p2m: Always check to see if removing a p2m entry actually worked

The PoD zero-check functions speculatively remove memory from the p2m,
then check to see if it's completely zeroed, before putting it in the
cache.

Unfortunately, the p2m_set_entry() calls may fail if the underlying
pagetable structure needs to change and the domain has exhausted its
p2m memory pool: for instance, if we're removing a 2MiB region out of
a 1GiB entry (in the p2m_pod_zero_check_superpage() case), or a 4k
region out of a 2MiB or larger entry (in the p2m_pod_zero_check()
case); and the return value is not checked.

The underlying mfn will then be added into the PoD cache, and at some
point mapped into another location in the p2m.  If the guest
afterwards ballons out this memory, it will be freed to the hypervisor
and potentially reused by another domain, in spite of the fact that
the original domain still has writable mappings to it.

There are several places where p2m_set_entry() shouldn't be able to
fail, as it is guaranteed to write an entry of the same order that
succeeded before.  Add a backstop of crashing the domain just in case,
and an ASSERT_UNREACHABLE() to flag up the broken assumption on debug
builds.

While we're here, use PAGE_ORDER_2M rather than a magic constant.

This is part of XSA-247.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 92790672dedf2eab042e04ecc277c19d40fd348a
master date: 2017-11-28 13:13:03 +0100

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index 9c08797a9d5a7402ee309b65de32104036895573..d2931c530646380736560f4e5228117b5a13e652 100644 (file)
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -730,8 +730,9 @@ p2m_pod_zero_check_superpage(struct p2m_domain *p2m, unsigned long gfn)
     }
 
     /* Try to remove the page, restoring old mapping if it fails. */
-    p2m_set_entry(p2m, gfn, _mfn(INVALID_MFN), PAGE_ORDER_2M,
-                  p2m_populate_on_demand, p2m->default_access);
+    if ( p2m_set_entry(p2m, gfn, _mfn(INVALID_MFN), PAGE_ORDER_2M,
+                       p2m_populate_on_demand, p2m->default_access) )
+        goto out;
 
     /* Make none of the MFNs are used elsewhere... for example, mapped
      * via the grant table interface, or by qemu.  Allow one refcount for
@@ -787,9 +788,18 @@ p2m_pod_zero_check_superpage(struct p2m_domain *p2m, unsigned long gfn)
     ret = SUPERPAGE_PAGES;
 
 out_reset:
-    if ( reset )
-        p2m_set_entry(p2m, gfn, mfn0, 9, type0, p2m->default_access);
-    
+    /*
+     * This p2m_set_entry() call shouldn't be able to fail, since the same order
+     * on the same gfn succeeded above.  If that turns out to be false, crashing
+     * the domain should be the safest way of making sure we don't leak memory.
+     */
+    if ( reset && p2m_set_entry(p2m, gfn, mfn0, PAGE_ORDER_2M,
+                                type0, p2m->default_access) )
+    {
+        ASSERT_UNREACHABLE();
+        domain_crash(d);
+    }
+
 out:
     gfn_unlock(p2m, gfn, SUPERPAGE_ORDER);
     return ret;
@@ -846,19 +856,30 @@ p2m_pod_zero_check(struct p2m_domain *p2m, unsigned long *gfns, int count)
         }
 
         /* Try to remove the page, restoring old mapping if it fails. */
-        p2m_set_entry(p2m, gfns[i], _mfn(INVALID_MFN), PAGE_ORDER_4K,
-                      p2m_populate_on_demand, p2m->default_access);
+        if ( p2m_set_entry(p2m, gfns[i], _mfn(INVALID_MFN), PAGE_ORDER_4K,
+                           p2m_populate_on_demand, p2m->default_access) )
+            goto skip;
 
         /* See if the page was successfully unmapped.  (Allow one refcount
          * for being allocated to a domain.) */
         if ( (mfn_to_page(mfns[i])->count_info & PGC_count_mask) > 1 )
         {
+            /*
+             * If the previous p2m_set_entry call succeeded, this one shouldn't
+             * be able to fail.  If it does, crashing the domain should be safe.
+             */
+            if ( p2m_set_entry(p2m, gfns[i], mfns[i], PAGE_ORDER_4K,
+                               types[i], p2m->default_access) )
+            {
+                ASSERT_UNREACHABLE();
+                domain_crash(d);
+                goto out_unmap;
+            }
+
+        skip:
             unmap_domain_page(map[i]);
             map[i] = NULL;
 
-            p2m_set_entry(p2m, gfns[i], mfns[i], PAGE_ORDER_4K,
-                types[i], p2m->default_access);
-
             continue;
         }
     }
@@ -875,12 +896,25 @@ p2m_pod_zero_check(struct p2m_domain *p2m, unsigned long *gfns, int count)
 
         unmap_domain_page(map[i]);
 
-        /* See comment in p2m_pod_zero_check_superpage() re gnttab
-         * check timing.  */
-        if ( j < PAGE_SIZE/sizeof(*map[i]) )
+        map[i] = NULL;
+
+        /*
+         * See comment in p2m_pod_zero_check_superpage() re gnttab
+         * check timing.
+         */
+        if ( j < (PAGE_SIZE / sizeof(*map[i])) )
         {
-            p2m_set_entry(p2m, gfns[i], mfns[i], PAGE_ORDER_4K,
-                types[i], p2m->default_access);
+            /*
+             * If the previous p2m_set_entry call succeeded, this one shouldn't
+             * be able to fail.  If it does, crashing the domain should be safe.
+             */
+            if ( p2m_set_entry(p2m, gfns[i], mfns[i], PAGE_ORDER_4K,
+                               types[i], p2m->default_access) )
+            {
+                ASSERT_UNREACHABLE();
+                domain_crash(d);
+                goto out_unmap;
+            }
         }
         else
         {
@@ -904,7 +938,17 @@ p2m_pod_zero_check(struct p2m_domain *p2m, unsigned long *gfns, int count)
             p2m->pod.entry_count++;
         }
     }
-    
+
+    return;
+
+out_unmap:
+    /*
+     * Something went wrong, probably crashing the domain.  Unmap
+     * everything and return.
+     */
+    for ( i = 0; i < count; i++ )
+        if ( map[i] )
+            unmap_domain_page(map[i]);
 }
 
 #define POD_SWEEP_LIMIT 1024