linux-kernel - Re: Linux 3.6

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Message-ID: <20121003200515.GZ9092@outflux.net>
Date:	Wed, 3 Oct 2012 13:05:15 -0700
From:	Kees Cook <kees@...flux.net>
To:	Nick Bowler <nbowler@...iptictech.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 3.6

Hi Nick,

3.6 introduced link restrictions:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=800179c9b8a1e796e441674776d11cd4c05d61d7

It sounds like you've got symlinks in a world-writable directory, and
you're following those symlinks across mis-matched uids. You can either
have the symlinks be owned by the directory owner, or you can turn off
symlink restrictions in sysctl:

# echo 0 > /proc/sys/fs/protected_symlinks

-Kees

On Wed, Oct 03, 2012 at 03:46:14PM -0400, Nick Bowler wrote:
> On 2012-09-30 17:38 -0700, Linus Torvalds wrote:
> > So here it is, 3.6 final. Sure, I'd have been happier with even fewer
> > changes, but that just never happens. And holding off the release
> > until people get too bored to send me the small stuff just makes the
> > next merge window more painful.
> 
> Just upgraded to 3.6 from 3.5, and now some of my kernel build scripts
> are throwing "permission denied" errors.  Apparently symlinks are
> broken somehow?
> 
>   # id
>   uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
> 
>   # ls -l /scratch_space/linux
>   drwxr-xr-x 24 nbowler eng 4096 2012-10-03 13:41 /scratch_space/linux
> 
>   # readlink /scratch_space/linux-2.6
>   linux
> 
>   # cd /scratch_space/linux
>   # pwd
>   /scratch_space/linux
> 
>   # cd /scratch_space/linux-2.6
>   cd: permission denied: /scratch_space/linux-2.6
> 
> WTF?  3.5 is fine.  I will try to bisect this later, but I figured I'd
> throw this out there now in case anyone has any ideas...
> 
> Cheers,
> -- 
> Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
> 
> --
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-- 
Kees Cook                                            @outflux.net
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