vt: selection, push console lock down

commit 4b70dd57a15d2f4685ac6e38056bad93e81e982f upstream.

We need to nest the console lock in sel_lock, so we have to push it down
a bit. Fortunately, the callers of set_selection_* just lock the console
lock around the function call. So moving it down is easy.

In the next patch, we switch the order.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Fixes: 07e6124a1a46 ("vt: selection, close sel_buffer race")
Link: https://lore.kernel.org/r/20200228115406.5735-1-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
CVE-2020-8648
Signed-off-by: Kevin F. Haggerty <haggertk@lineageos.org>
Change-Id: If732e6acb1aab4d19379e2494e10cf2085e6ac65

drivers/tty/vt/selection.c

@@ -157,7 +157,7 @@ static int store_utf8(u16 c, char *p)
  *	The entire selection process is managed under the console_lock. It's
  *	 a lot under the lock but its hardly a performance path
  */
-int set_selection(const struct tiocl_selection __user *sel, struct tty_struct *tty)
+static int __set_selection(const struct tiocl_selection __user *sel, struct tty_struct *tty)
 {
 	struct vc_data *vc = vc_cons[fg_console].d;
 	int sel_mode, new_sel_start, new_sel_end, spc;
@@ -333,6 +333,17 @@ unlock:
 	return ret;
 }
 
+int set_selection(const struct tiocl_selection __user *v, struct tty_struct *tty)
+{
+	int ret;
+
+	console_lock();
+	ret = __set_selection(v, tty);
+	console_unlock();
+
+	return ret;
+}
+
 /* Insert the contents of the selection buffer into the
  * queue of the tty associated with the current console.
  * Invoked by ioctl().

drivers/tty/vt/vt.c

@@ -2611,9 +2611,7 @@ int tioclinux(struct tty_struct *tty, unsigned long arg)
 	switch (type)
 	{
 		case TIOCL_SETSEL:
-			console_lock();
 			ret = set_selection((struct tiocl_selection __user *)(p+1), tty);
-			console_unlock();
 			break;
 		case TIOCL_PASTESEL:
 			ret = paste_selection(tty);