From 7f1265b917aba4436653aa8e7bf90976b82b77ee Mon Sep 17 00:00:00 2001
From: Jann Horn <jann@thejh.net>
Date: Fri, 14 Aug 2015 17:47:01 +0200
Subject: [PATCH] drivers/tty: require read access for controlling terminal

This is mostly a hardening fix, given that write-only access to other
users' ttys is usually only given through setgid tty executables.

Signed-off-by: Jann Horn <jann@thejh.net>
---
 drivers/tty/tty_io.c | 48 ++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 44 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 57fc6ee..dcd6a83 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2136,8 +2136,31 @@ retry_open:
 	if (!noctty &&
 	    current->signal->leader &&
 	    !current->signal->tty &&
-	    tty->session == NULL)
-		__proc_set_tty(tty);
+	    tty->session == NULL) {
+		/*
+		 * Don't let a process that only has write access to the tty
+		 * obtain the privileges associated with having a tty as
+		 * controlling terminal (being able to reopen it with full
+		 * access through /dev/tty, being able to perform pushback).
+		 * Many distributions set the group of all ttys to "tty" and
+		 * grant write-only access to all terminals for setgid tty
+		 * binaries, which should not imply full privileges on all ttys.
+		 *
+		 * This could theoretically break old code that performs open()
+		 * on a write-only file descriptor. In that case, it might be
+		 * necessary to also permit this if
+		 * inode_permission(inode, MAY_READ) == 0.
+		 */
+		if (filp->f_mode & FMODE_READ)
+			__proc_set_tty(tty);
+		else {
+			char comm[sizeof(current->comm)];
+
+			pr_warn_once("%s: silently refused to set controlling terminal on open() - tty is not open for reading. Offending process: %d (%s)\n",
+			    tty->name, current->pid,
+			    get_task_comm(comm, current));
+		}
+	}
 	spin_unlock_irq(&current->sighand->siglock);
 	read_unlock(&tasklist_lock);
 	tty_unlock(tty);
@@ -2426,7 +2449,7 @@ static int fionbio(struct file *file, int __user *p)
  *		Takes ->siglock() when updating signal->tty
  */

-static int tiocsctty(struct tty_struct *tty, int arg)
+static int tiocsctty(struct tty_struct *tty, struct file *file, int arg)
 {
 	int ret = 0;

@@ -2460,6 +2483,23 @@ static int tiocsctty(struct tty_struct *tty, int arg)
 			goto unlock;
 		}
 	}
+
+	/* See the comment in tty_open(). */
+	if ((file->f_mode & FMODE_READ) == 0) {
+		char comm[sizeof(current->comm)];
+
+		get_task_comm(comm, current);
+		if (capable(CAP_SYS_ADMIN)) {
+			pr_warn_once("%s: TIOCSCTTY on a write-only fd was only allowed because it was performed by root. Offending process: %d (%s)\n",
+			    tty->name, current->pid, comm);
+		} else {
+			pr_warn_once("%s: TIOCSCTTY on a write-only fd was refused for security reasons! This might break old code. Offending process: %d (%s)\n",
+			    tty->name, current->pid, comm);
+			ret = -EPERM;
+			goto unlock;
+		}
+	}
+
 	proc_set_tty(tty);
 unlock:
 	read_unlock(&tasklist_lock);
@@ -2852,7 +2892,7 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		no_tty();
 		return 0;
 	case TIOCSCTTY:
-		return tiocsctty(tty, arg);
+		return tiocsctty(tty, file, arg);
 	case TIOCGPGRP:
 		return tiocgpgrp(tty, real_tty, p);
 	case TIOCSPGRP:
--
2.1.4