linux-kernel - [PATCH v3 2/2] PCI: Add sysfs "removable" attribute

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Date:   Wed, 12 May 2021 14:34:57 -0700
From:   Rajat Jain <rajatja@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Rajat Jain <rajatja@...gle.com>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-usb@...r.kernel.org,
        helgaas@...nel.org, Oliver Neukum <oneukum@...e.com>,
        David Laight <David.Laight@...lab.com>,
        "Krzysztof Wilczyński" <kw@...ux.com>
Cc:     rajatxjain@...il.com, jsbarnes@...gle.com, dtor@...gle.com
Subject: [PATCH v3 2/2] PCI: Add sysfs "removable" attribute

A PCI device is "external_facing" if it's a Root Port with the ACPI
"ExternalFacingPort" property or if it has the DT "external-facing"
property.  We consider everything downstream from such a device to
be removable by user.

We're mainly concerned with consumer platforms with user accessible
thunderbolt ports that are vulnerable to DMA attacks, and we expect those
ports to be identified as "ExternalFacingPort". Devices in traditional
hotplug slots can technically be removed, but the expectation is that
unless the port is marked with "ExternalFacingPort", such devices are less
accessible to user / may not be removed by end user, and thus not exposed
as "removable" to userspace.

Set pci_dev_type.supports_removable so the device core exposes the
"removable" file in sysfs, and tell the device core about removable
devices.

This can be used by userspace to implment any policies it wants to,
tailored specifically for user removable devices. Eg usage:
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2591812
https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2795038
(code uses such an attribute to remove external PCI devicces or disable
features on them as needed by the policy desired)

Signed-off-by: Rajat Jain <rajatja@...gle.com>
---
v3: - commit log updated
    - Rename set_pci_dev_removable() -> pci_set_removable()
    - Call it after applying early PCI quirks.
v2: Add documentation

 Documentation/ABI/testing/sysfs-devices-removable |  3 ++-
 drivers/pci/pci-sysfs.c                           |  1 +
 drivers/pci/probe.c                               | 12 ++++++++++++
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/Documentation/ABI/testing/sysfs-devices-removable b/Documentation/ABI/testing/sysfs-devices-removable
index 9dabcad7cdcd..ec0b243f5db4 100644
--- a/Documentation/ABI/testing/sysfs-devices-removable
+++ b/Documentation/ABI/testing/sysfs-devices-removable
@@ -14,4 +14,5 @@ Description:
 
 		Currently this is only supported by USB (which infers the
 		information from a combination of hub descriptor bits and
-		platform-specific data such as ACPI).
+		platform-specific data such as ACPI) and PCI (which gets this
+		from ACPI / device tree).
diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index beb8d1f4fafe..38b3259ba333 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c
@@ -1541,4 +1541,5 @@ static const struct attribute_group *pci_dev_attr_groups[] = {
 
 const struct device_type pci_dev_type = {
 	.groups = pci_dev_attr_groups,
+	.supports_removable = true,
 };
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 3a62d09b8869..3515afeeaba8 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1575,6 +1575,16 @@ static void set_pcie_untrusted(struct pci_dev *dev)
 		dev->untrusted = true;
 }
 
+static void pci_set_removable(struct pci_dev *dev)
+{
+	struct pci_dev *parent = pci_upstream_bridge(dev);
+	if (parent &&
+	    (parent->external_facing || dev_is_removable(&parent->dev)))
+		dev_set_removable(&dev->dev, DEVICE_REMOVABLE);
+	else
+		dev_set_removable(&dev->dev, DEVICE_FIXED);
+}
+
 /**
  * pci_ext_cfg_is_aliased - Is ext config space just an alias of std config?
  * @dev: PCI device
@@ -1822,6 +1832,8 @@ int pci_setup_device(struct pci_dev *dev)
 	/* Early fixups, before probing the BARs */
 	pci_fixup_device(pci_fixup_early, dev);
 
+	pci_set_removable(dev);
+
 	pci_info(dev, "[%04x:%04x] type %02x class %#08x\n",
 		 dev->vendor, dev->device, dev->hdr_type, dev->class);
 
-- 
2.31.1.607.g51e8a6a459-goog