USB serial ftdi_sio: Prevent userspace DoS (CVE-2006-2936)

This patch limits the amount of outstanding 'write' data that can be
queued up for the ftdi_sio driver, to prevent userspace DoS attacks (or
simple accidents) that use up all the system memory by writing lots of
data to the serial port.

The original patch was by Guillaume Autran, who in turn based it on the
same mechanism implemented in the 'visor' driver.  I (Ian Abbott)
re-targeted the patch to the latest sources, fixed a couple of errors,
renamed his new structure members, and updated the implementations of
the 'write_room' and 'chars_in_buffer' methods to take account of the
number of outstanding 'write' bytes.  It seems to work fine, though at
low baud rates it is still possible to queue up an amount of data that
takes an age to shift (a job for another day!).
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/ftdi_sio.c

@@ -553,6 +553,10 @@ struct ftdi_private {
 
 	int force_baud;		/* if non-zero, force the baud rate to this value */
 	int force_rtscts;	/* if non-zero, force RTS-CTS to always be enabled */
+
+	spinlock_t tx_lock;	/* spinlock for transmit state */
+	unsigned long tx_outstanding_bytes;
+	unsigned long tx_outstanding_urbs;
 };
 
 /* Used for TIOCMIWAIT */
@@ -626,6 +630,9 @@ static struct usb_serial_driver ftdi_sio_device = {
 #define HIGH 1
 #define LOW 0
 
+/* number of outstanding urbs to prevent userspace DoS from happening */
+#define URB_UPPER_LIMIT	42
+
 /*
  * ***************************************************************************
  * Utlity functions
@@ -1156,6 +1163,7 @@ static int ftdi_sio_attach (struct usb_serial *serial)
 	}
 
 	spin_lock_init(&priv->rx_lock);
+	spin_lock_init(&priv->tx_lock);
         init_waitqueue_head(&priv->delta_msr_wait);
 	/* This will push the characters through immediately rather
 	   than queue a task to deliver them */
@@ -1372,6 +1380,7 @@ static int ftdi_write (struct usb_serial_port *port,
 	int data_offset ;       /* will be 1 for the SIO and 0 otherwise */
 	int status;
 	int transfer_size;
+	unsigned long flags;
 
 	dbg("%s port %d, %d bytes", __FUNCTION__, port->number, count);
 
@@ -1379,6 +1388,13 @@ static int ftdi_write (struct usb_serial_port *port,
 		dbg("write request of 0 bytes");
 		return 0;
 	}
+	spin_lock_irqsave(&priv->tx_lock, flags);
+	if (priv->tx_outstanding_urbs > URB_UPPER_LIMIT) {
+		spin_unlock_irqrestore(&priv->tx_lock, flags);
+		dbg("%s - write limit hit\n", __FUNCTION__);
+		return 0;
+	}
+	spin_unlock_irqrestore(&priv->tx_lock, flags);
 	
 	data_offset = priv->write_offset;
         dbg("data_offset set to %d",data_offset);
@@ -1445,6 +1461,11 @@ static int ftdi_write (struct usb_serial_port *port,
 		err("%s - failed submitting write urb, error %d", __FUNCTION__, status);
 		count = status;
 		kfree (buffer);
+	} else {
+		spin_lock_irqsave(&priv->tx_lock, flags);
+		++priv->tx_outstanding_urbs;
+		priv->tx_outstanding_bytes += count;
+		spin_unlock_irqrestore(&priv->tx_lock, flags);
 	}
 
 	/* we are done with this urb, so let the host driver
@@ -1460,7 +1481,11 @@ static int ftdi_write (struct usb_serial_port *port,
 
 static void ftdi_write_bulk_callback (struct urb *urb, struct pt_regs *regs)
 {
+	unsigned long flags;
 	struct usb_serial_port *port = (struct usb_serial_port *)urb->context;
+	struct ftdi_private *priv;
+	int data_offset;       /* will be 1 for the SIO and 0 otherwise */
+	unsigned long countback;
 
 	/* free up the transfer buffer, as usb_free_urb() does not do this */
 	kfree (urb->transfer_buffer);
@@ -1472,34 +1497,67 @@ static void ftdi_write_bulk_callback (struct urb *urb, struct pt_regs *regs)
 		return;
 	}
 
+	priv = usb_get_serial_port_data(port);
+	if (!priv) {
+		dbg("%s - bad port private data pointer - exiting", __FUNCTION__);
+		return;
+	}
+	/* account for transferred data */
+	countback = urb->actual_length;
+	data_offset = priv->write_offset;
+	if (data_offset > 0) {
+		/* Subtract the control bytes */
+		countback -= (data_offset * ((countback + (PKTSZ - 1)) / PKTSZ));
+	}
+	spin_lock_irqsave(&priv->tx_lock, flags);
+	--priv->tx_outstanding_urbs;
+	priv->tx_outstanding_bytes -= countback;
+	spin_unlock_irqrestore(&priv->tx_lock, flags);
+
 	schedule_work(&port->work);
 } /* ftdi_write_bulk_callback */
 
 
 static int ftdi_write_room( struct usb_serial_port *port )
 {
+	struct ftdi_private *priv = usb_get_serial_port_data(port);
+	int room;
+	unsigned long flags;
+
 	dbg("%s - port %d", __FUNCTION__, port->number);
 
-	/*
-	 * We really can take anything the user throws at us
-	 * but let's pick a nice big number to tell the tty
-	 * layer that we have lots of free space
-	 */
-	return 2048;
+	spin_lock_irqsave(&priv->tx_lock, flags);
+	if (priv->tx_outstanding_urbs < URB_UPPER_LIMIT) {
+		/*
+		 * We really can take anything the user throws at us
+		 * but let's pick a nice big number to tell the tty
+		 * layer that we have lots of free space
+		 */
+		room = 2048;
+	} else {
+		room = 0;
+	}
+	spin_unlock_irqrestore(&priv->tx_lock, flags);
+	return room;
 } /* ftdi_write_room */
 
 
 static int ftdi_chars_in_buffer (struct usb_serial_port *port)
 { /* ftdi_chars_in_buffer */
+	struct ftdi_private *priv = usb_get_serial_port_data(port);
+	int buffered;
+	unsigned long flags;
+
 	dbg("%s - port %d", __FUNCTION__, port->number);
 
-	/* 
-	 * We can't really account for how much data we
-	 * have sent out, but hasn't made it through to the
-	 * device, so just tell the tty layer that everything
-	 * is flushed.
-	 */
-	return 0;
+	spin_lock_irqsave(&priv->tx_lock, flags);
+	buffered = (int)priv->tx_outstanding_bytes;
+	spin_unlock_irqrestore(&priv->tx_lock, flags);
+	if (buffered < 0) {
+		err("%s outstanding tx bytes is negative!", __FUNCTION__);
+		buffered = 0;
+	}
+	return buffered;
 } /* ftdi_chars_in_buffer */