Re: [PATCH v5 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

[Jann Horn]

On Tue, Apr 25, 2017 at 3:47 PM, Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx> wrote:
>> There could be a few user programs that would be effected by this
>> change.
>> See: <https://codesearch.debian.net/search?q=ioctl%5C%28.*TIOCSTI>
>> notable programs are: agetty, csh, xemacs and tcsh
>>
>> However, I still believe that this change is worth it given that the
>> Kconfig defaults to n. This will be a feature that is turned on for the
>> same reason that people activate it when using grsecurity. Users of this
>> opt-in feature will realize that they are choosing security over some OS
>> features
>
> Only in this case they are not.
>
> If I am at the point I have the ability to send you TIOCSTI you already
> lost because I can just open /dev/tty to get access to my controlling tty
> and use write().

In terms of PTYs, this patch does not try to prevent writes to a slave
device (which afaik is what /dev/tty will give you). It tries to prevent the
equivalent of writes to the master device. As far as I know, there is no
way to go from a slave to the corresponding master without having
access to the master in some other way already.