pty: Repair TIOCGPTPEER

The implementation of TIOCGPTPEER has two issues.

When /dev/ptmx (as opposed to /dev/pts/ptmx) is opened the wrong
vfsmount is passed to dentry_open.  Which results in the kernel displaying
the wrong pathname for the peer.

The second is simply by caching the vfsmount and dentry of the peer it leaves
them open, in a way they were not previously Which because of the inreased
reference counts can cause unnecessary behaviour differences resulting in
regressions.

To fix these move the ioctl into tty_io.c at a generic level allowing
the ioctl to have access to the struct file on which the ioctl is
being called.  This allows the path of the slave to be derived when
opening the slave through TIOCGPTPEER instead of requiring the path to
the slave be cached.  Thus removing the need for caching the path.

A new function devpts_ptmx_path is factored out of devpts_acquire and
used to implement a function devpts_mntget.   The new function devpts_mntget
takes a filp to perform the lookup on and fsi so that it can confirm
that the superblock that is found by devpts_ptmx_path is the proper superblock.

v2: Lots of fixes to make the code actually work
v3: Suggestions by Linus
    - Removed the unnecessary initialization of filp in ptm_open_peer
    - Simplified devpts_ptmx_path as gotos are no longer required

[ This is the fix for the issue that was reverted in commit
  143c97cc6529, but this time without breaking 'pbuilder' due to
  increased reference counts   - Linus ]

Fixes: 54ebbfb16034 ("tty: add TIOCGPTPEER ioctl")
Reported-by: Christian Brauner <christian.brauner@canonical.com>
Reported-and-tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 30 insertions, 37 deletions

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 284749fb0f6b..a6d5164c33a9 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -69,13 +69,8 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
 #ifdef CONFIG_UNIX98_PTYS
 		if (tty->driver == ptm_driver) {
 			mutex_lock(&devpts_mutex);
-			if (tty->link->driver_data) {
-				struct path *path = tty->link->driver_data;
-
-				devpts_pty_kill(path->dentry);
-				path_put(path);
-				kfree(path);
-			}
+			if (tty->link->driver_data)
+				devpts_pty_kill(tty->link->driver_data);
 			mutex_unlock(&devpts_mutex);
 		}
 #endif
@@ -607,25 +602,24 @@ static inline void legacy_pty_init(void) { }
 static struct cdev ptmx_cdev;
 
 /**
- *	pty_open_peer - open the peer of a pty
- *	@tty: the peer of the pty being opened
+ *	ptm_open_peer - open the peer of a pty
+ *	@master: the open struct file of the ptmx device node
+ *	@tty: the master of the pty being opened
+ *	@flags: the flags for open
  *
- *	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).
+ *	Provide a race free way for userspace to open 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 ptm_open_peer(struct file *master, struct tty_struct *tty, int flags)
 {
 	int fd = -1;
-	struct file *filp = NULL;
+	struct file *filp;
 	int retval = -EINVAL;
+	struct path path;
+
+	if (tty->driver != ptm_driver)
+		return -EIO;
 
 	fd = get_unused_fd_flags(0);
 	if (fd < 0) {
@@ -633,7 +627,16 @@ static int pty_get_peer(struct tty_struct *tty, int flags)
 		goto err;
 	}
 
-	filp = pty_open_peer(tty, flags);
+	/* Compute the slave's path */
+	path.mnt = devpts_mntget(master, tty->driver_data);
+	if (IS_ERR(path.mnt)) {
+		retval = PTR_ERR(path.mnt);
+		goto err_put;
+	}
+	path.dentry = tty->link->driver_data;
+
+	filp = dentry_open(&path, flags, current_cred());
+	mntput(path.mnt);
 	if (IS_ERR(filp)) {
 		retval = PTR_ERR(filp);
 		goto err_put;
@@ -662,8 +665,6 @@ static int pty_unix98_ioctl(struct tty_struct *tty,
 		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);
 	}
@@ -791,7 +792,6 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 {
 	struct pts_fs_info *fsi;
 	struct tty_struct *tty;
-	struct path *pts_path;
 	struct dentry *dentry;
 	int retval;
 	int index;
@@ -845,26 +845,16 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 		retval = PTR_ERR(dentry);
 		goto err_release;
 	}
-	/* 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;
+	tty->link->driver_data = dentry;
 
 	retval = ptm_driver->ops->open(tty, filp);
 	if (retval)
-		goto err_path_put;
+		goto err_release;
 
 	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
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 974b13d24401..10c4038c0e8d 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2518,6 +2518,9 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case TIOCSSERIAL:
 		tty_warn_deprecated_flags(p);
 		break;
+	case TIOCGPTPEER:
+		/* Special because the struct file is needed */
+		return ptm_open_peer(file, tty, (int)arg);
 	default:
 		retval = tty_jobctrl_ioctl(tty, real_tty, file, cmd, arg);
 		if (retval != -ENOIOCTLCMD)