Re: [PATCH v4 1/2] tiocsti-restrict : Add owner user namespace to tty_struct

From: Serge E. Hallyn
Date: Mon Apr 24 2017 - 09:58:06 EST


Quoting Matt Brown (matt@xxxxxxxxx):
> This patch adds struct user_namespace *owner_user_ns to the tty_struct.
> Then it is set to current_user_ns() in the alloc_tty_struct function.
>
> This is done to facilitate capability checks against the original user
> namespace that allocated the tty.
>
> E.g. ns_capable(tty->owner_user_ns,CAP_SYS_ADMIN)
>
> This combined with the use of user namespace's will allow hardening
> protections to be built to mitigate container escapes that utilize TTY
> ioctls such as TIOCSTI.
>
> See: https://bugzilla.redhat.com/show_bug.cgi?id=1411256
>
> Signed-off-by: Matt Brown <matt@xxxxxxxxx>

Acked-by: Serge Hallyn <serge@xxxxxxxxxx>

> ---
> drivers/tty/tty_io.c | 2 ++
> include/linux/tty.h | 2 ++
> 2 files changed, 4 insertions(+)
>
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index e6d1a65..c276814 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -171,6 +171,7 @@ static void free_tty_struct(struct tty_struct *tty)
> put_device(tty->dev);
> kfree(tty->write_buf);
> tty->magic = 0xDEADDEAD;
> + put_user_ns(tty->owner_user_ns);
> kfree(tty);
> }
>
> @@ -3191,6 +3192,7 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
> tty->index = idx;
> tty_line_name(driver, idx, tty->name);
> tty->dev = tty_get_device(tty);
> + tty->owner_user_ns = get_user_ns(current_user_ns());
>
> return tty;
> }
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index 1017e904..d902d42 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -12,6 +12,7 @@
> #include <uapi/linux/tty.h>
> #include <linux/rwsem.h>
> #include <linux/llist.h>
> +#include <linux/user_namespace.h>
>
>
> /*
> @@ -333,6 +334,7 @@ struct tty_struct {
> /* If the tty has a pending do_SAK, queue it here - akpm */
> struct work_struct SAK_work;
> struct tty_port *port;
> + struct user_namespace *owner_user_ns;
> };
>
> /* Each of a tty's open files has private_data pointing to tty_file_private */
> --
> 2.10.2