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>
master commit: 111469cc7b3f586c2335e70205320ed3c828b89e
master date: 2021-09-07 09:39:38 +0200

diff --git a/xen/arch/x86/mm/p2m.c b/xen/arch/x86/mm/p2m.c
index 3bdfc3f274e09cb41cc415cb4d54a422283cdb3a..871f3a04e8c5db132cb96a3302edf79b7f0d8ee6 100644 (file)
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -945,9 +945,13 @@ 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) )
@@ -1291,9 +1295,24 @@ 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) )
     {