Re: [PATCH v5 06/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Jan 10, 2025 at 11:27:53AM -0800, Nicolin Chen wrote:
> On Fri, Jan 10, 2025 at 01:48:42PM -0400, Jason Gunthorpe wrote:
> > On Tue, Jan 07, 2025 at 09:10:09AM -0800, Nicolin Chen wrote:
> > 
> > > +static ssize_t iommufd_veventq_fops_read(struct iommufd_eventq *eventq,
> > > +					 char __user *buf, size_t count,
> > > +					 loff_t *ppos)
> > > +{
> > > +	size_t done = 0;
> > > +	int rc = 0;
> > > +
> > > +	if (*ppos)
> > > +		return -ESPIPE;
> > > +
> > > +	mutex_lock(&eventq->mutex);
> > > +	while (!list_empty(&eventq->deliver) && count > done) {
> > > +		struct iommufd_vevent *cur = list_first_entry(
> > > +			&eventq->deliver, struct iommufd_vevent, node);
> > > +
> > > +		if (cur->data_len > count - done)
> > > +			break;
> > > +
> > > +		if (copy_to_user(buf + done, cur->event_data, cur->data_len)) {
> > > +			rc = -EFAULT;
> > > +			break;
> > > +		}
> > 
> > Now that I look at this more closely, the fault path this is copied
> > from is not great.
> > 
> > This copy_to_user() can block while waiting on a page fault, possibily
> > for a long time. While blocked the mutex is held and we can't add more
> > entries to the list.
> >
> > That will cause the shared IRQ handler in the iommu driver to back up,
> > which would cause a global DOS.
> >
> > This probably wants to be organized to look more like
> > 
> > while (itm = eventq_get_next_item(eventq)) {
> >    if (..) {
> >        eventq_restore_failed_item(eventq);
> >        return -1;
> >    }
> > }
> > 
> > Where the next_item would just be a simple spinlock across the linked
> > list manipulation.
> 
> Would it be simpler by just limiting one node per read(), i.e.
> no "while (!list_empty)" and no block?
> 
> The report() adds one node at a time, and wakes up the poll()
> each time of adding a node. And user space could read one event
> at a time too?

That doesn't really help, the issue is it holds the lock over the
copy_to_user() which it is doing because it doesn't want pull the item off
the list and then try to handle the failure and put it back.

Jason