linux-kernel - Re: [patch 2/2] fs, proc: Introduce the /proc/<pid>/map

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Date:	Wed, 14 Sep 2011 20:21:43 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>, Pavel Machek <pavel@....cz>,
	Andrew Morton <akpm00@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"containers@...ts.osdl.org" <containers@...ts.osdl.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Kirill Shutemov <kirill@...temov.name>,
	James Bottomley <jbottomley@...allels.com>,
	Nathan Lynch <ntl@...ox.com>, Zan Lynx <zlynx@....org>,
	Daniel Lezcano <dlezcano@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch 2/2] fs, proc: Introduce the /proc/<pid>/map_files/
 directory v12

On Wed, Sep 14, 2011 at 20:13 +0400, Pavel Emelyanov wrote:
> > No, I mean something else.  Assume you have a task, which does the
> > steps:
> > 
> > 1) opens some sensitive file as root.  This file is e.g. 0700.
> > 
> > 2) mmaps the file via opened fd, either RO or RW.
> > 
> > 3) closes fd.
> > 
> > 4) drops root.
> > 
> > Now it has a mapping of a privileged file, but cannot get fd of it
> > anyhow.  With map_files/ he may open his own /proc/$$/map_files/, pass
> > ptrace() check, and get fd of the privileged file.  He cannot explicitly
> > open it as it is 0700, but he may open it via map_files/ and get RO/RW
> > fd.
> > 
> 
> What is the problem here - the fact that we have some file considered to
> be private be open-able by somebody else, or the fact that we can truncate
> the file being mapped?

The latter - the file, which is considered to be restricted to a process
as W only without ability to truncate it, now can be truncated.  The
process after (4) had no such ability without map_files/ with current
permission model of mmap'ed files.  Or I am missing something?

FWIW, ftruncate() might be not the only syscall which makes sense to use
in this case, I just thought about it.

Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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