Flush writable pagetable state before emulating a PT

update. Avoids possibility of updating a PTE temporarily
marked writable by ptwr batching logic, which can corrupt
page reference counts. Aiee!

Signed-off-by: Keir Fraser <keir@xensource.com>

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index b37a16a4b0a3cdbfbd2d8f6beff3bc43476ebde9..79af9319a8c99524835625fe5ea4756a37ec4917 100644 (file)
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -3358,6 +3358,13 @@ int ptwr_do_page_fault(struct domain *d, unsigned long addr,
     return EXCRET_fault_fixed;
 
  emulate:
+    /*
+     * Cleaning up avoids emulating an update to a PTE that is temporarily
+     * marked writable (_PAGE_RW) by the batched ptwr logic. If this were
+     * performance critical then the check could compare addr against l1va's in
+     * ptwr_emulated_update(). Without this flush we can corrupt page refcnts!
+     */
+    cleanup_writable_pagetable(d);
     if ( x86_emulate_memop(guest_cpu_user_regs(), addr,
                            &ptwr_mem_emulator, BITS_PER_LONG/8) )
         return 0;