can: peak_usb: fix a potential out-of-sync while decoding packets

When decoding a buffer received from PCAN-USB, the first timestamp read in
a packet is a 16-bit coded time base, and the next ones are an 8-bit
offset to this base, regardless of the type of packet read.

This patch corrects a potential loss of synchronization by using a
timestamp index read from the buffer, rather than an index of received
data packets, to determine on the sizeof the timestamp to be read from the
packet being decoded.
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Fixes: 46be265d ("can: usb: PEAK-System Technik PCAN-USB specific part")
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/usb/peak_usb/pcan_usb.c

@@ -100,7 +100,7 @@ struct pcan_usb_msg_context {
 	u8 *end;
 	u8 rec_cnt;
 	u8 rec_idx;
-	u8 rec_data_idx;
+	u8 rec_ts_idx;
 	struct net_device *netdev;
 	struct pcan_usb *pdev;
 };
@@ -547,10 +547,15 @@ static int pcan_usb_decode_status(struct pcan_usb_msg_context *mc,
 	mc->ptr += PCAN_USB_CMD_ARGS;
 
 	if (status_len & PCAN_USB_STATUSLEN_TIMESTAMP) {
-		int err = pcan_usb_decode_ts(mc, !mc->rec_idx);
+		int err = pcan_usb_decode_ts(mc, !mc->rec_ts_idx);
 
 		if (err)
 			return err;
+
+		/* Next packet in the buffer will have a timestamp on a single
+		 * byte
+		 */
+		mc->rec_ts_idx++;
 	}
 
 	switch (f) {
@@ -632,10 +637,13 @@ static int pcan_usb_decode_data(struct pcan_usb_msg_context *mc, u8 status_len)
 
 	cf->can_dlc = get_can_dlc(rec_len);
 
-	/* first data packet timestamp is a word */
-	if (pcan_usb_decode_ts(mc, !mc->rec_data_idx))
+	/* Only first packet timestamp is a word */
+	if (pcan_usb_decode_ts(mc, !mc->rec_ts_idx))
 		goto decode_failed;
 
+	/* Next packet in the buffer will have a timestamp on a single byte */
+	mc->rec_ts_idx++;
+
 	/* read data */
 	memset(cf->data, 0x0, sizeof(cf->data));
 	if (status_len & PCAN_USB_STATUSLEN_RTR) {
@@ -688,7 +696,6 @@ static int pcan_usb_decode_msg(struct peak_usb_device *dev, u8 *ibuf, u32 lbuf)
 		/* handle normal can frames here */
 		} else {
 			err = pcan_usb_decode_data(&mc, sl);
-			mc.rec_data_idx++;
 		}
 	}