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

[Andy Lutomirski <luto@kernel.org>]

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:
> >> > On Thu, 7 Jun 2018, H.J. Lu wrote:
> >> >> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@kernel.org> wrote:
> >> >> > Why is the lockout necessary?  If user code enables CET and tries to
> >> >> > run code that doesn't support CET, it will crash.  I don't see why we
> >> >> > need special code in the kernel to prevent a user program from calling
> >> >> > arch_prctl() and crashing itself.  There are already plenty of ways to
> >> >> > do that :)
> >> >>
> >> >> On CET enabled machine, not all programs nor shared libraries are
> >> >> CET enabled.  But since ld.so is CET enabled, all programs start
> >> >> as CET enabled.  ld.so will disable CET if a program or any of its shared
> >> >> libraries aren't CET enabled.  ld.so will lock up CET once it is done CET
> >> >> checking so that CET can't no longer be disabled afterwards.
> >> >
> >> > 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.

I'm concerned that the entire concept of locking CET is there to solve
a security problem that doesn't actually exist.