linux-kernel - [PATCH 3.4 74/92] drivers/tty: require read access for controlling terminal

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Date:	Mon, 18 Apr 2016 18:46:19 +0800
From:	lizf@...nel.org
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Jann Horn <jann@...jh.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Zefan Li <lizefan@...wei.com>
Subject: [PATCH 3.4 74/92] drivers/tty: require read access for controlling terminal

From: Jann Horn <jann@...jh.net>

3.4.112-rc1 review patch.  If anyone has any objections, please let me know.

------------------


commit 0c55627167870255158db1cde0d28366f91c8872 upstream.

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@...jh.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@...wei.com>
---
 drivers/tty/tty_io.c | 31 +++++++++++++++++++++++++++----
 1 file changed, 27 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 5f0b4a4..3ea4150 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2018,8 +2018,24 @@ retry_open:
 	if (!noctty &&
 	    current->signal->leader &&
 	    !current->signal->tty &&
-	    tty->session == NULL)
-		__proc_set_tty(current, 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(current, tty);
+	}
 	spin_unlock_irq(&current->sighand->siglock);
 	tty_unlock();
 	mutex_unlock(&tty_mutex);
@@ -2308,7 +2324,7 @@ 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;
 	if (current->signal->leader && (task_session(current) == tty->session))
@@ -2341,6 +2357,13 @@ static int tiocsctty(struct tty_struct *tty, int arg)
 			goto unlock;
 		}
 	}
+
+	/* See the comment in tty_open(). */
+	if ((file->f_mode & FMODE_READ) == 0 && !capable(CAP_SYS_ADMIN)) {
+		ret = -EPERM;
+		goto unlock;
+	}
+
 	proc_set_tty(current, tty);
 unlock:
 	mutex_unlock(&tty_mutex);
@@ -2695,7 +2718,7 @@ 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:
-- 
1.9.1