kernel-hardening - Re: exec_id protection from bad child exit signals (was: Re: [PATCH 0/9] proc: protect /proc/<pid>/* files across execve)

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Date: Sun, 11 Mar 2012 19:20:07 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Solar Designer <solar@...nwall.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
        Djalal Harouni <tixxdz@...ndz.org>, linux-kernel@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Vasiliy Kulikov <segoon@...nwall.com>,
        Kees Cook <keescook@...omium.org>,
        WANG Cong <xiyou.wangcong@...il.com>,
        James Morris <james.l.morris@...cle.com>,
        linux-security-module@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alan Cox <alan@...rguk.ukuu.org.uk>,
        Greg KH <gregkh@...uxfoundation.org>, Ingo Molnar <mingo@...e.hu>,
        Stephen Wilson <wilsons@...rt.ca>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: exec_id protection from bad child exit signals (was: Re:
	[PATCH 0/9] proc: protect /proc/<pid>/* files across execve)

On 03/11, Solar Designer wrote:
>
> Actually, the original/historical purpose of the exec_id stuff was to
> protect privileged parent processes (those having done a SUID/SGID exec)
> from non-standard child exit signals, which could be set with clone().
> I think we may want to audit the current implementation and see if it
> still fully achieves the goal or maybe not (and fix it if not).

Funny that, I noticed this message only after I sent the question about
the current exec_id stuff.

> I include below pieces of the prototype implementation from
> linux-2.2.12-ow6.tar.gz released in 1999.

Perhaps I missed something, but ignoring the "cap_raised" issues, this
all is very simple. de_thread() should simply do:

	current->exit_signal = SIGCHLD;

	read_lock(&tasklist_lock);
	list_for_each_entry(p, &current->children, sibling)
		p->exit_signal = SIGCHILD;
	read_unlock(&tasklist_lock);

The only problem is CLONE_PARENT.

Oleg.

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