p2m: Check return value of p2m_set_entry() when decreasing reservation

If the entire range specified to p2m_pod_decrease_reservation() is marked
populate-on-demand, then it will make a single p2m_set_entry() call,
reducing its PoD entry count.

Unfortunately, in the right circumstances, this p2m_set_entry() call
may fail.  It that case, repeated calls to decrease_reservation() may
cause p2m->pod.entry_count to fall below zero, potentially tripping
over BUG_ON()s to the contrary.

Instead, check to see if the entry succeeded, and return false if not.
The caller will then call guest_remove_page() on the gfns, which will
return -EINVAL upon finding no valid memory there to return.

Unfortunately if the order > 0, the entry may have partially changed.
A domain_crash() is probably the safest thing in that case.

Other p2m_set_entry() calls in the same function should be fine,
because they are writing the entry at its current order.  Nonetheless,
check the return value and crash if our assumption turns otu to be
wrong.

This is part of XSA-247.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: a3d64de8e86f5812917d2d0af28298f80debdf9a
master date: 2017-11-28 13:13:26 +0100

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index d2931c530646380736560f4e5228117b5a13e652..b29043c6a177bcf1609b627d59ecad0900364a66 100644 (file)
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -560,11 +560,23 @@ recount:
 
     if ( !nonpod )
     {
-        /* All PoD: Mark the whole region invalid and tell caller
-         * we're done. */
-        p2m_set_entry(p2m, gpfn, _mfn(INVALID_MFN), order, p2m_invalid,
-                      p2m->default_access);
-        p2m->pod.entry_count-=(1<<order);
+        /*
+         * All PoD: Mark the whole region invalid and tell caller
+         * we're done.
+         */
+        if ( p2m_set_entry(p2m, gpfn, _mfn(INVALID_MFN), order, p2m_invalid,
+                           p2m->default_access) )
+        {
+            /*
+             * If this fails, we can't tell how much of the range was changed.
+             * Best to crash the domain unless we're sure a partial change is
+             * impossible.
+             */
+            if ( order != 0 )
+                domain_crash(d);
+            goto out_unlock;
+        }
+        p2m->pod.entry_count -= 1UL << order;
         BUG_ON(p2m->pod.entry_count < 0);
         ret = 1;
         goto out_entry_check;
@@ -596,8 +608,14 @@ recount:
         mfn = p2m->get_entry(p2m, gpfn + i, &t, &a, 0, NULL);
         if ( t == p2m_populate_on_demand )
         {
-            p2m_set_entry(p2m, gpfn + i, _mfn(INVALID_MFN), 0, p2m_invalid,
-                          p2m->default_access);
+            /* This shouldn't be able to fail */
+            if ( p2m_set_entry(p2m, gpfn + i, _mfn(INVALID_MFN), 0,
+                               p2m_invalid, p2m->default_access) )
+            {
+                ASSERT_UNREACHABLE();
+                domain_crash(d);
+                goto out_unlock;
+            }
             p2m->pod.entry_count--;
             BUG_ON(p2m->pod.entry_count < 0);
             pod--;
@@ -610,8 +628,14 @@ recount:
 
             page = mfn_to_page(mfn);
 
-            p2m_set_entry(p2m, gpfn + i, _mfn(INVALID_MFN), 0, p2m_invalid,
-                          p2m->default_access);
+            /* This shouldn't be able to fail */
+            if ( p2m_set_entry(p2m, gpfn + i, _mfn(INVALID_MFN), 0,
+                               p2m_invalid, p2m->default_access) )
+            {
+                ASSERT_UNREACHABLE();
+                domain_crash(d);
+                goto out_unlock;
+            }
             set_gpfn_from_mfn(mfn_x(mfn), INVALID_M2P_ENTRY);
 
             p2m_pod_cache_add(p2m, page, 0);