[xen master] x86/PVH: de-duplicate mappings for first Mb of Dom0 memory

[patchbot@xxxxxxx]

commit 6b4f6a31ace125d658a581e8d10809e4fccdc272
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Aug 31 17:43:36 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Aug 31 17:43:36 2021 +0200

    x86/PVH: de-duplicate mappings for first Mb of Dom0 memory
    
    One of the changes comprising the fixes for XSA-378 disallows replacing
    MMIO mappings by code paths not intended for this purpose. This means we
    need to be more careful about the mappings put in place in this range -
    mappings should be created exactly once:
    - iommu_hwdom_init() comes first; it should avoid the first Mb,
    - pvh_populate_p2m() should insert identity mappings only into ranges
      not populated as RAM,
    - pvh_setup_acpi() should again avoid the first Mb, which was already
      dealt with at that point.
    
    Fixes: 753cb68e6530 ("x86/p2m: guard (in particular) identity mapping 
entries")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/hvm/dom0_build.c       | 39 ++++++++++++++++++++++++++-----------
 xen/drivers/passthrough/x86/iommu.c |  8 +++++++-
 2 files changed, 35 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
index c24b9efdb0..43e1bf1248 100644
--- a/xen/arch/x86/hvm/dom0_build.c
+++ b/xen/arch/x86/hvm/dom0_build.c
@@ -430,17 +430,6 @@ static int __init pvh_populate_p2m(struct domain *d)
     int rc;
 #define MB1_PAGES PFN_DOWN(MB(1))
 
-    /*
-     * Memory below 1MB is identity mapped initially. RAM regions are
-     * populated and copied below, replacing the respective mappings.
-     */
-    rc = modify_identity_mmio(d, 0, MB1_PAGES, true);
-    if ( rc )
-    {
-        printk("Failed to identity map low 1MB: %d\n", rc);
-        return rc;
-    }
-
     /* Populate memory map. */
     for ( i = 0; i < d->arch.nr_e820; i++ )
     {
@@ -472,6 +461,23 @@ static int __init pvh_populate_p2m(struct domain *d)
         }
     }
 
+    /* Non-RAM regions of space below 1MB get identity mapped. */
+    for ( i = rc = 0; i < MB1_PAGES; ++i )
+    {
+        p2m_type_t p2mt;
+
+        if ( mfn_eq(get_gfn_query(d, i, &p2mt), INVALID_MFN) )
+            rc = set_mmio_p2m_entry(d, _gfn(i), _mfn(i), PAGE_ORDER_4K);
+        else
+            ASSERT(p2mt == p2m_ram_rw);
+        put_gfn(d, i);
+        if ( rc )
+        {
+            printk("Failed to identity map PFN %x: %d\n", i, rc);
+            return rc;
+        }
+    }
+
     if ( cpu_has_vmx && paging_mode_hap(d) && !vmx_unrestricted_guest(v) )
     {
         /*
@@ -1095,6 +1101,17 @@ static int __init pvh_setup_acpi(struct domain *d, 
paddr_t start_info)
         nr_pages = PFN_UP((d->arch.e820[i].addr & ~PAGE_MASK) +
                           d->arch.e820[i].size);
 
+        /* Memory below 1MB has been dealt with by pvh_populate_p2m(). */
+        if ( pfn < PFN_DOWN(MB(1)) )
+        {
+            if ( pfn + nr_pages <= PFN_DOWN(MB(1)) )
+                continue;
+
+            /* This shouldn't happen, but is easy to deal with. */
+            nr_pages -= PFN_DOWN(MB(1)) - pfn;
+            pfn = PFN_DOWN(MB(1));
+        }
+
         rc = modify_identity_mmio(d, pfn, nr_pages, true);
         if ( rc )
         {
diff --git a/xen/drivers/passthrough/x86/iommu.c 
b/xen/drivers/passthrough/x86/iommu.c
index 65ed4a7f9f..01dbd9b098 100644
--- a/xen/drivers/passthrough/x86/iommu.c
+++ b/xen/drivers/passthrough/x86/iommu.c
@@ -337,7 +337,13 @@ void __hwdom_init arch_iommu_hwdom_init(struct domain *d)
     max_pfn = (GB(4) >> PAGE_SHIFT) - 1;
     top = max(max_pdx, pfn_to_pdx(max_pfn) + 1);
 
-    for ( i = 0; i < top; i++ )
+    /*
+     * First Mb will get mapped in one go by pvh_populate_p2m(). Avoid
+     * setting up potentially conflicting mappings here.
+     */
+    i = paging_mode_translate(d) ? PFN_DOWN(MB(1)) : 0;
+
+    for ( ; i < top; i++ )
     {
         unsigned long pfn = pdx_to_pfn(i);
         int rc;
--
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