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

[Andy Lutomirski]

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 :)

> When the next exec() is called, CET
> feature is turned on/off based on the values set by ARCH_CET_EXEC.

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.

--Andy