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

From: Rajat Jain
Date: Tue Jul 14 2020 - 16:24:18 EST

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In reply to: Rajat Jain: "[PATCH v5] PCI/ACS: Enable PCI_ACS_TB and disable only when needed for ATS"
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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>
> ---
> 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
>

Next message: Sedat Dilek: "Re: [PATCH v4 0/7] x86/boot: Remove runtime relocations from compressed kernel"
Previous message: David Miller: "Re: [PATCH v4 09/10] net: eth: altera: add msgdma prefetcher"
In reply to: Rajat Jain: "[PATCH v5] PCI/ACS: Enable PCI_ACS_TB and disable only when needed for ATS"
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