Re: [PATCH 01/13] devpts: Teach /dev/ptmx to find the associated devpts via path lookup

[Andy Lutomirski]

On Sat, Apr 9, 2016 at 6:27 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Apr 9, 2016 5:45 PM, "Andy Lutomirski" <luto@xxxxxxxxxxxxxx> wrote:
>>
>>
>> What we *do* want to do, though, is to prevent the following:
>
> I don't see the point. Why do you bring up this insane scenario that nobody
> can possibly care about?
>
> So you actually have any reason to believe somebody does that?
>
> I already asked about that earlier, and the silence was deafening.

I have no idea, but I'm generally uncomfortable with magical things
that bypass normal security policy.

That being said, here's an idea for fixing this, at least in the long
run.  Add a new devpts mount option "no_ptmx_redirect" that turns off
this behavior for the super in question.  That is, opening /dev/ptmx
if "pts/ptmx" points to something with no_ptmx_redirect set will fail.
Distros shipping new kernels could be encouraged to (finally!) make
/dev/ptmx a symlink and set this option.

We just might be able to get away with spelling that option "newinstance".