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Subject: Re: [klibc] process '/usr/bin/rsync' started with executable stack
To:     Kees Cook <keescook@chromium.org>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Ben Hutchings <ben@decadent.org.uk>
Cc:     Christian Kujau <lists@nerdbynature.de>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Willy Tarreau <w@1wt.eu>, linux-kernel@vger.kernel.org,
        klibc@lists.zytor.com, "H. Peter Anvin" <hpa@zytor.com>
References: <alpine.DEB.2.22.1.446.2006231023390.3892@trent.utfs.org>
 <20200624165148.GD31008@kadam> <202006241238.E9CB1CE85B@keescook>
 <20200625100429.GB2571@kadam> <202006251253.2893D4F67@keescook>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <46c65832-6677-f644-30f5-d7554826c9c4@csgroup.eu>
Date:   Fri, 26 Jun 2020 06:42:13 +0200
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Le 25/06/2020 à 22:20, Kees Cook a écrit :
> On Thu, Jun 25, 2020 at 01:04:29PM +0300, Dan Carpenter wrote:
>> On Wed, Jun 24, 2020 at 12:39:24PM -0700, Kees Cook wrote:
>>> On Wed, Jun 24, 2020 at 07:51:48PM +0300, Dan Carpenter wrote:
>>>> In Debian testing the initrd triggers the warning.
>>>>
>>>> [   34.529809] process '/usr/bin/fstype' started with executable stack
>>>
>>> Where does fstype come from there? I am going to guess it is either
>>> busybox or linked against klibc?
>>>
>>> klibc has known problems with executable stacks due to its trampoline
>>> implementation:
>>> https://wiki.ubuntu.com/SecurityTeam/Roadmap/ExecutableStacks
>>
>> Yeah.  It comes from klibc-utils.
> 
> This is exactly what I was worried about back in Feb:
> https://lore.kernel.org/lkml/202002251341.48BC06E@keescook/
> 
> This warning, combined with klibc-based initrds, makes the whole thing
> pointless because it will always warn once on boot for the klibc stack,
> and then not warn about anything else after that.
> 
> It looks like upstream klibc hasn't been touched in about 4 years, and
> it's been up to Ben to keep it alive in Debian.
> 
> A couple ideas, in order of my preference:
> 
> 1) stop using klibc-utils[1]. initramfs-tools-core is the only thing with a
>     dependency on klibc-utils. Only a few things are missing from busybox.

Does busybox cleanly build with klibc lib ?
If it does, is the result as small ?

> 
> 2) make the warning rate-limited instead?
> 
> 3) fix the use of trampolines in klibc

That's done, see 
https://git.kernel.org/pub/scm/libs/klibc/klibc.git/commit/?id=9d8d648e604026b32cad00a84ed6c29cbd157641

Discussed here 
https://lists.zytor.com/archives/klibc/2020-February/004271.html

Christophe

> 
> Thoughts?
> 
> -Kees
> 
> 
> [1] Ben appears well aware of this idea, as he suggested it in 2018. :)
>      https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=887159
>