mei: limit number of bytes in mei header.

The MEI message header provides only 9 bits for storing
the message size, limiting to 511.
In theory the host buffer (hbuf) can contain up to 1020 bytes
(limited by byte =  255 * 4)
With the current hardware and hbuf size 512, this is not a real issue,
but as hardening approach we enforce the limit.
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20200211160522.7562-1-tomas.winkler@intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/mei/client.c

@@ -1585,7 +1585,7 @@ int mei_cl_irq_write(struct mei_cl *cl, struct mei_cl_cb *cb,
 		goto err;
 	}
 
-	hbuf_len = mei_slots2data(hbuf_slots);
+	hbuf_len = mei_slots2data(hbuf_slots) & MEI_MSG_MAX_LEN_MASK;
 	dr_slots = mei_dma_ring_empty_slots(dev);
 	dr_len = mei_slots2data(dr_slots);
 
@@ -1718,7 +1718,7 @@ ssize_t mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb)
 		goto out;
 	}
 
-	hbuf_len = mei_slots2data(hbuf_slots);
+	hbuf_len = mei_slots2data(hbuf_slots) & MEI_MSG_MAX_LEN_MASK;
 	dr_slots = mei_dma_ring_empty_slots(dev);
 	dr_len =  mei_slots2data(dr_slots);
 

drivers/misc/mei/hw.h

@@ -209,6 +209,9 @@ struct mei_msg_hdr {
 	u32 extension[0];
 } __packed;
 
+/* The length is up to 9 bits */
+#define MEI_MSG_MAX_LEN_MASK GENMASK(9, 0)
+
 #define MEI_MSG_HDR_MAX 2
 
 struct mei_bus_message {