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

lizf
2016-04-18T10:56:18Z

From: Jann Horn <jann@thejh.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@thejh.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[lizf: Backported to 3.4: adjust context]
Signed-off-by: Zefan Li <lizefan@huawei.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