linux-kernel - Re: [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20180522161305.04ec9817@w520.home>
Date:   Tue, 22 May 2018 16:13:05 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Doug Meyer <dmeyer@...aio.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        kurt.schwemmer@...rosemi.com, linux-pci@...r.kernel.org,
        linux-ntb <linux-ntb@...glegroups.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jon Mason <jdmason@...zu.us>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        Allen Hubbe <allenbh@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality
 with IOMMU On

On Tue, 22 May 2018 16:51:26 -0500
Bjorn Helgaas <helgaas@...nel.org> wrote:

> [+cc Alex]
> 
> On Tue, May 22, 2018 at 02:09:59PM -0700, Doug Meyer wrote:
> > Logan answered the questions quite thoroughly. (Thanks, Logan!)  
> 
> When you repost it, please rework the commit log so it answers the
> questions directly.  Otherwise the next reader may have the same
> questions again.  E.g., say something about how the proxy IDs are not
> programmable and are fixed in the hardware so all we have to do is
> read them.
> 
> I don't think the question of when the aliases need to be added is
> quite closed.  Logan said "it seems pci_add_dma_alias() must be called
> before the driver is initialized and therefore in a quirk", but that
> doesn't make clear *why* the alias needs to be added before the driver
> is initialized.  The alias shouldn't be needed until the device does a
> DMA, and it shouldn't do that until after the driver initializes.

Aliases for devices that don't have a representation on the bus is only
one use for pci_add_dma_alias(), we can also use it when the aliased
device is visible on the bus and then it factors not only into the IOMMU
context entries for a given device, but also the grouping of multiple
devices that must be done without a host endpoint driver.
 
> I suspect the reason the existing quirks are in drivers/pci/quirks.c
> is because the IOMMU driver is in the host OS, but the host may not
> have a driver for the device if the device is passed through to a
> guest OS.  In that case, the only way to add the alias is by using a
> quirk that is always built into the host OS.
> 
> We could argue that the driver in the guest should be able to tell the
> host's IOMMU driver about these aliases, but I doubt there's an
> interface for that.

Sounds like a dangerous interface, imagine two physical functions on a
device, each assigned to separate guests where one guest could usurp
context entries for hidden devices from the other guest.  Thanks,

Alex