devpts_get_tty() should validate inode

devpts_get_tty() assumes that the inode passed in is associated with a valid
pty.  But if the only reference to the pty is via a bind-mount, the inode
passed to devpts_get_tty() while valid, would refer to a pty that no longer
exists.

With a lot of debug effort, Grzegorz Nosek developed a small program (see
below) to reproduce a crash on recent kernels. This crash is a regression
introduced by the commit:

	commit 527b3e47
	Author: Sukadev Bhattiprolu <sukadev@us.ibm.com>
	Date:   Mon Oct 13 10:43:08 2008 +0100

To fix, ensure that the dentry associated with the inode has not yet been
deleted/unhashed by devpts_pty_kill().

See also:
https://lists.linux-foundation.org/pipermail/containers/2009-July/019273.html 

tty-bug.c:

#define _GNU_SOURCE
#include <fcntl.h>
#include <sched.h>
#include <stdlib.h>
#include <sys/mount.h>
#include <sys/signal.h>
#include <unistd.h>
#include <stdio.h>

#include <linux/fs.h>

void dummy(int sig)
{
}

s...

fs/devpts/inode.c

@@ -517,11 +517,23 @@ int devpts_pty_new(struct inode *ptmx_inode, struct tty_struct *tty)
 
 struct tty_struct *devpts_get_tty(struct inode *pts_inode, int number)
 {
+	struct dentry *dentry;
+	struct tty_struct *tty;
+
 	BUG_ON(pts_inode->i_rdev == MKDEV(TTYAUX_MAJOR, PTMX_MINOR));
 
+	/* Ensure dentry has not been deleted by devpts_pty_kill() */
+	dentry = d_find_alias(pts_inode);
+	if (!dentry)
+		return NULL;
+
+	tty = NULL;
 	if (pts_inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
-		return (struct tty_struct *)pts_inode->i_private;
-	return NULL;
+		tty = (struct tty_struct *)pts_inode->i_private;
+
+	dput(dentry);
+
+	return tty;
 }
 
 void devpts_pty_kill(struct tty_struct *tty)