linux-kernel - [PATCH 06/13] devpts: Use the same default mode for both /dev/ptmx and dev/pts/ptmx

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Date:	Mon,  4 Apr 2016 20:29:22 -0500
From:	"Eric W. Biederman" <ebiederm@...ssion.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <greg@...ah.com>, Jiri Slaby <jslaby@...e.com>,
	Aurelien Jarno <aurelien@...el32.net>,
	Andy Lutomirski <luto@...capital.net>,
	Florian Weimer <fw@...eb.enyo.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Jann Horn <jann@...jh.net>,
	"security@...nel.org" <security@...nel.org>, security@...ntu.com,
	security@...ian.org, Willy Tarreau <w@....eu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 06/13] devpts: Use the same default mode for both /dev/ptmx and dev/pts/ptmx

The default mode is 0666 on /dev/ptmx so we really don't gain anything
except a bunch of hassle by not having the default mode on
/dev/pts/ptmx also being 0666.

Leave the default of ptmxmode at 0000 on the system mount of devpts to
avoid the off chance that it would open an security hole on existing
setups that change the permissions of devpts.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 drivers/tty/tty_io.c      |  5 +++--
 fs/devpts/inode.c         | 11 +++--------
 include/linux/devpts_fs.h |  3 +++
 3 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 8d26ed79bb4c..438b2209ea41 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3597,9 +3597,10 @@ static char *tty_devnode(struct device *dev, umode_t *mode)
 {
 	if (!mode)
 		return NULL;
-	if (dev->devt == MKDEV(TTYAUX_MAJOR, 0) ||
-	    dev->devt == MKDEV(TTYAUX_MAJOR, 2))
+	if (dev->devt == MKDEV(TTYAUX_MAJOR, 0))
 		*mode = 0666;
+	if (dev->devt == MKDEV(TTYAUX_MAJOR, PTMX_MINOR))
+		*mode = DEVPTS_DEFAULT_PTMX_MODE;
 	return NULL;
 }
 
diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index cb0cc4e33c3f..80c78bb472a9 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -30,14 +30,6 @@
 #include <linux/seq_file.h>
 
 #define DEVPTS_DEFAULT_MODE 0600
-/*
- * ptmx is a new node in /dev/pts and will be unused in legacy (single-
- * instance) mode. To prevent surprises in user space, set permissions of
- * ptmx to 0. Use 'chmod' or remount with '-o ptmxmode' to set meaningful
- * permissions.
- */
-#define DEVPTS_DEFAULT_PTMX_MODE 0000
-#define PTMX_MINOR	2
 
 /*
  * sysctl support for setting limits on the number of Unix98 ptys allocated.
@@ -260,6 +252,9 @@ static int parse_mount_options(char *data, struct pts_mount_opts *opts)
 	opts->mode    = DEVPTS_DEFAULT_MODE;
 	opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
 	opts->max     = NR_UNIX98_PTY_MAX;
+	if (!devpts_mnt ||
+	    (&DEVPTS_SB(devpts_mnt->mnt_sb)->mount_opts == opts))
+		opts->ptmxmode = 0;
 
 	while ((p = strsep(&data, ",")) != NULL) {
 		substring_t args[MAX_OPT_ARGS];
diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h
index 260190690674..80908db52667 100644
--- a/include/linux/devpts_fs.h
+++ b/include/linux/devpts_fs.h
@@ -15,6 +15,9 @@
 
 #include <linux/errno.h>
 
+#define DEVPTS_DEFAULT_PTMX_MODE 0666
+#define PTMX_MINOR	2
+
 #ifdef CONFIG_UNIX98_PTYS
 
 struct inode *devpts_ptmx(struct inode *inode, struct file *filp);
-- 
2.6.3