linux-kernel - Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if file's opener may access task

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Message-ID: <20131009111556.GA8781@dztty>
Date:	Wed, 9 Oct 2013 12:15:56 +0100
From:	Djalal Harouni <tixxdz@...ndz.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Kees Cook <keescook@...omium.org>,
	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>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Djalal Harouni <tixxdz@...il.com>
Subject: Re: [PATCH v2 2/9] procfs: add proc_allow_access() to check if
 file's opener may access task

On Wed, Oct 09, 2013 at 11:54:02AM +0100, Djalal Harouni wrote:
> On Mon, Oct 07, 2013 at 02:41:33PM -0700, Andy Lutomirski wrote:
> > On Sat, Oct 5, 2013 at 6:23 AM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> > > On Fri, Oct 04, 2013 at 03:17:08PM -0700, Andy Lutomirski wrote:
> > >>
> > >> Exactly.  Hence the NAK.
> > > But Having two LSM Hooks there is really not practical!
> > 
> > It'd doable *if* it turns out that it's the right solution.
> > 
> > But revoke seems much more likely to be simple, comprehensible, and
> > obviously correct to me.
> Yes Andy, I agree, revoke is much better!
> 
> But it will not handle or fix all the situations, as I've said what if
> revoke is not invloved here? there is no an execve from the target task!
> 
> 
> Remember:
> /proc/*/{stat,maps} and perhaps others have 0444 and don't have ptrace
> checks during ->open() to not break some userspace... especially
> /proc/*/stat file
> 
> 
> So you will have an fd on these privileged files!
> 
> Current will execve a privileged process, and pass ptrace_may_access()
> checks during ->read()...
> 
> Here revoke is not involved at all! so it will not fix these files and
> they will continue to be vulnerable.
> 
> IMO to fix them, we must have the correct ptrace_may_access() check and
> this involves: current doing an execve + current's cred
> 
> 
> 
> BTW, Andy we already return 0 (end of file) for /proc/*/mem
>   ->read()
>     ->mem_read()
>       ->mem_rw()
>         if (!atomic_inc_not_zero(&mm->mm_users))
> 		return 0
> 
> So can this be considered some sort of simple revoke?
Or create dummy compat-quirk maps inode as Ingo put it in the other mail:
00000000-00000000 ---p 00000000 00:00 0
...

For /proc/*/maps files, to not break userspace


-- 
Djalal Harouni
http://opendz.org
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