Re: [PATCH] devpts: Make each mount of devpts an independent filesystem.

From: Konstantin Khlebnikov
Date: Wed Apr 20 2016 - 11:34:39 EST

On Wed, Apr 20, 2016 at 5:55 PM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
>> On Tue, Apr 19, 2016 at 9:36 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>>> On Wed, Apr 20, 2016 at 6:04 AM, Eric W. Biederman
>>>> The kernel.pty.reserve sysctl is neutered with no way currently
>>>> implemented to be able to use the reserved ptys.
>>> I think we could convert this into reserve for init user namespace,
>>> ssh in host will work even if containers eaten all ptys.
>> Yes. That's basically how it effectively worked before (ie everything
>> but the initial non-newinstance devpts mount would be limited to the
>> non-reserved numbers).
>> We required the non-init namespaces to do a newinstance mount, so the
>> whole test for "newinstance" was effectively the same thing as just
>> checking for the init namespace from a security standpoint.
>> And in fact, rewriting it in that form (ie checking for init_ns) would
>> just make it much more obvious what the intent it.
> How does this sound.
> When mounting a devpts filesystem. We look at the caller (aka current)
> and if we are in the initial mount namespace set a flag in fsi that
> allows that instance of devpts to draw into the reserve pool.

Maybe just check current user namespace when task opens /dev/ptmx?

IIRR now check looks like: count < limit - (newinstance ? reserved : 0).
So, it will be count < limit - (current_in_init_userns ? 0 : newinstance).

> That will still allow crazy pieces of code like xen-create-instance run
> by root that mount a devpts filesystem in a chroot environment to draw
> into the reserved pool, but any sane users that set up their own mount
> namespace won't be able to user the reserve pool.
> I believe that will give an almost identical policy to what we have
> today, and it certainly makes a good default test for a container. Just
> for cleanliness containers (of anyone's definition) almost always use
> mount namespaces instead of chroots.
> Sigh one last past through all of the distros, to confirm that this
> works in practice.
> Eric