Re: [Xen-devel] [PATCH] evtchn: clean last_vcpu_id on EVTCHNOP_reset to avoid crash

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

David Vrabel <david.vrabel@xxxxxxxxxx> writes:

> On 08/08/14 15:22, Vitaly Kuznetsov wrote:
>> When EVTCHNOP_reset is being performed last_vcpu_id attribute is not being
>> cleaned by __evtchn_close(). In case last_vcpu_id != 0 for a particular
>> event channel and this event channel is going to be used for event delivery
>> (for another vcpu) before EVTCHNOP_init_control for vcpu == last_vcpu_id
>> was done the following crash is observed:
>> 
>>  ...
>>  (XEN) Xen call trace:
>>  (XEN)    [<ffff82d080127785>] _spin_lock_irqsave+0x5/0x70
>>  (XEN)    [<ffff82d0801097db>] evtchn_fifo_set_pending+0xdb/0x370
>>  (XEN)    [<ffff82d080107146>] evtchn_send+0xd6/0x160
>>  (XEN)    [<ffff82d080107df9>] do_event_channel_op+0x6a9/0x16c0
>>  (XEN)    [<ffff82d0801ce800>] vmx_intr_assist+0x30/0x480
>>  (XEN)    [<ffff82d080219e99>] syscall_enter+0xa9/0xae
>> 
>> This happens because lock_old_queue() does not check VCPU's control
>> block existence and after EVTCHNOP_reset they are all cleaned.
>> 
>> I suggest we fix the issue twice: reset last_vcpu_id to 0 in __evtchn_close()
>> and add appropriate check to lock_old_queue() as lost event is much better
>> than hypervisor crash.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>>  xen/common/event_channel.c | 3 +++
>>  xen/common/event_fifo.c    | 9 +++++++++
>>  2 files changed, 12 insertions(+)
>> 
>> diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
>> index a7becae..67b9d53 100644
>> --- a/xen/common/event_channel.c
>> +++ b/xen/common/event_channel.c
>> @@ -578,6 +578,9 @@ static long __evtchn_close(struct domain *d1, int port1)
>>      chn1->state          = ECS_FREE;
>>      chn1->notify_vcpu_id = 0;
>>  
>> +    /* Reset last_vcpu_id to vcpu0 as control block can be freed */
>> +    chn1->last_vcpu_id = 0;
>
> This is broken if the event channel is closed and rebound while the
> event is linked.
>
> You can only safely clear chn->last_vcpu_id during evtchn_fifo_destroy().
>
> You also need to clear last_priority.
>

Thanks, alternatively I can do that in evtchn_reset() after
evtchn_fifo_destroy() as it is the only path leading to the issue. I
wanted to avoid that to exclude additional loop for all event channels.

>> +
>>      xsm_evtchn_close_post(chn1);
>>  
>>   out:
>> diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
>> index 51b4ff6..e4bef80 100644
>> --- a/xen/common/event_fifo.c
>> +++ b/xen/common/event_fifo.c
>> @@ -61,6 +61,15 @@ static struct evtchn_fifo_queue *lock_old_queue(const 
>> struct domain *d,
>>      for ( try = 0; try < 3; try++ )
>>      {
>>          v = d->vcpu[evtchn->last_vcpu_id];
>> +
>> +        if ( !v->evtchn_fifo )
>> +        {
>> +            gdprintk(XENLOG_ERR,
>> +                     "domain %d vcpu %d has no control block!\n",
>> +                     d->domain_id, v->vcpu_id);
>> +            return NULL;
>> +        }
>
> I think this check needs to be in evtchn_fifo_init() to prevent the
> event from being bound to VCPU that does not have a control block.
>

I *think* it is not the issue here - the event is being bound to VCPU
with this block initialized. But last_vcpu_id for this particular event
channel points to some other VCPU which has not initialized its control
block yet (so d->vcpu[evtchn->last_vcpu_id]->evtchn_fifo is NULL). There
is no path to get in such situation (after we clear last_vcpu_id), I
just wanted to put reasonable message here in case something will change
in future.

>> +
>>          old_q = &v->evtchn_fifo->queue[evtchn->last_priority];
>>  
>>          spin_lock_irqsave(&old_q->lock, *flags);
>
> David

-- 
  Vitaly

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