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

[Eric W. Biederman]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:

> 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".

Interesting point.  Very interesting point.  At this point I don't know
that it is worth it, but that would trivially prevent any non-sense,
that might possibly happen.   The downside would be that the semantics
of /dev/ptmx would be more complicated.

Eric