[SCSI] erase invalid data returned by device

This patch (as1108) fixes a problem that can occur with certain USB
mass-storage devices: They return invalid data together with a residue
indicating that the data should be ignored.  Rather than leave the
invalid data in a transfer buffer, where it can get misinterpreted,
the patch clears the invalid portion of the buffer.

This solves a problem (wrong write-protect setting detected) reported
by Maciej Rutecki and Peter Teoh.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Peter Teoh <htmldeveloper@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/scsi_lib.c

@@ -207,6 +207,15 @@ int scsi_execute(struct scsi_device *sdev, const unsigned char *cmd,
 	 */
 	blk_execute_rq(req->q, NULL, req, 1);
 
+	/*
+	 * Some devices (USB mass-storage in particular) may transfer
+	 * garbage data together with a residue indicating that the data
+	 * is invalid.  Prevent the garbage from being misinterpreted
+	 * and prevent security leaks by zeroing out the excess data.
+	 */
+	if (unlikely(req->data_len > 0 && req->data_len <= bufflen))
+		memset(buffer + (bufflen - req->data_len), 0, req->data_len);
+
 	ret = req->errors;
  out:
 	blk_put_request(req);