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

Farhan Ali
2019-06-21T14:26:26Z

Hey Halil,

On 06/21/2019 10:00 AM, Halil Pasic wrote:
> On Thu, 20 Jun 2019 17:07:09 -0400
> Farhan Ali <alifm@linux.ibm.com> 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
>> 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.
>>
>> 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);
>> - if (is_final)
>> + if (is_final && private->state != VFIO_CCW_STATE_CP_PROCESSING)
>
> How is access to private->state correctly synchronized? And don't we
> expect private->state == VFIO_CCW_STATE_CP_PENDING in case the cp was
> submitted successfully with a ssch() and is done now (one way or the
> other)?

I think the interrupt that we are processing could be for a csch/hsch
and not an ssch. So we could have one thread handling an ssch and
another thread handling a csch/hsch interrupt.

>
> Does this have something to do with 71189f2 "vfio-ccw: make it safe to
> access channel programs" (Cornelia Huck, 2019-01-21)?

It's related. Though that patch handles freeing cp when we have
successfully issued a ssch and then issue a csch/hsch. But it leaves a
tiny window where if the reverse happens where we get an csch/hsch
first, and then ssch there could be a scenario where we can end up
calling cp_free in 2 different threads.

I have responded to Eric's email explaining further, so if you could
kindly refer to that and continue the discussion there because I think
he had similar questions and concern as you :)

Thanks
Farhan

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