x86/pod: prevent infinite loop when shattering large pages

When populating pages, the PoD may need to split large ones using
p2m_set_entry and request the caller to retry (see ept_get_entry for
instance).

p2m_set_entry may fail to shatter if it is not possible to allocate
memory for the new page table. However, the error is not propagated
resulting to the callers to retry infinitely the PoD.

Prevent the infinite loop by return false when it is not possible to
shatter the large mapping.

This is XSA-246.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
master commit: a1c6c6768971ea387d7eba0803908ef0928b43ac
master date: 2017-11-28 13:11:55 +0100

diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
index b7ab16916baec3624ce1cdc2b41d65bcf9d3f793..87082cf65ff0bb61146b798ca87c13b89c5571c5 100644 (file)
--- a/xen/arch/x86/mm/p2m-pod.c
+++ b/xen/arch/x86/mm/p2m-pod.c
@@ -1073,9 +1073,8 @@ p2m_pod_demand_populate(struct p2m_domain *p2m, unsigned long gfn,
          * NOTE: In a fine-grained p2m locking scenario this operation
          * may need to promote its locking from gfn->1g superpage
          */
-        p2m_set_entry(p2m, gfn_aligned, _mfn(INVALID_MFN), PAGE_ORDER_2M,
-                      p2m_populate_on_demand, p2m->default_access);
-        return 0;
+        return p2m_set_entry(p2m, gfn_aligned, _mfn(INVALID_MFN), PAGE_ORDER_2M,
+                             p2m_populate_on_demand, p2m->default_access);
     }
 
     /* Only reclaim if we're in actual need of more cache. */
@@ -1106,8 +1105,12 @@ p2m_pod_demand_populate(struct p2m_domain *p2m, unsigned long gfn,
 
     gfn_aligned = (gfn >> order) << order;
 
-    p2m_set_entry(p2m, gfn_aligned, mfn, order, p2m_ram_rw,
-                  p2m->default_access);
+    if ( p2m_set_entry(p2m, gfn_aligned, mfn, order, p2m_ram_rw,
+                       p2m->default_access) )
+    {
+        p2m_pod_cache_add(p2m, p, order);
+        goto out_fail;
+    }
 
     for( i = 0; i < (1UL << order); i++ )
     {
@@ -1152,13 +1155,18 @@ remap_and_retry:
     BUG_ON(order != PAGE_ORDER_2M);
     pod_unlock(p2m);
 
-    /* Remap this 2-meg region in singleton chunks */
-    /* NOTE: In a p2m fine-grained lock scenario this might
-     * need promoting the gfn lock from gfn->2M superpage */
+    /*
+     * Remap this 2-meg region in singleton chunks. See the comment on the
+     * 1G page splitting path above for why a single call suffices.
+     *
+     * NOTE: In a p2m fine-grained lock scenario this might
+     * need promoting the gfn lock from gfn->2M superpage.
+     */
     gfn_aligned = (gfn>>order)<<order;
-    for(i=0; i<(1<<order); i++)
-        p2m_set_entry(p2m, gfn_aligned + i, _mfn(INVALID_MFN), PAGE_ORDER_4K,
-                      p2m_populate_on_demand, p2m->default_access);
+    if ( p2m_set_entry(p2m, gfn_aligned, _mfn(INVALID_MFN), PAGE_ORDER_4K,
+                       p2m_populate_on_demand, p2m->default_access) )
+        return -1;
+
     if ( tb_init_done )
     {
         struct {