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

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Jun 7, 2018 at 9:35 PM, Andy Lutomirski <luto@kernel.org> wrote:
> On Thu, Jun 7, 2018 at 9:22 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>>
>> On Thu, Jun 7, 2018 at 3:02 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> > On Thu, Jun 7, 2018 at 2:01 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> >> On Thu, Jun 7, 2018 at 1:33 PM Yu-cheng Yu <yu-cheng.yu@intel.com> 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@intel.com> 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?

We need to enable shadow stack from the start.  Otherwise function
return will fail when returning from callee with shadow stack to caller
without shadow stack.

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

We'd like to provide protection as much as we can.

-- 
H.J.