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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