scsi: sg: sg_write(): __get_user() can fail...

Link: https://lore.kernel.org/r/20191017193925.25539-3-viro@ZenIV.linux.org.ukSigned-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/sg.c

@@ -640,13 +640,15 @@ sg_write(struct file *filp, const char __user *buf, size_t count, loff_t * ppos)
 	if (count < (SZ_SG_HEADER + 6))
 		return -EIO;	/* The minimum scsi command length is 6 bytes. */
 
+	buf += SZ_SG_HEADER;
+	if (__get_user(opcode, buf))
+		return -EFAULT;
+
 	if (!(srp = sg_add_request(sfp))) {
 		SCSI_LOG_TIMEOUT(1, sg_printk(KERN_INFO, sdp,
 					      "sg_write: queue full\n"));
 		return -EDOM;
 	}
-	buf += SZ_SG_HEADER;
-	__get_user(opcode, buf);
 	mutex_lock(&sfp->f_mutex);
 	if (sfp->next_cmd_len > 0) {
 		cmd_size = sfp->next_cmd_len;