Re: [RFC v1 1/1] vfio-ccw: Don't call cp_free if we are processing a channel program

Farhan Ali
2019-06-21T14:17:20Z

On 06/20/2019 04:27 PM, Eric Farman wrote:
>
>
> On 6/20/19 3:40 PM, Farhan Ali wrote:
>> There is a small window where it's possible that an interrupt can
>> arrive and can call cp_free, while we are still processing a channel
>> program (i.e allocating memory, pinnging pages, translating
>
> s/pinnging/pinning/
>
>> addresses etc). This can lead to allocating and freeing at the same
>> time and can cause memory corruption.
>>
>> Let's not call cp_free if we are currently processing a channel program.
>
> The check around this cp_free() call is for a solicited interrupt, so
> it's presumably in response to a SSCH we issued. But if we're still
> processing a CP, then we hadn't issued the SSCH to the hardware yet. So
> what is this interrupt for? Do the contents of irb.cpa provide any
> clues, perhaps if it's in the current cp or for someone else?
>

I don't think the interrupt is in response to an ssch but rather due to
an csch/hsch.

>>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> ---
>>
>> I have been running my test overnight with this patch and I haven't
>> seen the stack traces that I mentioned about earlier. I would like
>> to get some reviews on this and also if this is the right thing to
>> do?
>>
>> Thanks
>> Farhan
>>
>> drivers/s390/cio/vfio_ccw_drv.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
>> index 66a66ac..61ece3f 100644
>> --- a/drivers/s390/cio/vfio_ccw_drv.c
>> +++ b/drivers/s390/cio/vfio_ccw_drv.c
>> @@ -88,7 +88,7 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
>> (SCSW_ACTL_DEVACT | SCSW_ACTL_SCHACT));
>> if (scsw_is_solicited(&irb->scsw)) {
>> cp_update_scsw(&private->cp, &irb->scsw);
>
> As I alluded earlier, do we know this irb is for this cp? If no, what
> does this function end up putting in the scsw?
>
>> - if (is_final)
>> + if (is_final && private->state != VFIO_CCW_STATE_CP_PROCESSING)
>
> In looking at how we set this state, and how we exit it, I see we do:
>
> if SSCH got CC0, CP_PROCESSING -> CP_PENDING
> if SSCH got !CC0, CP_PROCESSING -> IDLE
>
> While the first scenario happens immediately after the SSCH instruction,
> I guess it could be just tiny enough, like the io_trigger FSM patch I
> sent a few weeks ago.
>
> Meanwhile, the latter happens way after we return from the jump table.
> So that scenario leaves considerable time for such an interrupt to
> occur, though I don't understand why it would if we got a CC(1-3) on the
> SSCH.
>
> And anyway, the return from fsm_io_helper() in that case will also call
> cp_free(). So why does the cp->initialized check provide protection
> from a double-free in that direction, but not here? I'm confused.

I have a theory where I think it's possible to have 2 different threads
executing cp_free

If we start with private->state == IDLE and the guest issues a
clear/halt and then an ssch

- clear/halt will be issued to hardware, and if succeeds we will return
cc=0 to guest

- the guest can then issue ssch

- we get an interrupt for csch/hsch and we queue the interrupt in the
workqueue

- we start processing the ssch and then at the same time another cpu
could be working on the
interrupt

Thread 1 Thread 2
-------- --------

fsm_io_request vfio_ccw_sch_io_todo

cp_init cp_free
cp_prefetch
fsm_io_helper
cp_free

The test that I am trying is with a guest running an fio workload, while
at the same time stressing the error recovery path in the guest. So
there is a lot of ssch and lot of csch.

Of course I don't think my patch completely solves the problem, I think
it just makes the window narrower. I just wanted to get a discussion
started :)

Now that I am thinking more about it, I think we might have to protect
cp with it's own mutex.

Thanks
Farhan

>
>> cp_free(&private->cp);
>> }
>> mutex_lock(&private->io_mutex);
>>
>