Re: [RFC v1 4/4] vfio-ccw: Don't call cp_free if we are processing a channel program

Cornelia Huck
2019-07-02T09:51:45Z

On Mon, 1 Jul 2019 12:23:46 -0400
Farhan Ali <alifm@linux.ibm.com> wrote:

> There is a small window where it's possible that we could be working
> on an interrupt (queued in the workqueue) and setting up a channel
> program (i.e allocating memory, pinning pages, translating address).
> This can lead to allocating and freeing the channel program at the
> same time and can cause memory corruption.

This can only happen if the interrupt is for a halt/clear operation,
right?

>
> Let's not call cp_free if we are currently processing a channel program.
> The only way we know for sure that we don't have a thread setting
> up a channel program is when the state is set to VFIO_CCW_STATE_CP_PENDING.

I have looked through the code again and I think you are right.

>
> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> ---
> 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 4e3a903..0357165 100644
> --- a/drivers/s390/cio/vfio_ccw_drv.c
> +++ b/drivers/s390/cio/vfio_ccw_drv.c
> @@ -92,7 +92,7 @@ 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_PENDING)

Do we actually want to call cp_update_scsw() unconditionally?

At this point, we know that we have a solicited interrupt; that may be
for several reasons:
- Interrupt for something we issued via ssch; it makes sense to update
the scsw with the cpa address.
- Interrupt for a csch; the cpa address will be unpredictable, even if
we did a ssch before. cp_update_scsw() hopefully can deal with that?
Given that its purpose is to translate the cpa back, any
unpredictable value in the scsw should be fine in the end.
- Interrupt for a hsch after we did a ssch; the cpa might be valid (see
figure 16-6).
- Interrupt for a hsch without a prior ssch; we'll end up with an
unpredictable cpa, again.

So I *think* we're fine with calling cp_update_scsw() in all cases,
even if there's junk in the cpa of the scsw we get from the hardware.
Opinions?

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