vhost: use USER_DS in vhost_worker thread

On some architectures address spaces are set up in a way that this is
not necessary to work properly but on some others (like s390) it is.
Make sure we operate on the user address space to allow copy_xxx_user()
from the vhost_worker() thread by setting it explicitly before calling
use_mm() and revert it after unuse_mm().

Signed-off-by: Jens Freimann <[email protected]>
Signed-off-by: Christian Borntraeger <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Jens Freimann

drivers/vhost/vhost.c

@@ -191,7 +191,9 @@ static int vhost_worker(void *data)
 	struct vhost_dev *dev = data;
 	struct vhost_work *work = NULL;
 	unsigned uninitialized_var(seq);
+	mm_segment_t oldfs = get_fs();
 
+	set_fs(USER_DS);
 	use_mm(dev->mm);
 
 	for (;;) {
@@ -229,6 +231,7 @@ static int vhost_worker(void *data)
 
 	}
 	unuse_mm(dev->mm);
+	set_fs(oldfs);
 	return 0;
 }