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 527b3e4773628b30d03323a2cb5fb0d84441990f
	Author: Sukadev Bhattiprolu <[email protected]>
	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)
{
}

static int child(void *unused)

{
	int fd;

	signal(SIGINT, dummy); signal(SIGHUP, dummy);
	pause(); /* cheesy synchronisation to wait for /dev/pts/0 to appear */

	mount("/dev/pts/0", "/dev/console", NULL, MS_BIND, NULL);
	sleep(2);

	fd = open("/dev/console", O_RDWR);
	dup(0); dup(0);
	write(1, "Hello world!\n", sizeof("Hello world!\n")-1);
	return 0;
}

int main(void)

{
	pid_t pid;
	char *stack;

	stack = malloc(16384);
	pid = clone(child, stack+16384, CLONE_NEWNS|SIGCHLD, NULL);

	open("/dev/ptmx", O_RDWR|O_NOCTTY|O_NONBLOCK);

	unlockpt(fd); grantpt(fd);

	sleep(2);
	kill(pid, SIGHUP);
	sleep(1);
	return 0; /* exit before child opens /dev/console */
}

---

Index: linux-2.6/fs/devpts/inode.c
===================================================================
@@ -517,11 +517,23 @@
 
 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)

Good debugging, I'll queue this up.

greg k-h

This is a note to let you know that I've just added the patch titled

Subject: devpts_get_tty() should validate inode

to my gregkh-2.6 tree. Its filename is

devpts_get_tty-should-validate-inode.patch

This tree can be found at
http://www.kernel.org/pub/linux/kernel/people/gregkh/gregkh-2.6/patches/

From [email protected] Thu Nov 19 15:40:33 2009
From: Sukadev Bhattiprolu <[email protected]>
Date: Tue, 17 Nov 2009 18:35:43 -0800
Subject: devpts_get_tty() should validate inode
To: [email protected]
Cc: Alan Cox <alan-qBU/[email protected]>, [email protected], [email protected], [email protected], Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>
Message-ID: <20091118023541.GA14430-r/[email protected]>

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 527b3e4773628b30d03323a2cb5fb0d84441990f
	Author: Sukadev Bhattiprolu <sukadev-r/[email protected]>
	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)
{
}

static int child(void *unused)

{
	int fd;

	signal(SIGINT, dummy); signal(SIGHUP, dummy);
	pause(); /* cheesy synchronisation to wait for /dev/pts/0 to appear */

	mount("/dev/pts/0", "/dev/console", NULL, MS_BIND, NULL);
	sleep(2);

	fd = open("/dev/console", O_RDWR);
	dup(0); dup(0);
	write(1, "Hello world!\n", sizeof("Hello world!\n")-1);
	return 0;
}

int main(void)

{
	pid_t pid;
	char *stack;

	stack = malloc(16384);
	pid = clone(child, stack+16384, CLONE_NEWNS|SIGCHLD, NULL);

	open("/dev/ptmx", O_RDWR|O_NOCTTY|O_NONBLOCK);

	unlockpt(fd); grantpt(fd);

	sleep(2);
	kill(pid, SIGHUP);
	sleep(1);
	return 0; /* exit before child opens /dev/console */
}

---
 fs/devpts/inode.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

@@ -517,11 +517,23 @@ int devpts_pty_new(struct inode *ptmx_in
 
 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)

Good debugging, I'll queue this up.

greg k-h

fwiw,