[xen master] gnttab: fix GNTTABOP_copy continuation handling

[patchbot@xxxxxxx]

commit d6f22d5d9e8d6848ec229083ac9fb044f0adea93
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Apr 14 14:42:32 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Apr 14 14:42:32 2020 +0200

    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@xxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    Tested-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
---
 xen/common/grant_table.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 4b5344dc21..96080b3dec 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -3576,8 +3576,7 @@ 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;
@@ -3644,6 +3643,9 @@ 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",
--
generated by git-patchbot for /home/xen/git/xen.git#master