mei: do not access freed cb in blocking write

The mei_cl_write function is giving up on a write cb ownership after it
was sent or queued. The write cb is then freed in the completion
handler. Especially during blocking write mei_cl_write function waits
for the completion handler and then access the freed memory to fetch the
written size.  The quick fix is to store the buffer size prior to
sending, the size is not altered during the flow.
Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/client.c

@@ -1356,6 +1356,7 @@ int mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb, bool blocking)
 	struct mei_device *dev;
 	struct mei_msg_data *buf;
 	struct mei_msg_hdr mei_hdr;
+	int size;
 	int rets;
 
 
@@ -1367,10 +1368,10 @@ int mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb, bool blocking)
 
 	dev = cl->dev;
 
-
 	buf = &cb->buf;
+	size = buf->size;
 
-	cl_dbg(dev, cl, "size=%d\n", buf->size);
+	cl_dbg(dev, cl, "size=%d\n", size);
 
 	rets = pm_runtime_get(dev->dev);
 	if (rets < 0 && rets != -EINPROGRESS) {
@@ -1394,21 +1395,21 @@ int mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb, bool blocking)
 
 	if (rets == 0) {
 		cl_dbg(dev, cl, "No flow control credentials: not sending.\n");
-		rets = buf->size;
+		rets = size;
 		goto out;
 	}
 	if (!mei_hbuf_acquire(dev)) {
 		cl_dbg(dev, cl, "Cannot acquire the host buffer: not sending.\n");
-		rets = buf->size;
+		rets = size;
 		goto out;
 	}
 
 	/* Check for a maximum length */
-	if (buf->size > mei_hbuf_max_len(dev)) {
+	if (size > mei_hbuf_max_len(dev)) {
 		mei_hdr.length = mei_hbuf_max_len(dev);
 		mei_hdr.msg_complete = 0;
 	} else {
-		mei_hdr.length = buf->size;
+		mei_hdr.length = size;
 		mei_hdr.msg_complete = 1;
 	}
 
@@ -1430,6 +1431,7 @@ int mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb, bool blocking)
 	else
 		list_add_tail(&cb->list, &dev->write_list.list);
 
+	cb = NULL;
 	if (blocking && cl->writing_state != MEI_WRITE_COMPLETE) {
 
 		mutex_unlock(&dev->device_lock);
@@ -1444,7 +1446,7 @@ int mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb, bool blocking)
 		}
 	}
 
-	rets = buf->size;
+	rets = size;
 err:
 	cl_dbg(dev, cl, "rpm: autosuspend\n");
 	pm_runtime_mark_last_busy(dev->dev);