gnttab: fix GNTTABOP_copy continuation handling

The XSA-226 fix was flawed - the backwards transformation on rc was done
too early, causing a continuation to not get invoked when the need for
preemption was determined at the very first iteration of the request.
This in particular means that all of the status fields of the individual
operations would be left untouched, i.e. set to whatever the caller may
or may not have initialized them to.

This is part of XSA-318.

Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Tested-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
master commit: d6f22d5d9e8d6848ec229083ac9fb044f0adea93
master date: 2020-04-14 14:42:32 +0200

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index e272daef5a0fbe5fb3294b8ff90fe9b0f86136a0..7d464a96415730c5eec88eb0111e06503b5312b1 100644 (file)
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -3509,8 +3509,7 @@ do_grant_table_op(
         rc = gnttab_copy(copy, count);
         if ( rc > 0 )
         {
-            rc = count - rc;
-            guest_handle_add_offset(copy, rc);
+            guest_handle_add_offset(copy, count - rc);
             uop = guest_handle_cast(copy, void);
         }
         break;
@@ -3577,6 +3576,9 @@ do_grant_table_op(
   out:
     if ( rc > 0 || opaque_out != 0 )
     {
+        /* Adjust rc, see gnttab_copy() for why this is needed. */
+        if ( cmd == GNTTABOP_copy )
+            rc = count - rc;
         ASSERT(rc < count);
         ASSERT((opaque_out & GNTTABOP_CMD_MASK) == 0);
         rc = hypercall_create_continuation(__HYPERVISOR_grant_table_op, "ihi",