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SubjectRe: [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@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

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