linux-kernel - Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for untrusted/external-facing devices

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Date:   Sat, 11 Jul 2020 14:53:46 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Rajat Jain <rajatja@...gle.com>
Cc:     "Raj, Ashok" <ashok.raj@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "Krishnakumar, Lalithambika" <lalithambika.krishnakumar@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Prashant Malani <pmalani@...gle.com>,
        Benson Leung <bleung@...gle.com>,
        Todd Broch <tbroch@...gle.com>,
        Alex Levin <levinale@...gle.com>,
        Mattias Nissler <mnissler@...gle.com>,
        Rajat Jain <rajatxjain@...il.com>,
        Bernie Keany <bernie.keany@...el.com>,
        Aaron Durbin <adurbin@...gle.com>,
        Diego Rivas <diegorivas@...gle.com>,
        Duncan Laurie <dlaurie@...gle.com>,
        Furquan Shaikh <furquan@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Christian Kellner <christian@...lner.me>,
        Alex Williamson <alex.williamson@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Oliver O'Halloran <oohall@...il.com>,
        Saravana Kannan <saravanak@...gle.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for
 untrusted/external-facing devices

On Fri, Jul 10, 2020 at 03:53:59PM -0700, Rajat Jain wrote:
> On Fri, Jul 10, 2020 at 2:29 PM Raj, Ashok <ashok.raj@...el.com> wrote:
> > On Fri, Jul 10, 2020 at 03:29:22PM -0500, Bjorn Helgaas wrote:
> > > On Tue, Jul 07, 2020 at 03:46:04PM -0700, Rajat Jain wrote:
> > > > When enabling ACS, enable translation blocking for external facing ports
> > > > and untrusted devices.
> > > >
> > > > Signed-off-by: Rajat Jain <rajatja@...gle.com>
> > > > ---
> > > > v4: Add braces to avoid warning from kernel robot
> > > >     print warning for only external-facing devices.
> > > > v3: print warning if ACS_TB not supported on external-facing/untrusted ports.
> > > >     Minor code comments fixes.
> > > > v2: Commit log change
> > > >
> > > >  drivers/pci/pci.c    |  8 ++++++++
> > > >  drivers/pci/quirks.c | 15 +++++++++++++++
> > > >  2 files changed, 23 insertions(+)
> > > >
> > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > index 73a8627822140..a5a6bea7af7ce 100644
> > > > --- a/drivers/pci/pci.c
> > > > +++ b/drivers/pci/pci.c
> > > > @@ -876,6 +876,14 @@ static void pci_std_enable_acs(struct pci_dev *dev)
> > > >     /* Upstream Forwarding */
> > > >     ctrl |= (cap & PCI_ACS_UF);
> > > >
> > > > +   /* Enable Translation Blocking for external devices */
> > > > +   if (dev->external_facing || dev->untrusted) {
> > > > +           if (cap & PCI_ACS_TB)
> > > > +                   ctrl |= PCI_ACS_TB;
> > > > +           else if (dev->external_facing)
> > > > +                   pci_warn(dev, "ACS: No Translation Blocking on external-facing dev\n");
> > > > +   }
> > >
> > > IIUC, this means that external devices can *never* use ATS and
> > > can never cache translations.
> 
> Yes, but it already exists today (and this patch doesn't change that):
> 521376741b2c2 "PCI/ATS: Only enable ATS for trusted devices"
> 
> IMHO any external device trying to send ATS traffic despite having ATS
> disabled should count as a bad intent. And this patch is trying to
> plug that loophole, by blocking the AT traffic from devices that we do
> not expect to see AT from anyway.

Thinking about this some more, I wonder if Linux should:

  - Explicitly disable ATS for every device at enumeration-time, e.g.,
    in pci_init_capabilities(), 

  - Enable PCI_ACS_TB for every device (not just external-facing or
    untrusted ones),

  - Disable PCI_ACS_TB for the relevant devices along the path only
    when enabling ATS.

One nice thing about doing that is that the "untrusted" test would be
only in pci_enable_ats(), and we wouldn't need one in
pci_std_enable_acs().

It's possible BIOS gives us devices with ATS enabled, and this might
break them, but that seems like something we'd want to find out about.

Bjorn