linux-kernel - Re: [PATCH 03/11] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming

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Message-ID: <YZJe1jquP+osF+Wn@infradead.org>
Date:   Mon, 15 Nov 2021 05:21:26 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>, kvm@...r.kernel.org,
        rafael@...nel.org, linux-pci@...r.kernel.org,
        Cornelia Huck <cohuck@...hat.com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Diana Craciun <diana.craciun@....nxp.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 03/11] PCI: pci_stub: Suppress kernel DMA ownership
 auto-claiming

On Mon, Nov 15, 2021 at 10:05:44AM +0800, Lu Baolu wrote:
> pci_stub allows the admin to block driver binding on a device and make
> it permanently shared with userspace. Since pci_stub does not do DMA,
> it is safe.

If an IOMMU is setup and dma-iommu or friends are not used nothing is
unsafe anyway, it just is that IOMMU won't work..

> However the admin must understand that using pci_stub allows
> userspace to attack whatever device it was bound to.

I don't understand this sentence at all.