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

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Message-ID: <Z4VnG2PSeH0MCfGs@Asurada-Nvidia>
Date: Mon, 13 Jan 2025 11:18:51 -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 Mon, Jan 13, 2025 at 03:12:25PM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 10, 2025 at 01:58:21PM -0800, Nicolin Chen wrote:
> > 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?
> 
> copy_to_user() is a sleeping function that sleeps if the user memory
> is non-present. So userspace can cause copy_to_user to copy to
> anything, including memory that is non-present and will take along
> time to page fault in. Eg perhaps by abusing userfaultfd.
> 
> We should not allow userspace to globally DOS the iommu driver this
> way.
> 
> So do not hold locks that are also held by the HW event path across
> copy_to_user().

I see. Thanks for explaining. I will add a patch fixing the fault
read() and change the veventq read() accordingly.

Nicolin