tty: Fix unsafe vt paste_selection()

Convert the tty_buffer_flush() exclusion mechanism to a
public interface - tty_buffer_lock/unlock_exclusive() - and use
the interface to safely write the paste selection to the line
discipline.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/vt/selection.c

@@ -24,6 +24,7 @@
 #include <linux/selection.h>
 #include <linux/tiocl.h>
 #include <linux/console.h>
+#include <linux/tty_flip.h>
 
 /* Don't take this from <ctype.h>: 011-015 on the screen aren't spaces */
 #define isspace(c)	((c) == ' ')
@@ -346,8 +347,8 @@ int paste_selection(struct tty_struct *tty)
 	console_unlock();
 
 	ld = tty_ldisc_ref_wait(tty);
+	tty_buffer_lock_exclusive(&vc->port);
 
-	/* FIXME: this is completely unsafe */
 	add_wait_queue(&vc->paste_wait, &wait);
 	while (sel_buffer && sel_buffer_lth > pasted) {
 		set_current_state(TASK_INTERRUPTIBLE);
@@ -363,6 +364,7 @@ int paste_selection(struct tty_struct *tty)
 	remove_wait_queue(&vc->paste_wait, &wait);
 	__set_current_state(TASK_RUNNING);
 
+	tty_buffer_unlock_exclusive(&vc->port);
 	tty_ldisc_deref(ld);
 	return 0;
 }