linux-kernel - Re: [PATCH 11/11] seccomp: Add tgid and tid into seccomp

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Date:	Tue, 29 Jul 2014 21:05:42 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Kees Cook <keescook@...omium.org>, Julien Tinnes <jln@...gle.com>,
	David Drysdale <drysdale@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Paolo Bonzini <pbonzini@...hat.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Moore <paul@...l-moore.com>,
	James Morris <james.l.morris@...cle.com>,
	Linux API <linux-api@...r.kernel.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/11] seccomp: Add tgid and tid into seccomp_data

On Mon, Jul 28, 2014 at 2:18 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Andy Lutomirski <luto@...capital.net> writes:
>
>> [cc: Eric Biederman]
>>
>
>> Can we do one better and add a flag to prevent any non-self pid
>> lookups?  This might actually be easy on top of the pid namespace work
>> (e.g. we could change the way that find_task_by_vpid works).
>>
>> It's far from just being signals.  There's access_process_vm, ptrace,
>> all the signal functions, clock_gettime (see CPUCLOCK_PID -- yes, this
>> is ridiculous), and probably some others that I've forgotten about or
>> never noticed in the first place.
>
> So here is the practical question.
>
> Are these processes that only can send signals to their thread group
> allowed to call fork()?
>
>
> If fork is allowed and all pid lookups are restricted to their own
> thread group that wait, waitpid, and all of the rest of the wait family
> will never return the pids of their children, and zombies will
> accumulate.  Aka the semantics are fundamentally broken.

Good point.

I can imagine at least three ways that fork() could continue working, though:

1. Allow lookups of immediate children, too.  (I don't love this one.)
2. Allow non-self pids to be translated in but not out.  This way
P_ALL will continue working.
3. Have the kernel treat any PID-restricted process as though it were NOCLDWAIT.

I think I like #3.  Thoughts?

>
> If fork is not allowed pid namespaces already solve this problem.

PID namespaces are fairly heavyweight.  Julien pointed out that using
PID namespaces requires a bunch of dummy PID 1 processes.

--Andy
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