linux-kernel - Re: [PATCH v5 06/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ

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Message-ID: <Z4GX/R6lRwuhlBJQ@Asurada-Nvidia>
Date: Fri, 10 Jan 2025 13:58:21 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <kevin.tian@...el.com>, <corbet@....net>, <will@...nel.org>,
	<joro@...tes.org>, <suravee.suthikulpanit@....com>, <robin.murphy@....com>,
	<dwmw2@...radead.org>, <baolu.lu@...ux.intel.com>, <shuah@...nel.org>,
	<linux-kernel@...r.kernel.org>, <iommu@...ts.linux.dev>,
	<linux-arm-kernel@...ts.infradead.org>, <linux-kselftest@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <eric.auger@...hat.com>,
	<jean-philippe@...aro.org>, <mdf@...nel.org>, <mshavit@...gle.com>,
	<shameerali.kolothum.thodi@...wei.com>, <smostafa@...gle.com>,
	<ddutile@...hat.com>, <yi.l.liu@...el.com>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v5 06/14] iommufd: Add IOMMUFD_OBJ_VEVENTQ and
 IOMMUFD_CMD_VEVENTQ_ALLOC

On Fri, Jan 10, 2025 at 03:49:50PM -0400, Jason Gunthorpe wrote:
> 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.

Hmm, it seems that I haven't got your first narrative straight..

Would you mind elaborate "copy_to_user() can block while waiting
on a page fault"? When would this happen?

Thanks
Nicolin