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

[H.J. Lu]

On Thu, Jun 7, 2018 at 4:01 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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
>> >> > >
>> >> > > 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*.

It is to prevent malicious code from disabling CET.

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

No. vDSO is loaded by kernel.  vDSO in CET kernel is CET
compatible.

-- 
H.J.