tty: add TIOCGPTPEER ioctl

When opening the slave end of a PTY, it is not possible for userspace to
safely ensure that /dev/pts/$num is actually a slave (in cases where the
mount namespace in which devpts was mounted is controlled by an
untrusted process). In addition, there are several unresolvable
race conditions if userspace were to attempt to detect attacks through
stat(2) and other similar methods [in addition it is not clear how
userspace could detect attacks involving FUSE].

Resolve this by providing an interface for userpace to safely open the
"peer" end of a PTY file descriptor by using the dentry cached by
devpts. Since it is not possible to have an open master PTY without
having its slave exposed in /dev/pts this interface is safe. This
interface currently does not provide a way to get the master pty (since
it is not clear whether such an interface is safe or even useful).

Cc: Christian Brauner <[e-mail]>
Cc: Valentin Rothberg <[e-mail]>
Signed-off-by: Aleksa Sarai <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

arch/alpha/include/uapi/asm/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 #define TIOCSERCONFIG	0x5453
 #define TIOCSERGWILD	0x5454

arch/mips/include/uapi/asm/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 /* I hope the range from 0x5480 on is free ... */
 #define TIOCSCTTY	0x5480		/* become controlling tty */

arch/parisc/include/uapi/asm/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 #define FIONCLEX	0x5450  /* these numbers need to be adjusted. */
 #define FIOCLEX		0x5451

arch/powerpc/include/uapi/asm/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 #define TIOCSERCONFIG	0x5453
 #define TIOCSERGWILD	0x5454

arch/sh/include/uapi/asm/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 #define TIOCSERCONFIG	_IO('T', 83) /* 0x5453 */
 #define TIOCSERGWILD	_IOR('T', 84,  int) /* 0x5454 */

arch/sparc/include/uapi/asm/ioctls.h

···
 #define TIOCGRS485	_IOR('T', 0x41, struct serial_rs485)
 #define TIOCSRS485	_IOWR('T', 0x42, struct serial_rs485)
 
-/* Note that all the ioctls that are not available in Linux have a 
+/* Note that all the ioctls that are not available in Linux have a
  * double underscore on the front to: a) avoid some programs to
  * think we support some ioctls under Linux (autoconfiguration stuff)
  */
···
 #define TIOCGPTN	_IOR('t', 134, unsigned int) /* Get Pty Number */
 #define TIOCSPTLCK	_IOW('t', 135, int) /* Lock/unlock PTY */
 #define TIOCSIG		_IOW('t', 136, int) /* Generate signal on Pty slave */
+#define TIOCGPTPEER	_IOR('t', 137, int) /* Safely open the slave */
 
 /* Little f */
 #define FIOCLEX		_IO('f', 1)

arch/xtensa/include/uapi/asm/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 #define TIOCSERCONFIG	_IO('T', 83)
 #define TIOCSERGWILD	_IOR('T', 84,  int)

drivers/tty/pty.c

···
 #include <linux/slab.h>
 #include <linux/mutex.h>
 #include <linux/poll.h>
+#include <linux/mount.h>
+#include <linux/file.h>
+#include <linux/ioctl.h>
 
 #undef TTY_DEBUG_HANGUP
 #ifdef TTY_DEBUG_HANGUP
···
 #ifdef CONFIG_UNIX98_PTYS
 		if (tty->driver == ptm_driver) {
 			mutex_lock(&devpts_mutex);
-			if (tty->link->driver_data)
-				devpts_pty_kill(tty->link->driver_data);
+			if (tty->link->driver_data) {
+				struct path *path = tty->link->driver_data;
+
+				devpts_pty_kill(path->dentry);
+				path_put(path);
+				kfree(path);
+			}
 			mutex_unlock(&devpts_mutex);
 		}
 #endif
···
 	return retval;
 }
 
+/**
+ *	pty_open_peer - open the peer of a pty
+ *	@tty: the peer of the pty being opened
+ *
+ *	Open the cached dentry in tty->link, providing a safe way for userspace
+ *	to get the slave end of a pty (where they have the master fd and cannot
+ *	access or trust the mount namespace /dev/pts was mounted inside).
+ */
+static struct file *pty_open_peer(struct tty_struct *tty, int flags)
+{
+	if (tty->driver->subtype != PTY_TYPE_MASTER)
+		return ERR_PTR(-EIO);
+	return dentry_open(tty->link->driver_data, flags, current_cred());
+}
+
+static int pty_get_peer(struct tty_struct *tty, int flags)
+{
+	int fd = -1;
+	struct file *filp = NULL;
+	int retval = -EINVAL;
+
+	fd = get_unused_fd_flags(0);
+	if (fd < 0) {
+		retval = fd;
+		goto err;
+	}
+
+	filp = pty_open_peer(tty, flags);
+	if (IS_ERR(filp)) {
+		retval = PTR_ERR(filp);
+		goto err_put;
+	}
+
+	fd_install(fd, filp);
+	return fd;
+
+err_put:
+	put_unused_fd(fd);
+err:
+	return retval;
+}
+
 static void pty_cleanup(struct tty_struct *tty)
 {
 	tty_port_put(tty->port);
···
 		return pty_get_pktmode(tty, (int __user *)arg);
 	case TIOCGPTN: /* Get PT Number */
 		return put_user(tty->index, (unsigned int __user *)arg);
+	case TIOCGPTPEER: /* Open the other end */
+		return pty_get_peer(tty, (int) arg);
 	case TIOCSIG:    /* Send signal to other side of pty */
 		return pty_signal(tty, (int) arg);
 	}
···
 {
 	struct pts_fs_info *fsi;
 	struct tty_struct *tty;
+	struct path *pts_path;
 	struct dentry *dentry;
 	int retval;
 	int index;
···
 		retval = PTR_ERR(dentry);
 		goto err_release;
 	}
-	tty->link->driver_data = dentry;
+	/* We need to cache a fake path for TIOCGPTPEER. */
+	pts_path = kmalloc(sizeof(struct path), GFP_KERNEL);
+	if (!pts_path)
+		goto err_release;
+	pts_path->mnt = filp->f_path.mnt;
+	pts_path->dentry = dentry;
+	path_get(pts_path);
+	tty->link->driver_data = pts_path;
 
 	retval = ptm_driver->ops->open(tty, filp);
 	if (retval)
-		goto err_release;
+		goto err_path_put;
 
 	tty_debug_hangup(tty, "opening (count=%d)\n", tty->count);
 
 	tty_unlock(tty);
 	return 0;
+err_path_put:
+	path_put(pts_path);
+	kfree(pts_path);
 err_release:
 	tty_unlock(tty);
 	// This will also put-ref the fsi

include/uapi/asm-generic/ioctls.h

···
 #define TIOCGPKT	_IOR('T', 0x38, int) /* Get packet mode state */
 #define TIOCGPTLCK	_IOR('T', 0x39, int) /* Get Pty lock state */
 #define TIOCGEXCL	_IOR('T', 0x40, int) /* Get exclusive mode state */
+#define TIOCGPTPEER	_IOR('T', 0x41, int) /* Safely open the slave */
 
 #define FIONCLEX	0x5450
 #define FIOCLEX		0x5451