Re: [PATCH 06/10] x86/cet: Add arch_prctl functions for shadow stack

[Andy Lutomirski <luto@kernel.org>]

On Tue, Jun 12, 2018 at 12:34 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Tue, Jun 12, 2018 at 11:59 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Tue, 12 Jun 2018, H.J. Lu wrote:
> >> On Tue, Jun 12, 2018 at 9:34 AM, Andy Lutomirski <luto@kernel.org> wrote:
> >> > On Tue, Jun 12, 2018 at 9:05 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >> >> On Tue, Jun 12, 2018 at 9:01 AM, Andy Lutomirski <luto@kernel.org> wrote:
> >> >> > On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >> >> >> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> >> >> >> > That works for stuff which loads all libraries at start time, but what
> >> >> >> > happens if the program uses dlopen() later on? If CET is force locked and
> >> >> >> > the library is not CET enabled, it will fail.
> >> >> >>
> >> >> >> That is to prevent disabling CET by dlopening a legacy shared library.
> >> >> >>
> >> >> >> > I don't see the point of trying to support CET by magic. It adds complexity
> >> >> >> > and you'll never be able to handle all corner cases correctly. dlopen() is
> >> >> >> > not even a corner case.
> >> >> >>
> >> >> >> That is a price we pay for security.  To enable CET, especially shadow
> >> >> >> shack, the program and all of shared libraries it uses should be CET
> >> >> >> enabled.  Most of programs can be enabled with CET by compiling them
> >> >> >> with -fcf-protection.
> >> >> >
> >> >> > If you charge too high a price for security, people may turn it off.
> >> >> > I think we're going to need a mode where a program says "I want to use
> >> >> > the CET, but turn it off if I dlopen an unsupported library".  There
> >> >> > are programs that load binary-only plugins.
> >> >>
> >> >> You can do
> >> >>
> >> >> # export GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK
> >> >>
> >> >> which turns off shadow stack.
> >> >>
> >> >
> >> > Which exactly illustrates my point.  By making your security story too
> >> > absolute, you'll force people to turn it off when they don't need to.
> >> > If I'm using a fully CET-ified distro and I'm using a CET-aware
> >> > program that loads binary plugins, and I may or may not have an old
> >> > (binary-only, perhaps) plugin that doesn't support CET, then the
> >> > behavior I want is for CET to be on until I dlopen() a program that
> >> > doesn't support it.  Unless there's some ABI reason why that can't be
> >> > done, but I don't think there is.
> >>
> >> We can make it opt-in via GLIBC_TUNABLES.  But by default, the legacy
> >> shared object is disallowed when CET is enabled.
> >
> > That's a bad idea. Stuff has launchers which people might not be able to
> > change. So they will simply turn of CET completely or it makes them hack
> > horrible crap into init, e.g. the above export.
> >
> > Give them sane kernel options:
> >
> >      cet = off, relaxed, forced
> >
> > where relaxed allows to run binary plugins. Then let dlopen() call into the
> > kernel with the filepath of the library to check for CET and that will tell
> > you whether its ok or or not and do the necessary magic in the kernel when
> > CET has to be disabled due to a !CET library/application.
> >
> > That's also making the whole thing independent of magic glibc environment
> > options and allows it to be used all over the place in the same way.
>
> This is very similar to our ARCH_CET_EXEC proposal which controls how
> CET should be enforced.   But Andy thinks it is a bad idea.
>

I do think it's a bad idea to have a new piece of state that survives
across exec().  It's going to have nasty usability problems and nasty
security problems.

We may need a mode by which glibc can turn CET *back off* even after a
program had it on if it dlopens() an old binary.  Or maybe there won't
be demand.  I can certainly understand why the CET_LOCK feature is
there, although I think we need a way to override it using something
like ptrace().  I'm not convinced that CET_LOCK is really needed, but
someone who understand the thread model should chime in.

Kees, do you know anyone who has a good enough understanding of
usermode exploits and how they'll interact with CET?

--Andy