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

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Message-ID: <20211115204821.GA1587269@bhelgaas>
Date:   Mon, 15 Nov 2021 14:48:21 -0600
From:   Bjorn Helgaas <helgaas@...nel.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>, Will Deacon <will@...nel.org>,
        rafael@...nel.org, Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.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. 

Can you elaborate on what "permanently shared with userspace" means
here?  I assume it's only permanent as long as pci-stub is bound to
the device?

Also, a few words about what "it is safe" means here would be helpful.

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

The admin isn't going to read this sentence.  Should there be a doc
update related to this?  What sort of attack does this refer to?

> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> ---
>  drivers/pci/pci-stub.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/pci/pci-stub.c b/drivers/pci/pci-stub.c
> index e408099fea52..6324c68602b4 100644
> --- a/drivers/pci/pci-stub.c
> +++ b/drivers/pci/pci-stub.c
> @@ -36,6 +36,9 @@ static struct pci_driver stub_driver = {
>  	.name		= "pci-stub",
>  	.id_table	= NULL,	/* only dynamic id's */
>  	.probe		= pci_stub_probe,
> +	.driver		= {
> +		.suppress_auto_claim_dma_owner = true,
> +	},
>  };
>  
>  static int __init pci_stub_init(void)
> -- 
> 2.25.1
>