xen/balloon: flush persistent kmaps in correct position

Xen balloon driver will update ballooned out pages' P2M entries to point
to scratch page for PV guests. In 24f69373e2 ("xen/balloon: don't alloc
page while non-preemptible", kmap_flush_unused was moved after updating
P2M table. In that case for 32 bit PV guest we might end up with

  P2M    X -----> S  (S is mfn of balloon scratch page)
  M2P    Y -----> X  (Y is mfn in persistent kmap entry)

kmap_flush_unused() iterates through all the PTEs in the kmap address
space, using pte_to_page() to obtain the page. If the p2m and the m2p
are inconsistent the incorrect page is returned.  This will clear
page->address on the wrong page which may cause subsequent oopses if
that page is currently kmap'ed.

Move the flush back between get_page and __set_phys_to_machine to fix
this.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Cc: stable@vger.kernel.org # 3.12+

1 files changed, 18 insertions, 6 deletions

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 37d06ea624aa..61a6ac8fa8fc 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -399,11 +399,25 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
                         state = BP_EAGAIN;
                         break;
                 }
+                scrub_page(page);
 
-                pfn = page_to_pfn(page);
-                frame_list[i] = pfn_to_mfn(pfn);
+                frame_list[i] = page_to_pfn(page);
+        }
 
-                scrub_page(page);
+        /*
+         * Ensure that ballooned highmem pages don't have kmaps.
+         *
+         * Do this before changing the p2m as kmap_flush_unused()
+         * reads PTEs to obtain pages (and hence needs the original
+         * p2m entry).
+         */
+        kmap_flush_unused();
+
+        /* Update direct mapping, invalidate P2M, and add to balloon. */
+        for (i = 0; i < nr_pages; i++) {
+                pfn = frame_list[i];
+                frame_list[i] = pfn_to_mfn(pfn);
+                page = pfn_to_page(pfn);
 
 #ifdef CONFIG_XEN_HAVE_PVMMU
                 /*
@@ -429,11 +443,9 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
                 }
 #endif
 
-                balloon_append(pfn_to_page(pfn));
+                balloon_append(page);
         }
 
-        /* Ensure that ballooned highmem pages don't have kmaps. */
-        kmap_flush_unused();
         flush_tlb_all();
 
         set_xen_guest_handle(reservation.extent_start, frame_list);