[PATCH] USB: usbserial: race-condition fix.

There is a race-condition in usb-serial driver that can be triggered if
a processes does 'port->tty->driver_data = NULL' in serial_close() while
other processes is in kernel-space about to call serial_ioctl() on the
same port.

This happens because a process can open the device while there is
another one closing it.

The patch below fixes that by adding a semaphore to ensure that no
process will open the device while another process is closing it.

Note that we can't use spinlocks here, since serial_open() and
serial_close() can sleep.

Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 17 insertions, 1 deletions

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index 5bc023c73893..8bc8337c99c4 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -30,6 +30,7 @@
 #include <linux/list.h>
 #include <linux/smp_lock.h>
 #include <asm/uaccess.h>
+#include <asm/semaphore.h>
 #include <linux/usb.h>
 #include "usb-serial.h"
 #include "pl2303.h"
@@ -190,6 +191,9 @@ static int serial_open (struct tty_struct *tty, struct file * filp)
         port = serial->port[portNumber];
         if (!port)
                 return -ENODEV;
+
+        if (down_interruptible(&port->sem))
+                return -ERESTARTSYS;
          
         ++port->open_count;
 
@@ -215,6 +219,7 @@ static int serial_open (struct tty_struct *tty, struct file * filp)
                         goto bailout_module_put;
         }
 
+        up(&port->sem);
         return 0;
 
 bailout_module_put:
@@ -222,6 +227,7 @@ bailout_module_put:
 bailout_kref_put:
         kref_put(&serial->kref, destroy_serial);
         port->open_count = 0;
+        up(&port->sem);
         return retval;
 }
 
@@ -234,8 +240,10 @@ static void serial_close(struct tty_struct *tty, struct file * filp)
 
         dbg("%s - port %d", __FUNCTION__, port->number);
 
+        down(&port->sem);
+
         if (port->open_count == 0)
-                return;
+                goto out;
 
         --port->open_count;
         if (port->open_count == 0) {
@@ -253,6 +261,9 @@ static void serial_close(struct tty_struct *tty, struct file * filp)
         }
 
         kref_put(&port->serial->kref, destroy_serial);
+
+out:
+        up(&port->sem);
 }
 
 static int serial_write (struct tty_struct * tty, const unsigned char *buf, int count)
@@ -774,6 +785,7 @@ int usb_serial_probe(struct usb_interface *interface,
                 port->number = i + serial->minor;
                 port->serial = serial;
                 spin_lock_init(&port->lock);
+                sema_init(&port->sem, 1);
                 INIT_WORK(&port->work, usb_serial_port_softint, port);
                 serial->port[i] = port;
         }
diff --git a/drivers/usb/serial/usb-serial.h b/drivers/usb/serial/usb-serial.h
index 238a5a871ed6..d7d27c3385b3 100644
--- a/drivers/usb/serial/usb-serial.h
+++ b/drivers/usb/serial/usb-serial.h
@@ -16,6 +16,7 @@
 
 #include <linux/config.h>
 #include <linux/kref.h>
+#include <asm/semaphore.h>
 
 #define SERIAL_TTY_MAJOR        188     /* Nice legal number now */
 #define SERIAL_TTY_MINORS       255     /* loads of devices :) */
@@ -30,6 +31,8 @@
  * @serial: pointer back to the struct usb_serial owner of this port.
  * @tty: pointer to the corresponding tty for this port.
  * @lock: spinlock to grab when updating portions of this structure.
+ * @sem: semaphore used to synchronize serial_open() and serial_close()
+ *      access for this port.
  * @number: the number of the port (the minor number).
  * @interrupt_in_buffer: pointer to the interrupt in buffer for this port.
  * @interrupt_in_urb: pointer to the interrupt in struct urb for this port.
@@ -60,6 +63,7 @@ struct usb_serial_port {
         struct usb_serial *     serial;
         struct tty_struct *     tty;
         spinlock_t              lock;
+        struct semaphore        sem;
         unsigned char           number;
 
         unsigned char *         interrupt_in_buffer;