Re: [PATCH v5] PCI/ACS: Enable PCI_ACS_TB and disable only when needed for ATS

From: Rajat Jain
Date: Sat Aug 01 2020 - 20:58:52 EST

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Hi Bjorn,

On Tue, Jul 14, 2020 at 1:24 PM Rajat Jain <rajatxjain@xxxxxxxxx> wrote:
>
> On Tue, Jul 14, 2020 at 1:15 PM Rajat Jain <rajatja@xxxxxxxxxx> wrote:
> >
> > The ACS "Translation Blocking" bit blocks the translated addresses from
> > the devices. We don't expect such traffic from devices unless ATS is
> > enabled on them. A device sending such traffic without ATS enabled,
> > indicates malicious intent, and thus should be blocked.
> >
> > Enable PCI_ACS_TB by default for all devices, and it stays enabled until
> > atleast one of the devices downstream wants to enable ATS. It gets
> > disabled to enable ATS on a device downstream it, and then gets enabled
> > back on once all the downstream devices don't need ATS.
> >
> > Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>

Just checking to see if you got a chance to look at this V5 patch.

Thanks & Best Regards,

Rajat

> > ---
> > Note that I'm ignoring the devices that require quirks to enable or
> > disable ACS, instead of using the standard way for ACS configuration.
> > The reason is that it would require adding yet another quirk table or
> > quirk function pointer, that I don't know how to implement for those
> > devices, and will neither have the devices to test that code.
> >
> > v5: Enable TB and disable ATS for all devices on boot. Disable TB later
> > only if needed to enable ATS on downstream devices.
> > 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/ats.c | 5 ++++
> > drivers/pci/pci.c | 57 +++++++++++++++++++++++++++++++++++++++++++++
> > drivers/pci/pci.h | 2 ++
> > drivers/pci/probe.c | 2 +-
> > include/linux/pci.h | 2 ++
> > 5 files changed, 67 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
> > index b761c1f72f67..e2ea9083f30f 100644
> > --- a/drivers/pci/ats.c
> > +++ b/drivers/pci/ats.c
> > @@ -28,6 +28,9 @@ void pci_ats_init(struct pci_dev *dev)
> > return;
> >
> > dev->ats_cap = pos;
> > +
> > + dev->ats_enabled = 1; /* To avoid WARN_ON from pci_disable_ats() */
> > + pci_disable_ats(dev);
> > }
> >
> > /**
> > @@ -82,6 +85,7 @@ int pci_enable_ats(struct pci_dev *dev, int ps)
> > }
> > pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
> >
> > + pci_disable_acs_trans_blocking(dev);
> > dev->ats_enabled = 1;
> > return 0;
> > }
> > @@ -102,6 +106,7 @@ void pci_disable_ats(struct pci_dev *dev)
> > ctrl &= ~PCI_ATS_CTRL_ENABLE;
> > pci_write_config_word(dev, dev->ats_cap + PCI_ATS_CTRL, ctrl);
> >
> > + pci_enable_acs_trans_blocking(dev);
> > dev->ats_enabled = 0;
> > }
> > EXPORT_SYMBOL_GPL(pci_disable_ats);
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index 73a862782214..614e3c1e8c56 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -876,6 +876,9 @@ static void pci_std_enable_acs(struct pci_dev *dev)
> > /* Upstream Forwarding */
> > ctrl |= (cap & PCI_ACS_UF);
> >
> > + /* Translation Blocking */
> > + ctrl |= (cap & PCI_ACS_TB);
> > +
> > pci_write_config_word(dev, pos + PCI_ACS_CTRL, ctrl);
> > }
> >
> > @@ -904,6 +907,60 @@ static void pci_enable_acs(struct pci_dev *dev)
> > pci_disable_acs_redir(dev);
> > }
> >
> > +void pci_disable_acs_trans_blocking(struct pci_dev *pdev)
> > +{
> > + u16 cap, ctrl, pos;
> > + struct pci_dev *dev;
> > +
> > + if (!pci_acs_enable)
> > + return;
> > +
> > + for (dev = pdev; dev; dev = pci_upstream_bridge(pdev)) {
> > +
> > + pos = dev->acs_cap;
> > + if (!pos)
> > + continue;
> > +
> > + /*
