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

[Andy Lutomirski]

On Thu, Jun 7, 2018 at 9:22 PM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
>
> 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
> >>> > >
>
> >> And why do we need ARCH_CET_EXEC?
> >>
> >> For background, I really really dislike adding new state that persists
> >> across exec().  It's nice to get as close to a clean slate as possible
> >> after exec() so that programs can run in a predictable environment.
> >> exec() is also a security boundary, and anything a task can do to
> >> affect itself after exec() needs to have its security implications
> >> considered very carefully.  (As a trivial example, you should not be
> >> able to use cetcmd ... sudo [malicious options here] to cause sudo to
> >> run with CET off and then try to exploit it via the malicious options.
> >>
> >> If a shutoff is needed for testing, how about teaching ld.so to parse
> >> LD_CET=no or similar and protect it the same way as LD_PRELOAD is
> >> protected.  Or just do LD_PRELOAD=/lib/libdoesntsupportcet.so.
> >>
> >
> > I will take a look.
>
> We can use LD_CET to turn off CET.   Since most of legacy binaries
> are compatible with shadow stack,  ARCH_CET_EXEC can be used
> to turn on shadow stack on legacy binaries:

Is there any reason you can't use LD_CET=force to do it for
dynamically linked binaries?

I find it quite hard to believe that forcibly CET-ifying a legacy
statically linked binary is a good idea.