USB: fix usb-serial generic recursive lock

Nobody should be using the generic usb-serial for anything other than
testing. Still, it's not a good thing that it's easy to lock up. There
is a traceback from NMI oopser here:
 https://bugzilla.redhat.com/show_bug.cgi?id=431379

But in short, if a line discipline has a chance to echo anything, input
can loop back a write method. So, don't call tty_flip_buffer_push from
under a lock taken on write path.

Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
index 97fa3c428435..7cfce9dabb90 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -323,7 +323,7 @@ static void flush_and_resubmit_read_urb (struct usb_serial_port *port)
                 room = tty_buffer_request_room(tty, urb->actual_length);
                 if (room) {
                         tty_insert_flip_string(tty, urb->transfer_buffer, room);
-                        tty_flip_buffer_push(tty); /* is this allowed from an URB callback ? */
+                        tty_flip_buffer_push(tty);
                 }
         }
 
@@ -349,10 +349,12 @@ void usb_serial_generic_read_bulk_callback (struct urb *urb)
 
         /* Throttle the device if requested by tty */
         spin_lock_irqsave(&port->lock, flags);
-        if (!(port->throttled = port->throttle_req))
-                /* Handle data and continue reading from device */
+        if (!(port->throttled = port->throttle_req)) {
+                spin_unlock_irqrestore(&port->lock, flags);
                 flush_and_resubmit_read_urb(port);
-        spin_unlock_irqrestore(&port->lock, flags);
+        } else {
+                spin_unlock_irqrestore(&port->lock, flags);
+        }
 }
 EXPORT_SYMBOL_GPL(usb_serial_generic_read_bulk_callback);