> > + * Disable translation blocking when first downstream
> > + * device that needs it (for ATS) wants to enable ATS
> > + */
> > + if (++dev->ats_dependencies == 1) {
>
> I am a little worried about a potential race condition here. I know
> that 2 PCI devices cannot be enumerating at the same time. Do we know
> if multiple pci_enable_ats() and pci_disable_ats() function calls can
> be simultaneously executing (even for different devices)? If so, we
> may need an atomic_t variable for ats_dependencies.
>
> Thanks,
>
> Rajat
>
>
> > + pci_read_config_word(dev, pos + PCI_ACS_CAP, &cap);
> > + pci_read_config_word(dev, pos + PCI_ACS_CTRL, &ctrl);
> > + ctrl &= ~(cap & PCI_ACS_TB);
> > + pci_write_config_word(dev, pos + PCI_ACS_CTRL, ctrl);
> > + }
> > + }
> > +}
> > +
> > +void pci_enable_acs_trans_blocking(struct pci_dev *pdev)
> > +{
> > + u16 cap, ctrl, pos;
> > + struct pci_dev *dev;
> > +
> > + if (!pci_acs_enable)
> > + return;
> > +
> > + for (dev = pdev; dev; dev = pci_upstream_bridge(pdev)) {
> > +
> > + pos = dev->acs_cap;
> > + if (!pos)
> > + continue;
> > +
> > + /*
> > + * Enable translation blocking when last downstream device
> > + * that depends on it (for ATS), doesn't need ATS anymore
> > + */
> > + if (--dev->ats_dependencies == 0) {
> > + pci_read_config_word(dev, pos + PCI_ACS_CAP, &cap);
> > + pci_read_config_word(dev, pos + PCI_ACS_CTRL, &ctrl);
> > + ctrl |= (cap & PCI_ACS_TB);
> > + pci_write_config_word(dev, pos + PCI_ACS_CTRL, ctrl);
> > + }
> > + }
> > +}
> > +
> > /**
> > * pci_restore_bars - restore a device's BAR values (e.g. after wake-up)
> > * @dev: PCI device to have its BARs restored
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index 12fb79fbe29d..f5d8ecb6ba96 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -552,6 +552,8 @@ static inline int pci_dev_specific_disable_acs_redir(struct pci_dev *dev)
> > return -ENOTTY;
> > }
> > #endif
> > +void pci_disable_acs_trans_blocking(struct pci_dev *dev);
> > +void pci_enable_acs_trans_blocking(struct pci_dev *dev);
> >
> > /* PCI error reporting and recovery */
> > pci_ers_result_t pcie_do_recovery(struct pci_dev *dev,
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 8c40c00413e7..e2ff3a94e621 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -2387,10 +2387,10 @@ static void pci_init_capabilities(struct pci_dev *dev)
> > pci_vpd_init(dev); /* Vital Product Data */
> > pci_configure_ari(dev); /* Alternative Routing-ID Forwarding */
> > pci_iov_init(dev); /* Single Root I/O Virtualization */
> > + pci_acs_init(dev); /* Access Control Services */
> > pci_ats_init(dev); /* Address Translation Services */
> > pci_pri_init(dev); /* Page Request Interface */
> > pci_pasid_init(dev); /* Process Address Space ID */
> > - pci_acs_init(dev); /* Access Control Services */
> > pci_ptm_init(dev); /* Precision Time Measurement */
> > pci_aer_init(dev); /* Advanced Error Reporting */
> > pci_dpc_init(dev); /* Downstream Port Containment */
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 7a40cd5caed0..31da4355f0fd 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -480,6 +480,8 @@ struct pci_dev {
> > u16 ats_cap; /* ATS Capability offset */
> > u8 ats_stu; /* ATS Smallest Translation Unit */
> > #endif
> > + /* Total number of downstream devices below a bridge that need ATS */
> > + u8 ats_dependencies;
> > #ifdef CONFIG_PCI_PRI
> > u16 pri_cap; /* PRI Capability offset */
> > u32 pri_reqs_alloc; /* Number of PRI requests allocated */
> > --
> > 2.27.0.389.gc38d7665816-goog
> >

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