linux-kernel - Re: [PATCH 0/9] proc: protect /proc/<pid>/* files across execve

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Message-ID: <20120311002725.GA4310@dztty>
Date:	Sun, 11 Mar 2012 01:27:25 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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>,
	Solar Designer <solar@...nwall.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	James Morris <james.l.morris@...cle.com>,
	Oleg Nesterov <oleg@...hat.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: [PATCH 0/9] proc: protect /proc/<pid>/* files across execve

On Sat, Mar 10, 2012 at 04:01:09PM -0800, Linus Torvalds wrote:
> On Sat, Mar 10, 2012 at 3:25 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> >
> > 1) Use the target exec_id to bind files to their exec_id task:
> >
> > For the REG files /proc/<pid>/{environ,pagemap,mem} we set the exec_id
> > of the proc_file_private to the target task, and we continue with
> > permission checks at open time, later on each read/write call the
> > permission checks are done + check the target exec_id if it equals the
> > exec_id of the proc_file_private that was set at open time, in other words
> > we bind the file to its task's exec_id, this way new exec programs can not
> > operate on the passed fd.
> 
> So the exec_id approach was totally broken when it was used for
> /proc/<pid>/mem, is there any reason to believe it's a good idea now?
Yes the previously one was broken since it was not a global uniq exec_id,
it was designed for threads tracking.

The current one is a global exec_id with uniq IDs, incremented on each
do_execve_common() call.

> It's entirely predictable, and you can make the exec_id match by
> simply forking elsewhere and then passing the fd around using unix
> domain sockets, since the exec_id is just updated by incrementing a
> counter.
For the fork one yes exec_id will match but we have the permission
checks (ptrace) at each syscall, so even if two processes share the same
exec_id the ptrace check should fail.

Yes it's predictable, but I don't see how you could pass the fd to another
extern privileged process without failing at the exec_id check.

-- 
tixxdz
http://opendz.org
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