[char-misc 4.2] mei: do not access freed cb in blocking write

[Tomas Winkler <tomas.winkler@intel.com>]

From: Alexander Usyskin <alexander.usyskin@intel.com>

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>
---
 drivers/misc/mei/client.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/misc/mei/client.c b/drivers/misc/mei/client.c
index 6decbe136ea7..1987a201c7f3 100644
--- a/drivers/misc/mei/client.c
+++ b/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 @@ out:
 	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 @@ out:
 		}
 	}
 
-	rets = buf->size;
+	rets = size;
 err:
 	cl_dbg(dev, cl, "rpm: autosuspend\n");
 	pm_runtime_mark_last_busy(dev->dev);
-- 
2.4.3