linux-kernel - Re: [PATCH v5 03/13] iommufd: Add iommufd_verify_unfinalized

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Message-ID: <ZyE4Zdxvge0vm6j0@Asurada-Nvidia>
Date: Tue, 29 Oct 2024 12:32:53 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <kevin.tian@...el.com>, <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>,
	<eric.auger@...hat.com>, <jean-philippe@...aro.org>, <mdf@...nel.org>,
	<mshavit@...gle.com>, <shameerali.kolothum.thodi@...wei.com>,
	<smostafa@...gle.com>, <yi.l.liu@...el.com>, <aik@....com>,
	<zhangfei.gao@...aro.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v5 03/13] iommufd: Add iommufd_verify_unfinalized_object

On Tue, Oct 29, 2024 at 03:55:58PM -0300, Jason Gunthorpe wrote:
> On Tue, Oct 29, 2024 at 09:18:05AM -0700, Nicolin Chen wrote:
> > On Tue, Oct 29, 2024 at 11:49:07AM -0300, Jason Gunthorpe wrote:
> > > On Fri, Oct 25, 2024 at 04:49:43PM -0700, Nicolin Chen wrote:
> > > > To support driver-allocated vIOMMU objects, it's suggested to call the
> > > > allocator helper in IOMMU dirvers. However, there is no guarantee that
> > > > drivers will all use it and allocate objects properly.
> > > > 
> > > > Add a helper for iommufd core to verify if an unfinalized object is at
> > > > least reserved in the ictx.
> > > 
> > > I don't think we need this..
> > > 
> > > iommufd_object_finalize() already does:
> > > 
> > > 	old = xa_store(&ictx->objects, obj->id, obj, GFP_KERNEL);
> > > 	/* obj->id was returned from xa_alloc() so the xa_store() cannot fail */
> > > 	WARN_ON(old);
> > 
> > It feels unsafe to carry on the iommufd_viommu_alloc_ioctl() until
> > iommufd_object_finalize() as the function would touch the returned
> > faulty viommu pointer? E.g. what if the viommu has an even smaller
> > size than struct iommufd_viommu?
> 
> This is Linux just because the output came from a driver doesn't mean
> we have to validate it somehow. It is reasonable to be helpful and
> detect driver bugs, but if the driver is buggy it is still OK to
> crash.
> 
> So you don't *have* to check any of this, if the driver didn't use the
> right function to allocate the memory then it will go bad pretty fast.
> 
> Improving the xa_store() is something that will detect more kinds of
> bugs everywhere, so seems more worthwhile

I see. Thanks!

Nicolin