linux-kernel - Re: [PATCH v2012.1] fs: symlink restrictions on sticky directories

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Message-ID: <CAGXu5jKe_qpQ9yG4f0J_94WNKYy_qdDNdVW4v74ZivwO4Xy0Rg@mail.gmail.com>
Date:	Thu, 5 Jan 2012 12:55:25 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Nick Bowler <nbowler@...iptictech.com>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Federica Teodori <federica.teodori@...glemail.com>,
	Lucian Adrian Grijincu <lucian.grijincu@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Eric Paris <eparis@...hat.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Dan Rosenberg <drosenberg@...curity.com>,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2012.1] fs: symlink restrictions on sticky directories

On Thu, Jan 5, 2012 at 12:08 PM, Nick Bowler <nbowler@...iptictech.com> wrote:
> On 2012-01-05 11:34 -0800, Kees Cook wrote:
>> On Thu, Jan 5, 2012 at 6:30 AM, Nick Bowler <nbowler@...iptictech.com> wrote:
>> > On 2012-01-04 12:18 -0800, Kees Cook wrote:
>> >> diff --git a/fs/Kconfig b/fs/Kconfig
>> >> index 5f4c45d..26ede24 100644
>> >> --- a/fs/Kconfig
>> >> +++ b/fs/Kconfig
>> >> @@ -278,3 +278,19 @@ source "fs/nls/Kconfig"
>> >>  source "fs/dlm/Kconfig"
>> >>
>> >>  endmenu
>> >> +
>> >> +config PROTECTED_STICKY_SYMLINKS
>> >> +     bool "Protect symlink following in sticky world-writable directories"
>> >> +     default y
>> > [...]
>> >
>> > Why do we need a config option for this?  What's wrong with just using
>> > the sysctl?
>>
>> This way the sysctl can configured directly without needing to have a
>> distro add a new item to sysctl.conf.
>
> This seems totally pointless to me.  There are tons of sysctls that
> don't have Kconfig options: what makes this one special?

Most are system tuning; this is directly related to vulnerability
mitigation. Besides, I like having CONFIGs for sysctls because then I
can build my kernel the way I want it without having to worry about
tweaking my userspace sysctl.conf file, or run newer kernels on older
userspaces, etc etc.

>> > Why have you made this option "default y", when enabling it clearly
>> > makes user-visible changes to kernel behaviour?
>>
>> Ingo specifically asked me to make it "default y".
>
> But this is a brand new feature that changes longstanding behaviour of
> various syscalls.  Making it default to enabled is rather mean to users
> (since it will tend to get enabled by "oldconfig") and seems almost
> guaranteed to cause regressions.

I couldn't disagree more. There has been zero evidence of this change
causing anything but regressions in _attacks_. :P If anything, I think
there should be no CONFIG and no sysctl, and it should be entirely
non-optional. But since this patch needs consensus, I have provided
knobs to control it. This is the way of security features. For
example, years back I added a knob for /proc/$pid/maps protection
being optional (and defaulted it to insecure because of people's fear
of regression), and eventually it changed to secure-by-default, and
then the knob went away completely because it didn't actually cause
problems.

-Kees

-- 
Kees Cook
ChromeOS Security
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