[PATCH 2/2] pci: Add parameter to disable attaching untrusted devices

[Rajat Jain]

Introduce a PCI parameter that disables the automatic attachment of
untrusted devices to their drivers.

Signed-off-by: Rajat Jain <rajatja@xxxxxxxxxx>
---
Context:

  I set out to implement the approach outlined in
    https://lkml.org/lkml/2020/6/9/1331
    https://lkml.org/lkml/2020/6/15/1453

  But to my surprise, I found that the new hotplugged PCI devices
  were getting automatically attached to drivers even though
  /sys/bus/pci/drivers_autoprobe was set to 0.

  I realized that the device core's "drivers_autoprobe":

  * only disables the *initial* probe of the device (i.e. from
    device_add()). If a subsystem calls device_attach() explicitly
    for its devices like PCI subsystem does, the drivers_autoprobe
    setting does not matter. The core will attach device to the driver.
    This looks like correct semantic behavior to me because PCI is
    explicitly calling device_attach(), which is a way to explicitly
    ask the core to find and attach a driver for a device.

  * "drivers_autoprobe" cannot be controlled at boot time (to restrict
    any drivers before userspace comes up).

  The options I considered were:

  1) Change device_attach() so that it takes into consideration the
     drivers_autoprobe property. Not sure if this is semantically correct
     thing to do though. If I do this, then the only way a driver can
     be attached to the drivers would be via userspace
     (/sys/bus/pci/drivers/bind) (Good for our use case though!).

  2) Make the drivers_autoprobe property available to PCI to use
     (currently it is private to device core). The PCI could use this
     to determine whether or not to call device_attach(). This still
     leaves the other problem (of not being able to set
     drivers_autoprobe via command line open).

  3) I found the pci_dev->match_driver, which seemed similar to what I
     am trying to do, but can't be controlled from userspace. I considered
     populating that field based on drivers_autoprobe (still need (2)).
     But the problem is that there is the AMD IOMMU driver which is setting
     this independently, so setting the match_driver based on
     drivers_autoprobe may not be a good idea. May be we can populate it
     for untrusted devicesi, based on the parameter that I'm introducing?

  4) This patch was my option 4 that helps fix both the problems for me.

 drivers/pci/bus.c | 11 ++++++++---
 drivers/pci/pci.c |  9 +++++++++
 drivers/pci/pci.h |  1 +
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
index 3cef835b375fd..336aeeb4c4ebf 100644
--- a/drivers/pci/bus.c
+++ b/drivers/pci/bus.c
@@ -321,9 +321,14 @@ void pci_bus_add_device(struct pci_dev *dev)
 	pci_bridge_d3_update(dev);
 
 	dev->match_driver = true;
-	retval = device_attach(&dev->dev);
-	if (retval < 0 && retval != -EPROBE_DEFER)
-		pci_warn(dev, "device attach failed (%d)\n", retval);
+
+	if (dev->untrusted && pci_dont_attach_untrusted_devs) {
+		pci_info(dev, "not attaching untrusted device\n");
+	} else {
+		retval = device_attach(&dev->dev);
+		if (retval < 0 && retval != -EPROBE_DEFER)
+			pci_warn(dev, "device attach failed (%d)\n", retval);
+	}
 
 	pci_dev_assign_added(dev, true);
 }
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index ce096272f52b1..dec1f9ef27d71 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -127,6 +127,13 @@ static bool pcie_ats_disabled;
 /* If set, the PCI config space of each device is printed during boot. */
 bool pci_early_dump;
 
+/*
+ * If set, the devices with "untrusted" flag shall not be attached automatically
+ * Userspace will need to attach them manually:
+ * echo <pci device>  > /sys/bus/pci/drivers/<driver>/bind
+ */
+bool pci_dont_attach_untrusted_devs;
+
 bool pci_ats_disabled(void)
 {
 	return pcie_ats_disabled;
@@ -6522,6 +6529,8 @@ static int __init pci_setup(char *str)
 				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
 			} else if (!strncmp(str, "disable_acs_redir=", 18)) {
 				disable_acs_redir_param = str + 18;
+			} else if (!strcmp(str, "dont_attach_untrusted_devs")) {
+				pci_dont_attach_untrusted_devs = true;
 			} else {
 				pr_err("PCI: Unknown option `%s'\n", str);
 			}
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 6d3f758671064..30ffad047d926 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -13,6 +13,7 @@
 
 extern const unsigned char pcie_link_speed[];
 extern bool pci_early_dump;
+extern bool pci_dont_attach_untrusted_devs;
 
 bool pcie_cap_has_lnkctl(const struct pci_dev *dev);
 bool pcie_cap_has_rtctl(const struct pci_dev *dev);
-- 
2.27.0.212.ge8ba1cc988-goog