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

[Andy Lutomirski]

On Tue, Jun 12, 2018 at 4:43 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>
> On Tue, Jun 12, 2018 at 3:03 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > On Thu, 7 Jun 2018, H.J. Lu wrote:
> >> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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.