tty: ldisc: add sysctl to prevent autoloading of ldiscs

commit 7c0cca7c847e6e019d67b7d793efbbe3b947d004 upstream.

By default, the kernel will automatically load the module of any line
dicipline that is asked for.  As this sometimes isn't the safest thing
to do, provide a sysctl to disable this feature.

By default, we set this to 'y' as that is the historical way that Linux
has worked, and we do not want to break working systems.  But in the
future, perhaps this can default to 'n' to prevent this functionality.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 0840d27381ea79fa501bee4e26f3f928fa58031f..e0a04bfc873e2f8c64c38e65724cd6bb4a758f36 100644 (file)
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -441,4 +441,28 @@ config VCC
        depends on SUN_LDOMS
        help
          Support for Sun logical domain consoles.
+
+config LDISC_AUTOLOAD
+       bool "Automatically load TTY Line Disciplines"
+       default y
+       help
+         Historically the kernel has always automatically loaded any
+         line discipline that is in a kernel module when a user asks
+         for it to be loaded with the TIOCSETD ioctl, or through other
+         means.  This is not always the best thing to do on systems
+         where you know you will not be using some of the more
+         "ancient" line disciplines, so prevent the kernel from doing
+         this unless the request is coming from a process with the
+         CAP_SYS_MODULE permissions.
+
+         Say 'Y' here if you trust your userspace users to do the right
+         thing, or if you have only provided the line disciplines that
+         you know you will be using, or if you wish to continue to use
+         the traditional method of on-demand loading of these modules
+         by any user.
+
+         This functionality can be changed at runtime with the
+         dev.tty.ldisc_autoload sysctl, this configuration option will
+         only set the default value of this functionality.
+
 endif # TTY

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e7d192ebecd7d5d1220ab11f197ee9f1357ed833..ac8025cd4a1fefa7f223e9ba12a6b30f9069ac73 100644 (file)
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -512,6 +512,8 @@ static const struct file_operations hung_up_tty_fops = {
 static DEFINE_SPINLOCK(redirect_lock);
 static struct file *redirect;
 
+extern void tty_sysctl_init(void);
+
 /**
  *     tty_wakeup      -       request more data
  *     @tty: terminal
@@ -3340,6 +3342,7 @@ void console_sysfs_notify(void)
  */
 int __init tty_init(void)
 {
+       tty_sysctl_init();
        cdev_init(&tty_cdev, &tty_fops);
        if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) ||
            register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index fc4c97cae01ef4812da9b1bf61d6efe5d9cf6784..53bb6d4e9e8d9098ae0dbd5e39204669d3b8d7cf 100644 (file)
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -156,6 +156,13 @@ static void put_ldops(struct tty_ldisc_ops *ldops)
  *             takes tty_ldiscs_lock to guard against ldisc races
  */
 
+#if defined(CONFIG_LDISC_AUTOLOAD)
+       #define INITIAL_AUTOLOAD_STATE  1
+#else
+       #define INITIAL_AUTOLOAD_STATE  0
+#endif
+static int tty_ldisc_autoload = INITIAL_AUTOLOAD_STATE;
+
 static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
 {
        struct tty_ldisc *ld;
@@ -170,6 +177,8 @@ static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
         */
        ldops = get_ldops(disc);
        if (IS_ERR(ldops)) {
+               if (!capable(CAP_SYS_MODULE) && !tty_ldisc_autoload)
+                       return ERR_PTR(-EPERM);
                request_module("tty-ldisc-%d", disc);
                ldops = get_ldops(disc);
                if (IS_ERR(ldops))
@@ -829,3 +838,41 @@ void tty_ldisc_deinit(struct tty_struct *tty)
                tty_ldisc_put(tty->ldisc);
        tty->ldisc = NULL;
 }
+
+static int zero;
+static int one = 1;
+static struct ctl_table tty_table[] = {
+       {
+               .procname       = "ldisc_autoload",
+               .data           = &tty_ldisc_autoload,
+               .maxlen         = sizeof(tty_ldisc_autoload),
+               .mode           = 0644,
+               .proc_handler   = proc_dointvec,
+               .extra1         = &zero,
+               .extra2         = &one,
+       },
+       { }
+};
+
+static struct ctl_table tty_dir_table[] = {
+       {
+               .procname       = "tty",
+               .mode           = 0555,
+               .child          = tty_table,
+       },
+       { }
+};
+
+static struct ctl_table tty_root_table[] = {
+       {
+               .procname       = "dev",
+               .mode           = 0555,
+               .child          = tty_dir_table,
+       },
+       { }
+};
+
+void tty_sysctl_init(void)
+{
+       register_sysctl_table(tty_root_table);
+}