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

[Eric W. Biederman]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Fri, Apr 8, 2016 at 11:51 AM, Eric W. Biederman
> <ebiederm@xxxxxxxxxxxx> wrote:
>>
>> Given that concern under the rule we don't break userspace we have to
>> check the permissions of /dev/pts/ptmx when we are creating a new pty,
>> on a instance of devpts that was created with newinstance.
>
> The rule is that we don't break existing installations.
>
> If somebody has root and installs a "ptmx" node in an existing mount
> space next to a pts subdirectory, that's not a security issue, nor is
> it going to break any existing installation.

Anyone can do that with "mount --bind".  All it takes is root in a user
namespace.  I can get root in a user namespace as no one special.

So someone may have set such a thing up, and it may now be possible
to defeat such a regime as anyone.

In practice I suspect all such cases are handled by actually hiding the
mount of devpts in another mount namespace.

> The whole point of the patch is that yes, we change semantics. A
> change of semantics means that people will see situations where the
> behavior is different. But that's not "breaking user space", that's
> just "ok, you can see a difference".

If we don't want to care about this case, and if someone complains about
a security regression readd my permission checks I am fine with that.

But I don't want to let a possibility of breaking someone (that I don't
know how to test for, and would be silent breakage) slip through.

Eric