fix serial buffer memory leak

Patch c5c34d4862e18ef07c1276d233507f540fb5a532 (tty: flush flip buffer on
ldisc input queue flush) introduces a race condition which can lead to memory
leaks.

The problem can be triggered when tcflush() is called when data are being
pushed to the line discipline driver by flush_to_ldisc().

flush_to_ldisc() releases tty->buf.lock when calling the line discipline
receive_buf function. At that poing tty_buffer_flush() kicks in and sets both
tty->buf.head and tty->buf.tail to NULL. When flush_to_ldisc() finishes, it
restores tty->buf.head but doesn't touch tty->buf.tail. This corrups the
buffer queue, and the next call to tty_buffer_request_room() will allocate a
new buffer and overwrite tty->buf.head. The previous buffer is then lost
forever without being released.

(Thanks to Laurent for the above text, for finding, disgnosing and reporting
the bug)

- Use tty->flags bits for the flush status.

- Wait for the flag to clear again before returning

- Fix the doc error noted

- Fix flush of empty queue leaving stale flushpending

[[e-mail]: cleanup]
Signed-off-by: Alan Cox <[e-mail]>
Acked-by: Paul Fulghum <[e-mail]>
Cc: Laurent Pinchart <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/tty_io.c

···
 }
 
 /**
+ *	__tty_buffer_flush		-	flush full tty buffers
+ *	@tty: tty to flush
+ *
+ *	flush all the buffers containing receive data. Caller must
+ *	hold the buffer lock and must have ensured no parallel flush to
+ *	ldisc is running.
+ *
+ *	Locking: Caller must hold tty->buf.lock
+ */
+
+static void __tty_buffer_flush(struct tty_struct *tty)
+{
+	struct tty_buffer *thead;
+
+	while((thead = tty->buf.head) != NULL) {
+		tty->buf.head = thead->next;
+		tty_buffer_free(tty, thead);
+	}
+	tty->buf.tail = NULL;
+}
+
+/**
  *	tty_buffer_flush		-	flush full tty buffers
  *	@tty: tty to flush
  *
- *	flush all the buffers containing receive data
+ *	flush all the buffers containing receive data. If the buffer is
+ *	being processed by flush_to_ldisc then we defer the processing
+ *	to that function
  *
  *	Locking: none
  */
 
 static void tty_buffer_flush(struct tty_struct *tty)
 {
-	struct tty_buffer *thead;
 	unsigned long flags;
-
 	spin_lock_irqsave(&tty->buf.lock, flags);
-	while((thead = tty->buf.head) != NULL) {
-		tty->buf.head = thead->next;
-		tty_buffer_free(tty, thead);
-	}
-	tty->buf.tail = NULL;
+
+	/* If the data is being pushed to the tty layer then we can't
+	   process it here. Instead set a flag and the flush_to_ldisc
+	   path will process the flush request before it exits */
+	if (test_bit(TTY_FLUSHING, &tty->flags)) {
+		set_bit(TTY_FLUSHPENDING, &tty->flags);
+		spin_unlock_irqrestore(&tty->buf.lock, flags);
+		wait_event(tty->read_wait,
+				test_bit(TTY_FLUSHPENDING, &tty->flags) == 0);
+		return;
+	} else
+		__tty_buffer_flush(tty);
 	spin_unlock_irqrestore(&tty->buf.lock, flags);
 }
 
···
 		return;
 
 	spin_lock_irqsave(&tty->buf.lock, flags);
+	set_bit(TTY_FLUSHING, &tty->flags);	/* So we know a flush is running */
 	head = tty->buf.head;
 	if (head != NULL) {
 		tty->buf.head = NULL;
···
 				tty_buffer_free(tty, tbuf);
 				continue;
 			}
+			/* Ldisc or user is trying to flush the buffers
+			   we are feeding to the ldisc, stop feeding the
+			   line discipline as we want to empty the queue */
+			if (test_bit(TTY_FLUSHPENDING, &tty->flags))
+				break;
 			if (!tty->receive_room) {
 				schedule_delayed_work(&tty->buf.work, 1);
 				break;
···
 			disc->receive_buf(tty, char_buf, flag_buf, count);
 			spin_lock_irqsave(&tty->buf.lock, flags);
 		}
+		/* Restore the queue head */
 		tty->buf.head = head;
 	}
+	/* We may have a deferred request to flush the input buffer,
+	   if so pull the chain under the lock and empty the queue */
+	if (test_bit(TTY_FLUSHPENDING, &tty->flags)) {
+		__tty_buffer_flush(tty);
+		clear_bit(TTY_FLUSHPENDING, &tty->flags);
+		wake_up(&tty->read_wait);
+	}
+	clear_bit(TTY_FLUSHING, &tty->flags);
 	spin_unlock_irqrestore(&tty->buf.lock, flags);
 
 	tty_ldisc_deref(disc);

include/linux/tty.h

···
 #define TTY_PTY_LOCK 		16	/* pty private */
 #define TTY_NO_WRITE_SPLIT 	17	/* Preserve write boundaries to driver */
 #define TTY_HUPPED 		18	/* Post driver->hangup() */
+#define TTY_FLUSHING		19	/* Flushing to ldisc in progress */
+#define TTY_FLUSHPENDING	20	/* Queued buffer flush pending */
 
 #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))