linux-kernel - Re: [PATCH v2 1/2] tiocsti-restrict : Add owner user namespace to tty

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Message-ID: <0c59f0c4-4202-86ad-eddf-c2c6ad11c69c@nmatt.com>
Date:   Sun, 23 Apr 2017 16:23:56 -0400
From:   Matt Brown <matt@...tt.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     serge@...lyn.com, jmorris@...ei.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        jslaby@...e.com, corbet@....net, Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel-hardening@...ts.openwall.com,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 1/2] tiocsti-restrict : Add owner user namespace to
 tty_struct

On 04/23/2017 01:02 PM, Jann Horn wrote:
> On Sun, Apr 23, 2017 at 9:24 AM, Matt Brown <matt@...tt.com> wrote:
>> 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@...tt.com>
>> ---
>>  drivers/tty/tty_io.c | 1 +
>>  include/linux/tty.h  | 2 ++
>>  2 files changed, 3 insertions(+)
>>
>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>> index e6d1a65..e774385 100644
>> --- a/drivers/tty/tty_io.c
>> +++ b/drivers/tty/tty_io.c
>> @@ -3191,6 +3191,7 @@ 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 = current_user_ns();
>
> Why are you not taking a reference to the userns?
>

current_user_ns() returns the user namespace of the current task. What
do you mean "not taking a reference to the userns?"