linux-kernel - RE: [PATCH v3 10/10] iommu/vt-d: Disallow nesting on domains with read-only mappings

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Date:   Wed, 24 May 2023 07:44:54 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>
CC:     "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>
Subject: RE: [PATCH v3 10/10] iommu/vt-d: Disallow nesting on domains with
 read-only mappings

> From: Liu, Yi L <yi.l.liu@...el.com>
> Sent: Thursday, May 11, 2023 10:51 PM
> 
> From: Lu Baolu <baolu.lu@...ux.intel.com>
> 
> When remapping hardware is configured by system software in scalable
> mode
> as Nested (PGTT=011b) and with PWSNP field Set in the PASID-table-entry,
> it may Set Accessed bit and Dirty bit (and Extended Access bit if enabled)
> in first-stage page-table entries even when second-stage mappings indicate
> that corresponding first-stage page-table is Read-Only.
> 
> As the result, contents of pages designated by VMM as Read-Only can be
> modified by IOMMU via PML5E (PML4E for 4-level tables) access as part of
> address translation process due to DMAs issued by Guest.
> 
> Disallow the nested translation when there are read-only pages in the
> corresponding second-stage mappings. And, no read-only pages are allowed
> to be configured in the second-stage table of a nested translation.
> For the latter, an alternative is to disallow read-only mappings in
> any stage-2 domain as long as it's ever been used as a parent. In this
> way, we can simply replace the user counter with a flag.
> 
> In concept if the user understands this errata and does expect to
> enable nested translation it should never install any RO mapping
> in stage-2 in the entire VM life cycle."

IMHO the alternative is reasonable and simpler. If the user decides to
enable nesting it should keep the nesting-friendly configuration static
since whether nesting is enabled on a device is according to viommu
configuration (i.e. whether the guest attaches the device to identity
domain or non-identity domain) and it's not good to break the nesting
setup just because the host inadvertently adds a RO mapping to s2 in
the middle between guest is detached/put back to identity domain
and then re-attach to an unmanaged domain.
> 
> +	if (!(prot & DMA_PTE_WRITE) && !domain->read_only_mapped) {
> +		spin_lock_irqsave(&domain->lock, flags);
> +		if (domain->nested_users > 0) {
> +			spin_unlock_irqrestore(&domain->lock, flags);
> +			return -EINVAL;
> +		}
> +

this is worth a one-off warning. Same in the other path.