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

[Rajat Jain via iommu <iommu@lists.linux-foundation.org>]

On Sat, Jul 11, 2020 at 12:53 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> 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().

Sent out v5 with this approach here:
https://patchwork.kernel.org/patch/11663515/

Thanks,

Rajat

>
>
> 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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