linux-kernel - Re: [PATCH 06/12] procfs: make /proc/*/stack 0400

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Date:	Wed, 2 Oct 2013 12:52:58 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Djalal Harouni <tixxdz@...il.com>
Subject: Re: [PATCH 06/12] procfs: make /proc/*/stack 0400

On Sat, Sep 28, 2013 at 7:35 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> On Thu, Sep 26, 2013 at 03:43:24PM -0500, Kees Cook wrote:
>> On Wed, Sep 25, 2013 at 3:14 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
>> > The /proc/*/stack contains sensitive information and currently its mode
>> > is 0444. Change this to 0400 so the VFS will be able to block
>> > unprivileged processes to get file descriptors on arbitrary privileged
>> > /proc/*/stack files.
>> >
>> > The /proc/*/stack is a /procfs ONE file that shares the same ->open()
>> > file operation with other ONE files. Doing a ptrace_may_access() check
>> > during open() might break userspace from accessing other ONE files
>> > like /proc/*/stat and /proc/*/statm.
>> >
>> > Therfore make it 0400 for now, and improve its check during ->read()
>> > in the next following patch.
>> >
>> > Cc: Kees Cook <keescook@...omium.org>
>> > Cc: Eric W. Biederman <ebiederm@...ssion.com>
>> > Signed-off-by: Djalal Harouni <tixxdz@...ndz.org>
>>
>> While the rest of the series is being discussed, I think it would be
>> nice to at least get this into the tree. Fixing this reduces which
>> processes are exposed to ASLR leaks. The rest of the series closes the
>> remaining holes.
>>
>> I would if it would be valuable adding a test for the identified leak
>> conditions to some test suite? LTP perhaps?
> I'm not familiar with LTP, but I guess a small program that perform I/O
> redirection and execve a suid-exec will do it?

It's actually a giant program. :)

http://ltp.sourceforge.net/

> I'll try to add code comment in fs/proc/base.c

Mostly I'm just thinking that this is a rather fragile area, and it
would be nice to have a set of tests that verify we haven't broken any
of the known test-cases.

-Kees

-- 
Kees Cook
Chrome OS Security
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