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

[Kees Cook <keescook@chromium.org>]

On Thu, Jan 5, 2012 at 2:18 PM, Nick Bowler <nbowler@elliptictech.com> wrote:
> On 2012-01-05 12:55 -0800, Kees Cook wrote:
>> On Thu, Jan 5, 2012 at 12:08 PM, Nick Bowler <nbowler@elliptictech.com> wrote:
>> > On 2012-01-05 11:34 -0800, Kees Cook wrote:
>> >> On Thu, Jan 5, 2012 at 6:30 AM, Nick Bowler <nbowler@elliptictech.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