mei: check for error returned from mei_hbuf_empty_slots()

mei_hbuf_empty_slots() may return with an error in case
of circular buffer overflow. This type of error may
be caused only by a bug. However currently, the error
won't be detected due signed type promotion in comparison to u32.
We add explicit check for less then zero and explicit cast
in comparison to suppress singn-compare warning.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/client.c

@@ -865,8 +865,10 @@ int mei_cl_irq_disconnect(struct mei_cl *cl, struct mei_cl_cb *cb,
 
 	msg_slots = mei_data2slots(sizeof(struct hbm_client_connect_request));
 	slots = mei_hbuf_empty_slots(dev);
+	if (slots < 0)
+		return -EOVERFLOW;
 
-	if (slots < msg_slots)
+	if ((u32)slots < msg_slots)
 		return -EMSGSIZE;
 
 	ret = mei_cl_send_disconnect(cl, cb);
@@ -1054,12 +1056,15 @@ int mei_cl_irq_connect(struct mei_cl *cl, struct mei_cl_cb *cb,
 	int rets;
 
 	msg_slots = mei_data2slots(sizeof(struct hbm_client_connect_request));
-	slots = mei_hbuf_empty_slots(dev);
 
 	if (mei_cl_is_other_connecting(cl))
 		return 0;
 
-	if (slots < msg_slots)
+	slots = mei_hbuf_empty_slots(dev);
+	if (slots < 0)
+		return -EOVERFLOW;
+
+	if ((u32)slots < msg_slots)
 		return -EMSGSIZE;
 
 	rets = mei_cl_send_connect(cl, cb);
@@ -1296,8 +1301,10 @@ int mei_cl_irq_notify(struct mei_cl *cl, struct mei_cl_cb *cb,
 
 	msg_slots = mei_data2slots(sizeof(struct hbm_client_connect_request));
 	slots = mei_hbuf_empty_slots(dev);
+	if (slots < 0)
+		return -EOVERFLOW;
 
-	if (slots < msg_slots)
+	if ((u32)slots < msg_slots)
 		return -EMSGSIZE;
 
 	request = mei_cl_notify_fop2req(cb->fop_type);
@@ -1573,6 +1580,9 @@ int mei_cl_irq_write(struct mei_cl *cl, struct mei_cl_cb *cb,
 	}
 
 	slots = mei_hbuf_empty_slots(dev);
+	if (slots < 0)
+		return -EOVERFLOW;
+
 	len = buf->size - cb->buf_idx;
 	msg_slots = mei_data2slots(len);
 
@@ -1581,11 +1591,11 @@ int mei_cl_irq_write(struct mei_cl *cl, struct mei_cl_cb *cb,
 	mei_hdr.reserved = 0;
 	mei_hdr.internal = cb->internal;
 
-	if (slots >= msg_slots) {
+	if ((u32)slots >= msg_slots) {
 		mei_hdr.length = len;
 		mei_hdr.msg_complete = 1;
 	/* Split the message only if we can write the whole host buffer */
-	} else if (slots == dev->hbuf_depth) {
+	} else if ((u32)slots == dev->hbuf_depth) {
 		msg_slots = slots;
 		len = (slots * sizeof(u32)) - sizeof(struct mei_msg_hdr);
 		mei_hdr.length = len;