x86/P2M: relax guarding of MMIO entries

One of the changes comprising the fixes for XSA-378 disallows replacing
MMIO mappings by code paths not intended for this purpose. At least in
the case of PVH Dom0 hitting an RMRR covered by an E820 ACPI region,
this is too strict. Generally short-circuit requests establishing the
same kind of mapping (mfn, type), but allow permissions to differ.

While there, also add a log message to the other domain_crash()
invocation that did prevent PVH Dom0 from coming up after the XSA-378
changes.

Fixes: 753cb68e6530 ("x86/p2m: guard (in particular) identity mapping entries")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 1d17499543c9f1c1029fa04e301758fb736f3744..0be252cc4b02c49f94891c26569830180e32d783 100644 (file)
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -958,9 +958,13 @@ guest_physmap_add_entry(struct domain *d, gfn_t gfn, mfn_t mfn,
         if ( p2m_is_special(ot) )
         {
             /* Don't permit unmapping grant/foreign/direct-MMIO this way. */
-            domain_crash(d);
             p2m_unlock(p2m);
-            
+            printk(XENLOG_G_ERR
+                   "%pd: GFN %#lx (%#lx,%u,%u) -> (%#lx,%u,%u) not permitted\n",
+                   d, gfn_x(gfn) + i,
+                   mfn_x(omfn), ot, a,
+                   mfn_x(mfn) + i, t, p2m->default_access);
+            domain_crash(d);
             return -EPERM;
         }
         else if ( p2m_is_ram(ot) && !p2m_is_paged(ot) )
@@ -1302,9 +1306,24 @@ static int set_typed_p2m_entry(struct domain *d, unsigned long gfn_l,
     }
     if ( p2m_is_special(ot) )
     {
-        gfn_unlock(p2m, gfn, order);
-        domain_crash(d);
-        return -EPERM;
+        /* Special-case (almost) identical mappings. */
+        if ( !mfn_eq(mfn, omfn) || gfn_p2mt != ot )
+        {
+            gfn_unlock(p2m, gfn, order);
+            printk(XENLOG_G_ERR
+                   "%pd: GFN %#lx (%#lx,%u,%u,%u) -> (%#lx,%u,%u,%u) not permitted\n",
+                   d, gfn_l,
+                   mfn_x(omfn), cur_order, ot, a,
+                   mfn_x(mfn), order, gfn_p2mt, access);
+            domain_crash(d);
+            return -EPERM;
+        }
+
+        if ( access == a )
+        {
+            gfn_unlock(p2m, gfn, order);
+            return 0;
+        }
     }
     else if ( p2m_is_ram(ot) )
     {