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Message-ID: <CAGXu5j++E2o+HajzdZdxvJ2cxw3aSLX6gHvXZnzKW56ZyuwjeQ@mail.gmail.com>
Date: Thu, 5 Jan 2012 16:08:51 -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 2:18 PM, Nick Bowler <nbowler@...iptictech.com> wrote:
> On 2012-01-05 12:55 -0800, Kees Cook wrote:
>> 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.
>
> I agree that having kconfig knobs for sysctls may be convenient for some
> users. But every kconfig option we add requires the user to make a
> decision before building their kernel. In this case, this decision is
> a waste of time because the option doesn't really affect the kernel in a
> meaningful way: either choice can be easily changed from userspace after
> booting. A similar argument could be applied to almost any sysctl, and
> we could add hundreds of new Kconfig options to control their default
> values. The result would be untenable.
>
> Perhaps what we need instead is a way to set arbitrary sysctls from the
> kernel command line. This could easily be done by an initramfs, and not
> require any changes to the kernel at all.
At present, I answer to Ingo and Al. I have no strong opinion on this
area of the patch. Ingo requested it be this way, so I'm leaving it.
:)
>> >> > 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_.
>
> We have absolutely no idea what applications people are running that
> will be affected by this change. Of course there's no evidence of
> breakage, because affected users (if any) have not had a single chance
> to try this new feature out: it's not in the kernel yet.
Ubuntu has been running with this restriction since Oct 2010. I've
seen 0 reports of this causing a regression. Openwall and grsecurity
have had this restriction for way longer without problem too.
>> 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/$pid/maps
>> protection being optional (and defaulted it to insecure because of
>> people's fear of regression), and eventually it changed to
>> secure-by-default, and then the knob went away completely because it
>> didn't actually cause problems.
>
> The process you describe above for /proc/$pid/maps is the right way to
> change kernel behaviour while mitigating the risk of regressions. With
> this patch, you've skipped all those important steps!
Like I said, I'm trying to keep the VFS maintainers happy. My original
patch had the default as 0 -- which was following my original
conservative approach.
-Kees
--
Kees Cook
ChromeOS Security
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