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>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 17b02724273440fa7b7b8504f7a0b86f539a66b2..2eefaa6e3e3a4af9a5ab2b03cf03f9e75a04ca1d 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2128,8 +2128,24 @@ 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);
+       }
        spin_unlock_irq(&current->sighand->siglock);
        read_unlock(&tasklist_lock);
        tty_unlock(tty);
@@ -2418,7 +2434,7 @@ 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;
 
@@ -2452,6 +2468,13 @@ 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(tty);
 unlock:
        read_unlock(&tasklist_lock);
@@ -2844,7 +2867,7 @@ 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: