[PATCH] drivers/char/lp.c race fix

In lp_write(), copy_from_user() is called to copy data into a statically
allocated kernel buffer before down_interruptible() is called.  If a second
thread of execution comes in between the copy_from_user() and the
down_interruptible() calls, silent data corruption could result.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/lp.c

@@ -314,12 +314,14 @@ static ssize_t lp_write(struct file * file, const char __user * buf,
 	if (copy_size > LP_BUFFER_SIZE)
 		copy_size = LP_BUFFER_SIZE;
 
-	if (copy_from_user (kbuf, buf, copy_size))
-		return -EFAULT;
-
 	if (down_interruptible (&lp_table[minor].port_mutex))
 		return -EINTR;
 
+	if (copy_from_user (kbuf, buf, copy_size)) {
+		retv = -EFAULT;
+		goto out_unlock;
+	}
+
  	/* Claim Parport or sleep until it becomes available
  	 */
 	lp_claim_parport_or_block (&lp_table[minor]);
@@ -398,7 +400,7 @@ static ssize_t lp_write(struct file * file, const char __user * buf,
 		lp_table[minor].current_mode = IEEE1284_MODE_COMPAT;
 		lp_release_parport (&lp_table[minor]);
 	}
-
+out_unlock:
 	up (&lp_table[minor].port_mutex);
 
  	return retv;