AMD/intremap: Prevent use of per-device vector maps until irq logic is fixed

XSA-36 changed the default vector map mode from global to per-device.  This is
because a global vector map does not prevent one PCI device from impersonating
another and launching a DoS on the system.

However, the per-device vector map logic is broken for devices with multiple
MSI-X vectors, which can either result in a failed ASSERT() or misprogramming
of a guests interrupt remapping tables.  The core problem is not trivial to
fix.

In an effort to get AMD systems back to a non-regressed state, introduce a new
type of vector map called per-device-global.  This uses per-device vector maps
in the IOMMU, but uses a single used_vector map for the core IRQ logic.

This patch is intended to be removed as soon as the per-device logic is fixed
correctly.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/xen/drivers/passthrough/amd/pci_amd_iommu.c b/xen/drivers/passthrough/amd/pci_amd_iommu.c
index 60696d74d8..5ea1a1d07d 100644
--- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
+++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
@@ -223,8 +223,19 @@ int __init amd_iov_detect(void)
     {
         if ( amd_iommu_perdev_intremap )
         {
-            printk("AMD-Vi: Enabling per-device vector maps\n");
-            opt_irq_vector_map = OPT_IRQ_VECTOR_MAP_PERDEV;
+            /* Per-device vector map logic is broken for devices with multiple
+             * MSI-X interrupts (and would also be for multiple MSI, if Xen
+             * supported it).
+             *
+             * Until this is fixed, use global vector tables as far as the irq
+             * logic is concerned to avoid the buggy behaviour of per-device
+             * maps in map_domain_pirq(), and use per-device tables as far as
+             * intremap code is concerned to avoid the security issue.
+             */
+            printk(XENLOG_WARNING "AMD-Vi: per-device vector map logic is broken.  "
+                   "Using per-device-global maps instead until a fix is found.\n");
+
+            opt_irq_vector_map = OPT_IRQ_VECTOR_MAP_GLOBAL;
         }
         else
         {
@@ -235,6 +246,10 @@ int __init amd_iov_detect(void)
     else
     {
         printk("AMD-Vi: Not overriding irq_vector_map setting\n");
+
+        if ( opt_irq_vector_map != OPT_IRQ_VECTOR_MAP_GLOBAL )
+            printk(XENLOG_WARNING "AMD-Vi: per-device vector map logic is broken.  "
+                   "Use irq_vector_map=global to work around.\n");
     }
     if ( !amd_iommu_perdev_intremap )
         printk(XENLOG_WARNING "AMD-Vi: Using global interrupt remap table is not recommended (see XSA-36)!\n");