Subject: AMD/IOMMU: re-arrange/complete re-assignment handling
From: Jan Beulich <[email-masked]>

Prior to the assignment step having completed successfully, devices
should not get associated with their new owner. Hand the device to DomIO
(perhaps temporarily), until after the de-assignment step has completed.

De-assignment of a device (from other than Dom0) as well as failure of
reassign_device() during assignment should result in unity mappings
getting torn down. This in turn requires switching to a refcounted
mapping approach, as was already used by VT-d for its RMRRs, to prevent
unmapping a region used by multiple devices.

This is CVE-2021-28696 / part of XSA-378.

Signed-off-by: Jan Beulich <[email-masked]>
Reviewed-by: Paul Durrant <[email-masked]>

--- a/xen/drivers/passthrough/amd/iommu_map.c
+++ b/xen/drivers/passthrough/amd/iommu_map.c
@@ -613,38 +613,49 @@ int amd_iommu_flush_iotlb_all(struct dom
     return 0;
 }
 
-int amd_iommu_reserve_domain_unity_map(struct domain *domain,
-                                       paddr_t phys_addr,
-                                       unsigned long size, int iw, int ir)
+int amd_iommu_reserve_domain_unity_map(struct domain *d,
+                                       const struct ivrs_unity_map *map,
+                                       unsigned int flag)
 {
-    unsigned long npages, i;
-    unsigned long gfn;
-    unsigned int flags = !!ir;
-    unsigned int flush_flags = 0;
-    int rt = 0;
-
-    if ( iw )
-        flags |= IOMMUF_writable;
-
-    npages = region_to_pages(phys_addr, size);
-    gfn = phys_addr >> PAGE_SHIFT;
-    for ( i = 0; i < npages; i++ )
+    int rc;
+
+    if ( d == dom_io )
+        return 0;
+
+    for ( rc = 0; !rc && map; map = map->next )
     {
-        unsigned long frame = gfn + i;
+        p2m_access_t p2ma = p2m_access_n;
+
+        if ( map->read )
+            p2ma |= p2m_access_r;
+        if ( map->write )
+            p2ma |= p2m_access_w;
 
-        rt = amd_iommu_map_page(domain, _dfn(frame), _mfn(frame), flags,
-                                &flush_flags);
-        if ( rt != 0 )
-            break;
+        rc = iommu_identity_mapping(d, p2ma, map->addr,
+                                    map->addr + map->length - 1, flag);
     }
 
-    /* Use while-break to avoid compiler warning */
-    while ( flush_flags &&
-            amd_iommu_flush_iotlb_pages(domain, _dfn(gfn),
-                                        npages, flush_flags) )
-        break;
+    return rc;
+}
+
+int amd_iommu_reserve_domain_unity_unmap(struct domain *d,
+                                         const struct ivrs_unity_map *map)
+{
+    int rc;
+
+    if ( d == dom_io )
+        return 0;
+
+    for ( rc = 0; map; map = map->next )
+    {
+        int ret = iommu_identity_mapping(d, p2m_access_x, map->addr,
+                                         map->addr + map->length - 1, 0);
+
+        if ( ret && ret != -ENOENT && !rc )
+            rc = ret;
+    }
 
-    return rt;
+    return rc;
 }
 
 /* Share p2m table with iommu. */
--- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
+++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
@@ -307,6 +307,7 @@ static int reassign_device(struct domain
     struct amd_iommu *iommu;
     int bdf, rc;
     struct domain_iommu *t = dom_iommu(target);
+    const struct ivrs_mappings *ivrs_mappings = get_ivrs_mappings(pdev->seg);
 
     bdf = PCI_BDF2(pdev->bus, pdev->devfn);
     iommu = find_iommu_for_device(pdev->seg, bdf);
@@ -321,10 +322,24 @@ static int reassign_device(struct domain
 
     amd_iommu_disable_domain_device(source, iommu, devfn, pdev);
 
