Re: [PATCH v3 1/2] tiocsti-restrict : Add owner user namespace to tty_struct

[Serge E. Hallyn]

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>
> ---
>  drivers/tty/tty_io.c | 4 ++++
>  include/linux/tty.h  | 2 ++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index e6d1a65..03d5ea2 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -161,6 +161,7 @@ static void release_tty(struct tty_struct *tty, int idx);
>   *	@tty: tty struct to free
>   *
>   *	Free the write buffers, tty queue and tty memory itself.
> + *	Decrement the owner_user_ns count.
>   *
>   *	Locking: none. Must be called after tty is definitely unused
>   */
> @@ -171,6 +172,7 @@ static void free_tty_struct(struct tty_struct *tty)
>  	put_device(tty->dev);
>  	kfree(tty->write_buf);
>  	tty->magic = 0xDEADDEAD;
> +	atomic_dec(&tty->owner_user_ns->count);
>  	kfree(tty);
>  }
>  
> @@ -3191,6 +3193,8 @@ 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 = current_user_ns();
> +	atomic_inc(&tty->owner_user_ns->count);

Hi,

CONFIG_USER_NS is an option, so unfortunately you need to do

	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