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

[Matt Brown <matt@nmatt.com>]

On 04/23/2017 09:09 PM, Serge E. Hallyn wrote:
> Quoting Matt Brown (matt@nmatt.com):
>> 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@nmatt.com>
>> ---
>>  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