tty: Fix ->session locking

Currently, locking of ->session is very inconsistent; most places
protect it using the legacy tty mutex, but disassociate_ctty(),
__do_SAK(), tiocspgrp() and tiocgsid() don't.
Two of the writers hold the ctrl_lock (because they already need it for
->pgrp), but __proc_set_tty() doesn't do that yet.

On a PREEMPT=y system, an unprivileged user can theoretically abuse
this broken locking to read 4 bytes of freed memory via TIOCGSID if
tiocgsid() is preempted long enough at the right point. (Other things
might also go wrong, especially if root-only ioctls are involved; I'm
not sure about that.)

Change the locking on ->session such that:

 - tty_lock() is held by all writers: By making disassociate_ctty()
   hold it. This should be fine because the same lock can already be
   taken through the call to tty_vhangup_session().
   The tricky part is that we need to shorten the area covered by
   siglock to be able to take tty_lock() without ugly retry logic; as
   far as I can tell, this should be fine, since nothing in the
   signal_struct is touched in the `if (tty)` branch.
 - ctrl_lock is held by all writers: By changing __proc_set_tty() to
   hold the lock a little longer.
 - All readers that aren't holding tty_lock() hold ctrl_lock: By
   adding locking to tiocgsid() and __do_SAK(), and expanding the area
   covered by ctrl_lock in tiocspgrp().

Cc: stable@kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -2897,10 +2897,14 @@ void __do_SAK(struct tty_struct *tty)
 	struct task_struct *g, *p;
 	struct pid *session;
 	int		i;
+	unsigned long flags;
 
 	if (!tty)
 		return;
-	session = tty->session;
+
+	spin_lock_irqsave(&tty->ctrl_lock, flags);
+	session = get_pid(tty->session);
+	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 
 	tty_ldisc_flush(tty);
 
@@ -2932,6 +2936,7 @@ void __do_SAK(struct tty_struct *tty)
 		task_unlock(p);
 	} while_each_thread(g, p);
 	read_unlock(&tasklist_lock);
+	put_pid(session);
 #endif
 }
 

drivers/tty/tty_jobctrl.c

@@ -103,8 +103,8 @@ static void __proc_set_tty(struct tty_struct *tty)
 	put_pid(tty->session);
 	put_pid(tty->pgrp);
 	tty->pgrp = get_pid(task_pgrp(current));
-	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 	tty->session = get_pid(task_session(current));
+	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 	if (current->signal->tty) {
 		tty_debug(tty, "current tty %s not NULL!!\n",
 			  current->signal->tty->name);
@@ -293,20 +293,23 @@ void disassociate_ctty(int on_exit)
 	spin_lock_irq(&current->sighand->siglock);
 	put_pid(current->signal->tty_old_pgrp);
 	current->signal->tty_old_pgrp = NULL;
-
 	tty = tty_kref_get(current->signal->tty);
+	spin_unlock_irq(&current->sighand->siglock);
+
 	if (tty) {
 		unsigned long flags;
+
+		tty_lock(tty);
 		spin_lock_irqsave(&tty->ctrl_lock, flags);
 		put_pid(tty->session);
 		put_pid(tty->pgrp);
 		tty->session = NULL;
 		tty->pgrp = NULL;
 		spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+		tty_unlock(tty);
 		tty_kref_put(tty);
 	}
 
-	spin_unlock_irq(&current->sighand->siglock);
 	/* Now clear signal->tty under the lock */
 	read_lock(&tasklist_lock);
 	session_clear_tty(task_session(current));
@@ -477,14 +480,19 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
 		return -ENOTTY;
 	if (retval)
 		return retval;
-	if (!current->signal->tty ||
+
-	    (current->signal->tty != real_tty) ||
-	    (real_tty->session != task_session(current)))
-		return -ENOTTY;
 	if (get_user(pgrp_nr, p))
 		return -EFAULT;
 	if (pgrp_nr < 0)
 		return -EINVAL;
+
+	spin_lock_irq(&real_tty->ctrl_lock);
+	if (!current->signal->tty ||
+	    (current->signal->tty != real_tty) ||
+	    (real_tty->session != task_session(current))) {
+		retval = -ENOTTY;
+		goto out_unlock_ctrl;
+	}
 	rcu_read_lock();
 	pgrp = find_vpid(pgrp_nr);
 	retval = -ESRCH;
@@ -494,12 +502,12 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
 	if (session_of_pgrp(pgrp) != task_session(current))
 		goto out_unlock;
 	retval = 0;
-	spin_lock_irq(&real_tty->ctrl_lock);
 	put_pid(real_tty->pgrp);
 	real_tty->pgrp = get_pid(pgrp);
-	spin_unlock_irq(&real_tty->ctrl_lock);
 out_unlock:
 	rcu_read_unlock();
+out_unlock_ctrl:
+	spin_unlock_irq(&real_tty->ctrl_lock);
 	return retval;
 }
 
@@ -511,20 +519,30 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
  *
  *	Obtain the session id of the tty. If there is no session
  *	return an error.
- *
- *	Locking: none. Reference to current->signal->tty is safe.
  */
 static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
 {
+	unsigned long flags;
+	pid_t sid;
+
 	/*
 	 * (tty == real_tty) is a cheap way of
 	 * testing if the tty is NOT a master pty.
 	*/
 	if (tty == real_tty && current->signal->tty != real_tty)
 		return -ENOTTY;
+
+	spin_lock_irqsave(&real_tty->ctrl_lock, flags);
 	if (!real_tty->session)
-		return -ENOTTY;
+		goto err;
-	return put_user(pid_vnr(real_tty->session), p);
+	sid = pid_vnr(real_tty->session);
+	spin_unlock_irqrestore(&real_tty->ctrl_lock, flags);
+
+	return put_user(sid, p);
+
+err:
+	spin_unlock_irqrestore(&real_tty->ctrl_lock, flags);
+	return -ENOTTY;
 }
 
 /*

include/linux/tty.h

@@ -306,6 +306,10 @@ struct tty_struct {
 	struct termiox *termiox;	/* May be NULL for unsupported */
 	char name[64];
 	struct pid *pgrp;		/* Protected by ctrl lock */
+	/*
+	 * Writes protected by both ctrl lock and legacy mutex, readers must use
+	 * at least one of them.
+	 */
 	struct pid *session;
 	unsigned long flags;
 	int count;