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

[Matt Brown]

On 04/26/2017 08:47 AM, One Thousand Gnomes wrote:
>> open() what? As far as I know, for System-V PTYs, there is no path you can
>> open() that will give you the PTY master. Am I missing something?
>
> Sorry brain fade - no.
>>
>>>>> If I want to do the equvalent of the TIOCSTI attack then I fork a process
>>>>> and exit the parent. The child can now use ptrace to reprogram your shell
>>>>> to do whatever interesting things it likes (eg running child processes
>>>>> called "su" via a second pty/tty pair). Not exactly rocket science.
>>>>
>>>> Why would the child be able to ptrace the shell? AFAICS, in the most
>>>> relevant scenarios, the child can't ptrace the shell because the
>>>> shell has a different UID (in the case of e.g. su or sudo). In other
>>>
>>> If I am the attacker wanting to type something into your su when you go
>>> and su from my account, or where the user account is trojanned I do the
>>> following
>>>
>>> fork
>>> exit parent
>>> child ptraces the shell (same uid as it's not setuid)
>>>
>>> You type "su" return
>>> The modified shell opens a new pty/tty pair and runs su over it
>>> My ptrace hooks watch the pty/tty traffic until you go to the loo
>>> My ptrace hooks switch the console
>>> My ptrace hooks type lots of stuff and hack your machine while eating the
>>> output
>>>
>>> and you come back, do stuff and then exit
>>>
>>> And if you are in X it's even easier and I don't even need to care about
>>> sessions or anything. X has no mechanism to sanely fix the problem, but
>>> Wayland does.
>>
>> I think the "When using a program like su or sudo" in the patch description
>> refers to the usecase where you go from a more privileged context (e.g. a
>> root shell) to a less privileged one (e.g. a shell as a service-specific
>> account used to run a daemon), not the other way around.
>
> Which is the sudo case and why sudo uses a separate pty/tty pair as it's
> not just TIOCSTI that's an issue but there are a load of ioctls that do
> things like cause signals to the process or are just annoying -
> vhangup(), changing the speed etc
>
> (And for console changing the keymap - which is a nasty one)
>

Are any of these annoyances potential security issues? I would be happy
to add patches or modify this one to include extra hardening measures.

>> [However, I do think that it's a nice side effect of this patch that it will
>> prevent a malicious program from directly injecting something like an
>> SSH command into my shell in a sufficiently hardened environment
>> (with LSM restrictions that prevent the malicious program from opening
>> SSH keyfiles or executing another program that can do that). Although
>> you could argue that in such a case, the LSM should be taking care of
>> blocking TIOCSTI.]
>
> I would submit that creating a new pty/tty pair is the proper answer for
> that case however. Making the tty calls respect namespaces is however
> still a no-brainer IMHO.
>
> Alan
>
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