xen/gntdev: avoid out of bounds access in case of partial gntdev_mmap()

In case gntdev_mmap() succeeds only partially in mapping grant pages
it will leave some vital information uninitialized needed later for
cleanup. This will lead to an out of bounds array access when unmapping
the already mapped pages.

So just initialize the data needed for unmapping the pages a little bit
earlier.

Cc: <[email protected]>
Reported-by: Arthur Borsboom <[email protected]>
Signed-off-by: Juergen Gross <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
Signed-off-by: Boris Ostrovsky <[email protected]>

Author: jgross1

drivers/xen/gntdev.c

@@ -1024,6 +1024,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	mutex_unlock(&priv->lock);
 
 	if (use_ptemod) {
+		map->pages_vm_start = vma->vm_start;
 		err = apply_to_page_range(vma->vm_mm, vma->vm_start,
 					  vma->vm_end - vma->vm_start,
 					  find_grant_ptes, map);
@@ -1061,7 +1062,6 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 					    set_grant_ptes_as_special, NULL);
 		}
 #endif
-		map->pages_vm_start = vma->vm_start;
 	}
 
 	return 0;