Re: [PATCH v2 1/2] xen/events: access last_priority and last_vcpu_id together

[Julien Grall <julien@xen.org>]

Hi Jan,

On 13/10/2020 15:26, Jan Beulich wrote:
> On 13.10.2020 16:20, Jürgen Groß wrote:
>> On 13.10.20 15:58, Jan Beulich wrote:
>>> On 12.10.2020 11:27, Juergen Gross wrote:
>>>> The queue for a fifo event is depending on the vcpu_id and the
>>>> priority of the event. When sending an event it might happen the
>>>> event needs to change queues and the old queue needs to be kept for
>>>> keeping the links between queue elements intact. For this purpose
>>>> the event channel contains last_priority and last_vcpu_id values
>>>> elements for being able to identify the old queue.
>>>>
>>>> In order to avoid races always access last_priority and last_vcpu_id
>>>> with a single atomic operation avoiding any inconsistencies.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>
>>> I seem to vaguely recall that at the time this seemingly racy
>>> access was done on purpose by David. Did you go look at the
>>> old commits to understand whether there really is a race which
>>> can't be tolerated within the spec?
>>
>> At least the comments in the code tell us that the race regarding
>> the writing of priority (not last_priority) is acceptable.
> 
> Ah, then it was comments. I knew I read something to this effect
> somewhere, recently.
> 
>> Especially Julien was rather worried by the current situation. In
>> case you can convince him the current handling is fine, we can
>> easily drop this patch.
> 
> Julien, in the light of the above - can you clarify the specific
> concerns you (still) have?

Let me start with that the assumption if evtchn->lock is not held when 
evtchn_fifo_set_pending() is called. If it is held, then my comment is moot.

 From my understanding, the goal of lock_old_queue() is to return the 
old queue used.

last_priority and last_vcpu_id may be updated separately and I could not 
convince myself that it would not be possible to return a queue that is 
neither the current one nor the old one.

The following could happen if evtchn->priority and 
evtchn->notify_vcpu_id keeps changing between calls.

pCPU0				| pCPU1
				|
evtchn_fifo_set_pending(v0,...)	|
				| evtchn_fifo_set_pending(v1, ...)
  [...]				|
  /* Queue has changed */	|
  evtchn->last_vcpu_id = v0 	|
				| -> evtchn_old_queue()
				| v = d->vcpu[evtchn->last_vcpu_id];
   				| old_q = ...
				| spin_lock(old_q->...)
				| v = ...
				| q = ...
				| /* q and old_q would be the same */
				|
  evtchn->las_priority = priority|

If my diagram is correct, then pCPU1 would return a queue that is 
neither the current nor old one.

In which case, I think it would at least be possible to corrupt the 
queue. From evtchn_fifo_set_pending():

         /*
          * If this event was a tail, the old queue is now empty and
          * its tail must be invalidated to prevent adding an event to
          * the old queue from corrupting the new queue.
          */
         if ( old_q->tail == port )
             old_q->tail = 0;

Did I miss anything?

Cheers,

-- 
Julien Grall