x86: enforce preemption in HVM_set_mem_access / p2m_set_mem_access()

Processing up to 4G PFNs may take almost arbitrarily long, so
preemption is needed here.

This is XSA-89.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4593,6 +4593,15 @@ long do_hvm_op(unsigned long op, XEN_GUE
             goto param_fail5;
             
         rc = p2m_set_mem_access(d, a.first_pfn, a.nr, a.hvmmem_access);
+        if ( rc > 0 )
+        {
+            a.first_pfn += a.nr - rc;
+            a.nr = rc;
+            if ( __copy_to_guest(arg, &a, 1) )
+                rc = -EFAULT;
+            else
+                rc = -EAGAIN;
+        }
 
     param_fail5:
         rcu_unlock_domain(d);
--- a/xen/arch/x86/mm/p2m.c
+++ b/xen/arch/x86/mm/p2m.c
@@ -1333,15 +1333,14 @@ void p2m_mem_access_resume(struct domain
 
 /* Set access type for a region of pfns.
  * If start_pfn == -1ul, sets the default access type */
-int p2m_set_mem_access(struct domain *d, unsigned long start_pfn, 
-                       uint32_t nr, hvmmem_access_t access) 
+long p2m_set_mem_access(struct domain *d, unsigned long pfn, uint32_t nr,
+                        hvmmem_access_t access)
 {
     struct p2m_domain *p2m = p2m_get_hostp2m(d);
-    unsigned long pfn;
     p2m_access_t a, _a;
     p2m_type_t t;
     mfn_t mfn;
-    int rc = 0;
+    long rc;
 
     /* N.B. _not_ static: initializer depends on p2m->default_access */
     p2m_access_t memaccess[] = {
@@ -1364,14 +1363,17 @@ int p2m_set_mem_access(struct domain *d,
     a = memaccess[access];
 
     /* If request to set default access */
-    if ( start_pfn == ~0ull ) 
+    if ( pfn == ~0ul )
     {
         p2m->default_access = a;
         return 0;
     }
 
+    if ( !nr )
+        return 0;
+
     p2m_lock(p2m);
-    for ( pfn = start_pfn; pfn < start_pfn + nr; pfn++ )
+    for ( ; ; ++pfn )
     {
         mfn = p2m->get_entry(p2m, pfn, &t, &_a, 0, NULL);
         if ( p2m->set_entry(p2m, pfn, mfn, PAGE_ORDER_4K, t, a) == 0 )
@@ -1379,6 +1381,13 @@ int p2m_set_mem_access(struct domain *d,
             rc = -ENOMEM;
             break;
         }
+
+        /* Check for continuation if it's not the last interation. */
+        if ( !--nr || hypercall_preempt_check() )
+        {
+            rc = nr;
+            break;
+        }
     }
     p2m_unlock(p2m);
     return rc;
--- a/xen/include/asm-x86/p2m.h
+++ b/xen/include/asm-x86/p2m.h
@@ -576,8 +576,8 @@ void p2m_mem_access_resume(struct domain
 
 /* Set access type for a region of pfns.
  * If start_pfn == -1ul, sets the default access type */
-int p2m_set_mem_access(struct domain *d, unsigned long start_pfn, 
-                       uint32_t nr, hvmmem_access_t access);
+long p2m_set_mem_access(struct domain *d, unsigned long start_pfn,
+                        uint32_t nr, hvmmem_access_t access);
 
 /* Get access type for a pfn
  * If pfn == -1ul, gets the default access type */