File xsa318.patch of Package xen.23721

Subject: gnttab: fix GNTTABOP_copy continuation handling
From: Jan Beulich jbeulich@suse.com Tue Apr 14 15:21:36 2020 +0200
Date: Tue Apr 14 15:21:36 2020 +0200:
Git: 45c90737d5f0c8bf479adcd8cb88450f1998e55c

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 3027478e30..90c09974f1 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -3256,8 +3256,7 @@ 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;
@@ -3323,6 +3322,9 @@ 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",