um: fix oopsable race in line_close()

tty->count is decremented only after ->close() had been called and
several tasks can hit it in parallel.  As the result, using tty->count
to check if you are the last one is broken.  We end up leaving line->tty
not reset to NULL and the next IRQ on that sucker will blow up trying to
dereference pointers from kfree'd struct tty.

Fix is obvious: we need to use a counter of our own.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/line.c

@@ -399,8 +399,8 @@ int line_setup_irq(int fd, int input, int output, struct line *line, void *data)
  * is done under a spinlock.  Checking whether the device is in use is
  * line->tty->count > 1, also under the spinlock.
  *
- * tty->count serves to decide whether the device should be enabled or
- * disabled on the host.  If it's equal to 1, then we are doing the
+ * line->count serves to decide whether the device should be enabled or
+ * disabled on the host.  If it's equal to 0, then we are doing the
  * first open or last close.  Otherwise, open and close just return.
  */
 
@@ -414,16 +414,16 @@ int line_open(struct line *lines, struct tty_struct *tty)
 		goto out_unlock;
 
 	err = 0;
-	if (tty->count > 1)
+	if (line->count++)
 		goto out_unlock;
 
-	spin_unlock(&line->count_lock);
-
+	BUG_ON(tty->driver_data);
 	tty->driver_data = line;
 	line->tty = tty;
 
+	spin_unlock(&line->count_lock);
 	err = enable_chan(line);
-	if (err)
+	if (err) /* line_close() will be called by our caller */
 		return err;
 
 	INIT_DELAYED_WORK(&line->task, line_timer_cb);
@@ -436,7 +436,7 @@ int line_open(struct line *lines, struct tty_struct *tty)
 	chan_window_size(&line->chan_list, &tty->winsize.ws_row,
 			 &tty->winsize.ws_col);
 
-	return err;
+	return 0;
 
 out_unlock:
 	spin_unlock(&line->count_lock);
@@ -460,17 +460,16 @@ void line_close(struct tty_struct *tty, struct file * filp)
 	flush_buffer(line);
 
 	spin_lock(&line->count_lock);
-	if (!line->valid)
-		goto out_unlock;
+	BUG_ON(!line->valid);
 
-	if (tty->count > 1)
+	if (--line->count)
 		goto out_unlock;
 
-	spin_unlock(&line->count_lock);
-
 	line->tty = NULL;
 	tty->driver_data = NULL;
 
+	spin_unlock(&line->count_lock);
+
 	if (line->sigio) {
 		unregister_winch(tty);
 		line->sigio = 0;
@@ -498,7 +497,7 @@ static int setup_one_line(struct line *lines, int n, char *init, int init_prio,
 
 	spin_lock(&line->count_lock);
 
-	if (line->tty != NULL) {
+	if (line->count) {
 		*error_out = "Device is already open";
 		goto out;
 	}

arch/um/include/shared/line.h

@@ -33,6 +33,7 @@ struct line_driver {
 struct line {
 	struct tty_struct *tty;
 	spinlock_t count_lock;
+	unsigned long count;
 	int valid;
 
 	char *init_str;