iommu/vt-d: Force IOMMU on for platform opt in hint

Intel VT-d spec added a new DMA_CTRL_PLATFORM_OPT_IN_FLAG flag in DMAR
ACPI table [1] for BIOS to report compliance about platform initiated
DMA restricted to RMRR ranges when transferring control to the OS. This
means that during OS boot, before it enables IOMMU none of the connected
devices can bypass DMA protection for instance by overwriting the data
structures used by the IOMMU. The OS also treats this as a hint that the
IOMMU should be enabled to prevent DMA attacks from possible malicious
devices.

A use of this flag is Kernel DMA protection for Thunderbolt [2] which in
practice means that IOMMU should be enabled for PCIe devices connected
to the Thunderbolt ports. With IOMMU enabled for these devices, all DMA
operations are limited in the range reserved for it, thus the DMA
attacks are prevented. All these devices are enumerated in the PCI/PCIe
module and marked with an untrusted flag.

This forces IOMMU to be enabled if DMA_CTRL_PLATFORM_OPT_IN_FLAG is set
in DMAR ACPI table and there are PCIe devices marked as untrusted in the
system. This can be turned off by adding "intel_iommu=off" in the kernel
command line, if any problems are found.

[1] https://software.intel.com/sites/default/files/managed/c5/15/vt-directed-io-spec.pdf
[2] https://docs.microsoft.com/en-us/windows/security/information-protection/kernel-dma-protection-for-thunderbolt

Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Ashok Raj <ashok.raj@intel.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>
Acked-by: Joerg Roedel <jroedel@suse.de>

1 files changed, 25 insertions, 0 deletions

diff --git a/drivers/iommu/dmar.c b/drivers/iommu/dmar.c
index d9c748b6f9e4..1edf2a251336 100644
--- a/drivers/iommu/dmar.c
+++ b/drivers/iommu/dmar.c
@@ -2042,3 +2042,28 @@ int dmar_device_remove(acpi_handle handle)
 {
 	return dmar_device_hotplug(handle, false);
 }
+
+/*
+ * dmar_platform_optin - Is %DMA_CTRL_PLATFORM_OPT_IN_FLAG set in DMAR table
+ *
+ * Returns true if the platform has %DMA_CTRL_PLATFORM_OPT_IN_FLAG set in
+ * the ACPI DMAR table. This means that the platform boot firmware has made
+ * sure no device can issue DMA outside of RMRR regions.
+ */
+bool dmar_platform_optin(void)
+{
+	struct acpi_table_dmar *dmar;
+	acpi_status status;
+	bool ret;
+
+	status = acpi_get_table(ACPI_SIG_DMAR, 0,
+				(struct acpi_table_header **)&dmar);
+	if (ACPI_FAILURE(status))
+		return false;
+
+	ret = !!(dmar->flags & DMAR_PLATFORM_OPT_IN);
+	acpi_put_table((struct acpi_table_header *)dmar);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(dmar_platform_optin);