linux-kernel - RE: [PATCH v2 7/8] iommu: Export iommu_get_dma

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Date:   Tue, 28 Mar 2023 07:52:23 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Jason Gunthorpe <jgg@...dia.com>,
        Joerg Roedel <joro@...tes.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>
CC:     Robin Murphy <robin.murphy@....com>, Will Deacon <will@...nel.org>,
        "David Woodhouse" <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>
Subject: RE: [PATCH v2 7/8] iommu: Export iommu_get_dma_domain

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Tuesday, March 28, 2023 2:04 PM
> 
> On 3/28/23 7:21 AM, Jacob Pan wrote:
> > Devices that use ENQCMDS to submit work needs to retrieve its DMA
> > domain. It can then attach PASID to the DMA domain for shared mapping
> > (with RID) established by DMA API.
> >
> > Signed-off-by: Jacob Pan<jacob.jun.pan@...ux.intel.com>
> > ---
> >   drivers/iommu/iommu.c | 1 +
> >   include/linux/iommu.h | 5 +++++
> >   2 files changed, 6 insertions(+)
> >
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 10db680acaed..c51d343a75d2 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -2118,6 +2118,7 @@ struct iommu_domain
> *iommu_get_dma_domain(struct device *dev)
> >   {
> >   	return dev->iommu_group->default_domain;
> >   }
> > +EXPORT_SYMBOL_GPL(iommu_get_dma_domain);
> 
> Directly exporting this function for external use seems unsafe. If the
> caller is the kernel driver for this device, it's fine because default
> domain remains unchanged during the life cycle of the driver. Otherwise,
> using this function may cause UAF. Keep in mind that group's default
> domain could be changed through sysfs.
> 
> However, iommu_get_domain_for_dev() has already done so and has been
> exported. Maybe I'm worried too much. :-)
> 

Agree. The kernel driver managing the device wants to get the current 
domain of the device then iommu_get_domain_for_dev() is the right
interface. It knows the domain is the dma domain.