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Date:   Sat, 25 Jul 2020 19:07:37 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Ben Hutchings <ben@decadent.org.uk>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        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>
Subject: Re: process '/usr/bin/rsync' started with executable stack
Message-ID: <202007251907.DC4D4C6C15@keescook>
References: <alpine.DEB.2.22.1.446.2006231023390.3892@trent.utfs.org>
 <20200624165148.GD31008@kadam>
 <202006241238.E9CB1CE85B@keescook>
 <20200625100429.GB2571@kadam>
 <202006251253.2893D4F67@keescook>
 <6898a5af75e165fb9524558804fca9f46f98b633.camel@decadent.org.uk>
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On Sat, Jul 25, 2020 at 10:27:56PM +0100, Ben Hutchings wrote:
> On Thu, 2020-06-25 at 13:20 -0700, Kees Cook wrote:
> > 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.
> > 
> > 2) make the warning rate-limited instead?
> > 
> > 3) fix the use of trampolines in klibc
> 
> It only uses trampolines on alpha, m68k, parisc, s390, and sparc32.  As
> of today, the master branch should correctly enable executable stacks
> on these and only these architecture.
> 
> I have a development branch that sets sa_restorer and disables
> executable stacks on alpha, s390, and sparc32:
> 
> https://git.kernel.org/pub/scm/libs/klibc/klibc.git/log/?h=execstack-fixes

Awesome! Thanks for tackling this. :)

-- 
Kees Cook