linux-kernel - Re: [PATCH 2/4] check_unsafe_exec: kill the dead -EAGAIN and clear_in

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Message-ID: <20131123153218.GA22071@redhat.com>
Date:	Sat, 23 Nov 2013 16:32:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	akpm@...ux-foundation.org, viro@...IV.linux.org.uk,
	keescook@...omium.org, mhocko@...e.cz, snanda@...omium.org,
	dserrg@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] check_unsafe_exec: kill the dead -EAGAIN and
	clear_in_exec logic

On 11/22, KOSAKI Motohiro wrote:
>
> >> I have found no problem in this patch. However, I have a very basic question.
> >> Why do we need to keep fs->in_exec?
> >
> > To ensure that a sub-thread can't create a new process with the same
> > ->fs while we are doing exec without LSM_UNSAFE_SHARE, I guess. This
> > is only for security/ code.
>
> But in LSM_UNSAFE_SHARE case, we have no check, right? I'm amazing why
> we don't need anything.

Yes. We rely on security/ code in this case, it can nack this exec if it
looks unsafe.

IOW. If LSM_UNSAFE_SHARE is not set, we promise that this fs has a single
user: the execing thread (it will kill other subthreads which can have the
same fs). That is why we need to cancel any attempt to create another
CLONE_FS process in between.

But let me repeat this is only my speculations, I know nothing about selinux
and selinux_bprm_set_creds() in particular. Although it looks obvious that
potentially exec with the shared ->fs has the additional security problems.

Kosaki, thank you for review!

Oleg.

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