-    if ( devfn == pdev->devfn )
+    /*
+     * If the device belongs to the hardware domain, and it has a unity mapping,
+     * don't remove it from the hardware domain, because BIOS may reference that
+     * mapping.
+     */
+    if ( !is_hardware_domain(source) )
+    {
+        rc = amd_iommu_reserve_domain_unity_unmap(
+                 source,
+                 ivrs_mappings[get_dma_requestor_id(pdev->seg, bdf)].unity_map);
+        if ( rc )
+            return rc;
+    }
+
+    if ( devfn == pdev->devfn && pdev->domain != dom_io )
     {
-        list_move(&pdev->domain_list, &target->arch.pdev_list);
-        pdev->domain = target;
+        list_move(&pdev->domain_list, &dom_io->arch.pdev_list);
+        pdev->domain = dom_io;
     }
 
     rc = allocate_domain_resources(t);
@@ -336,6 +351,12 @@ static int reassign_device(struct domain
                     pdev->seg, pdev->bus, PCI_SLOT(devfn), PCI_FUNC(devfn),
                     source->domain_id, target->domain_id);
 
+    if ( devfn == pdev->devfn && pdev->domain != target )
+    {
+        list_move(&pdev->domain_list, &target->arch.pdev_list);
+        pdev->domain = target;
+    }
+
     return 0;
 }
 
@@ -346,20 +367,28 @@ static int amd_iommu_assign_device(struc
     struct ivrs_mappings *ivrs_mappings = get_ivrs_mappings(pdev->seg);
     int bdf = PCI_BDF2(pdev->bus, devfn);
     int req_id = get_dma_requestor_id(pdev->seg, bdf);
-    const struct ivrs_unity_map *unity_map;
+    int rc = amd_iommu_reserve_domain_unity_map(
+                 d, ivrs_mappings[req_id].unity_map, flag);
+
+    if ( !rc )
+        rc = reassign_device(pdev->domain, d, devfn, pdev);
 
-    for ( unity_map = ivrs_mappings[req_id].unity_map; unity_map;
-          unity_map = unity_map->next )
+    if ( rc && !is_hardware_domain(d) )
     {
-        int rc = amd_iommu_reserve_domain_unity_map(
-                     d, unity_map->addr, unity_map->length,
-                     unity_map->write, unity_map->read);
+        int ret = amd_iommu_reserve_domain_unity_unmap(
+                      d, ivrs_mappings[req_id].unity_map);
 
-        if ( rc )
-            return rc;
+        if ( ret )
+        {
+            printk(XENLOG_ERR "AMD-Vi: "
+                   "unity-unmap for %pd/%04x:%02x:%02x.%u failed (%d)\n",
+                   d, pdev->seg, pdev->bus,
+                   PCI_SLOT(devfn), PCI_FUNC(devfn), ret);
+            domain_crash(d);
+        }
     }
 
-    return reassign_device(pdev->domain, d, devfn, pdev);
+    return rc;
 }
 
 static void deallocate_next_page_table(struct page_info *pg, int level)
@@ -425,6 +454,7 @@ static void deallocate_iommu_page_tables
 
 static void amd_iommu_domain_destroy(struct domain *d)
 {
+    iommu_identity_map_teardown(d);
     deallocate_iommu_page_tables(d);
     amd_iommu_flush_all_pages(d);
 }
--- a/xen/include/asm-x86/hvm/svm/amd-iommu-proto.h
+++ b/xen/include/asm-x86/hvm/svm/amd-iommu-proto.h
@@ -62,8 +62,10 @@ int __must_check amd_iommu_unmap_page(st
 uint64_t amd_iommu_get_address_from_pte(void *entry);
 int __must_check amd_iommu_alloc_root(struct domain_iommu *hd);
 int amd_iommu_reserve_domain_unity_map(struct domain *domain,
-                                       paddr_t phys_addr, unsigned long size,
-                                       int iw, int ir);
+                                       const struct ivrs_unity_map *map,
+                                       unsigned int flag);
+int amd_iommu_reserve_domain_unity_unmap(struct domain *d,
+                                         const struct ivrs_unity_map *map);
 int __must_check amd_iommu_flush_iotlb_pages(struct domain *d, dfn_t dfn,
                                              unsigned int page_count,
                                              unsigned int flush_flags);