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

[Andy Lutomirski]

On Thu, Jun 7, 2018 at 3:02 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>
> On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > On Thu, Jun 7, 2018 at 1:33 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> >>
> >> On Thu, 2018-06-07 at 11:48 -0700, Andy Lutomirski wrote:
> >> > On Thu, Jun 7, 2018 at 7:41 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> >> > >
> >> > > The following operations are provided.
> >> > >
> >> > > ARCH_CET_STATUS:
> >> > >         return the current CET status
> >> > >
> >> > > ARCH_CET_DISABLE:
> >> > >         disable CET features
> >> > >
> >> > > ARCH_CET_LOCK:
> >> > >         lock out CET features
> >> > >
> >> > > ARCH_CET_EXEC:
> >> > >         set CET features for exec()
> >> > >
> >> > > ARCH_CET_ALLOC_SHSTK:
> >> > >         allocate a new shadow stack
> >> > >
> >> > > ARCH_CET_PUSH_SHSTK:
> >> > >         put a return address on shadow stack
> >> > >
> >> > > ARCH_CET_ALLOC_SHSTK and ARCH_CET_PUSH_SHSTK are intended only for
> >> > > the implementation of GLIBC ucontext related APIs.
> >> >
> >> > Please document exactly what these all do and why.  I don't understand
> >> > what purpose ARCH_CET_LOCK and ARCH_CET_EXEC serve.  CET is opt in for
> >> > each ELF program, so I think there should be no need for a magic
> >> > override.
> >>
> >> CET is initially enabled if the loader has CET capability.  Then the
> >> loader decides if the application can run with CET.  If the application
> >> cannot run with CET (e.g. a dependent library does not have CET), then
> >> the loader turns off CET before passing to the application.  When the
> >> loader is done, it locks out CET and the feature cannot be turned off
> >> anymore until the next exec() call.
> >
> > 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.

Yeah, I got that.  No one has explained *why*.

(Also, shouldn't the vDSO itself be marked as supporting CET?)