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

[Matt Brown]

On 04/23/2017 09:09 PM, Serge E. Hallyn wrote:
> 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());

Does this mean I need to also use put_user_ns(tty->owner_user_ns) in
free_tty_struct instead of atomic_dec(&tty->owner_user_ns->count) ?

> >  	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