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

From: Bjorn Helgaas <helgaas@kernel.org>
To: Rajat Jain <rajatja@google.com>
Cc: "Raj, Ashok" <ashok.raj@intel.com>,
	David Woodhouse <dwmw2@infradead.org>,
	Lu Baolu <baolu.lu@linux.intel.com>,
	Joerg Roedel <joro@8bytes.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Len Brown <lenb@kernel.org>,
	"open list:AMD IOMMU (AMD-VI)" <iommu@lists.linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-pci <linux-pci@vger.kernel.org>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	"Krishnakumar,
	Lalithambika" <lalithambika.krishnakumar@intel.com>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	Prashant Malani <pmalani@google.com>,
	Benson Leung <bleung@google.com>, Todd Broch <tbroch@google.com>,
	Alex Levin <levinale@google.com>,
	Mattias Nissler <mnissler@google.com>,
	Rajat Jain <rajatxjain@gmail.com>,
	Bernie Keany <bernie.keany@intel.com>,
	Aaron Durbin <adurbin@google.com>,
	Diego Rivas <diegorivas@google.com>,
	Duncan Laurie <dlaurie@google.com>,
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	Saravana Kannan <saravanak@google.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Heikki Krogerus <heikki.krogerus@linux.intel.com>
Subject: Re: [PATCH v4 4/4] PCI/ACS: Enable PCI_ACS_TB for untrusted/external-facing devices
Date: Sat, 11 Jul 2020 14:53:46 -0500	[thread overview]
Message-ID: <20200711195346.GA132330@bjorn-Precision-5520> (raw)
In-Reply-To: <CACK8Z6F-8OZNJU8wqWuZq=moCaOi+3W=CzBeppfO31VZnkqBrg@mail.gmail.com>

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@intel.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@google.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