online(cpu) && cpu_present(cpu)) {
>  		xen_arch_unregister_cpu(cpu);
> -
> -	set_cpu_present(cpu, false);
> +		set_cpu_present(cpu, false);
> +	}
> +	unlock_device_hotplug();
>  }
>  
>  static int vcpu_online(unsigned int cpu)


I was hoping you'd respond to my question about warning.

root@haswell> xl vcpu-set 3 0


and in the guest

[root@vm-0238 ~]# [ÂÂ 32.866955] ------------[ cut here ]------------
[ÂÂ 32.866963] spinlock on CPU0 exists on IRQ1!
[ÂÂ 32.866984] WARNING: CPU: 0 PID: 14 at arch/x86/xen/spinlock.c:90
xen_init_lock_cpu+0xbf/0xd0
[ÂÂ 32.866990] Modules linked in:
[ÂÂ 32.866995] CPU: 0 PID: 14 Comm: cpuhp/0 Not tainted 4.19.0-rc2 #31
[ÂÂ 32.867001] RIP: e030:xen_init_lock_cpu+0xbf/0xd0
[ÂÂ 32.867005] Code: 4a 8b 0c e5 00 c7 14 82 48 c7 c2 90 4f 01 00 4c 89
2c 11 e9 85 00 00 00 8b 14 02 44 89 e6 48 c7 c7 a0 0f 08 82 e8 ab e3 05
00 <0f> 0b e9 7a ff ff ff 66 2e 0f 1f 84 00 00 00 00 00 80 3d 59 02 20
[ÂÂ 32.867015] RSP: e02b:ffffc900401ffe40 EFLAGS: 00010286
[ÂÂ 32.867019] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
0000000000000006
[ÂÂ 32.867024] RDX: 0000000000000007 RSI: 0000000000000001 RDI:
ffff88003d8168b0
[ÂÂ 32.867039] RBP: 0000000000014f98 R08: ffffffff81eb04a0 R09:
0000000000007f9b
[ÂÂ 32.867045] R10: 0000000000000065 R11: ffffffff82a9b7cd R12:
0000000000000000
[ÂÂ 32.867050] R13: ffffffff8101a820 R14: ffff88003d401280 R15:
ffffffff810aec10
[ÂÂ 32.867061] FS:Â 0000000000000000(0000) GS:ffff88003d800000(0000)
knlGS:0000000000000000
[ÂÂ 32.867066] CS:Â e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[ÂÂ 32.867081] CR2: 00005569b64e72b8 CR3: 000000002e902000 CR4:
0000000000042660
[ÂÂ 32.867089] Call Trace:
[ÂÂ 32.867096]Â ? cstate_cleanup+0x47/0x47
[ÂÂ 32.867101]Â xen_cpu_up_online+0xa/0x10
[ÂÂ 32.867107]Â cpuhp_invoke_callback+0x8d/0x500
[ÂÂ 32.867113]Â ? sort_range+0x20/0x20
[ÂÂ 32.867117]Â cpuhp_thread_fun+0xb0/0x110
[ÂÂ 32.867121]Â smpboot_thread_fn+0xc5/0x160
[ÂÂ 32.867126]Â kthread+0x112/0x130
[ÂÂ 32.867131]Â ? kthread_bind+0x30/0x30
[ÂÂ 32.867136]Â ret_from_fork+0x35/0x40
[ÂÂ 32.867141] ---[ end trace 15d4d7112a1b1cea ]---
[ÂÂ 32.867148] genirq: Flags mismatch irq 1. 0002cc00 (spinlock0) vs.
0002cc00 (spinlock0)
[ÂÂ 32.867154] CPU: 0 PID: 14 Comm: cpuhp/0 Tainted: GÂÂÂÂÂÂÂ WÂÂÂÂÂÂÂÂ
4.19.0-rc2 #31
[ÂÂ 32.867160] Call Trace:
[ÂÂ 32.867165]Â dump_stack+0x5c/0x80
[ÂÂ 32.867171]Â __setup_irq.cold.51+0x4e/0x9e
[ÂÂ 32.867177]Â request_threaded_irq+0xf5/0x160
[ÂÂ 32.867182]Â ? xen_qlock_wait+0x40/0x40
[ÂÂ 32.867188]Â bind_ipi_to_irqhandler+0xae/0x1d0
[ÂÂ 32.867194]Â ? sort_range+0x20/0x20
[ÂÂ 32.867198]Â xen_init_lock_cpu+0x74/0xd0
[ÂÂ 32.867202]Â ? cstate_cleanup+0x47/0x47
[ÂÂ 32.867206]Â xen_cpu_up_online+0xa/0x10
[ÂÂ 32.867210]Â cpuhp_invoke_callback+0x8d/0x500
[ÂÂ 32.867215]Â ? sort_range+0x20/0x20
[ÂÂ 32.867219]Â cpuhp_thread_fun+0xb0/0x110
[ÂÂ 32.867223]Â smpboot_thread_fn+0xc5/0x160
[ÂÂ 32.867227]Â kthread+0x112/0x130
[ÂÂ 32.867231]Â ? kthread_bind+0x30/0x30
[ÂÂ 32.867235]Â ret_from_fork+0x35/0x40
[ÂÂ 32.867249] cpu 0 spinlock event irq -16
[ÂÂ 32.880877] IRQ 16: no longer affine to CPU1
[ÂÂ 32.880879] IRQ 17: no longer affine to CPU1
[ÂÂ 32.880881] IRQ 18: no longer affine to CPU1
[ÂÂ 32.880882] IRQ 19: no longer affine to CPU1
[ÂÂ 32.880884] IRQ 20: no longer affine to CPU1
[ÂÂ 32.880885] IRQ 21: no longer affine to CPU1
[ÂÂ 32.880886] IRQ 22: no longer affine to CPU1
[ÂÂ 32.880888] IRQ 23: no longer affine to CPU1
[ÂÂ 32.880889] IRQ 24: no longer affine to CPU1
[ÂÂ 32.882202] smpboot: CPU 1 is now offline