Re: [PATCH v10 01/26] Documentation/x86: Add CET description

[H.J. Lu]

On Mon, May 18, 2020 at 6:41 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 5/15/20 7:53 PM, Yu-cheng Yu wrote:
> > On Fri, 2020-05-15 at 16:56 -0700, Dave Hansen wrote:
> >> What's my recourse as an end user?  I want to run my app and turn off
> >> CET for that app.  How can I do that?
> >
> > GLIBC_TUNABLES=glibc.tune.hwcaps=-SHSTK,-IBT
>
> Like I mentioned to H.J., this is something that we need to at least
> acknowledge the existence of in the changelog and probably even the
> Documentation/.
>
> >>>>  I think you're saying that the CET-enabled binary would do
> >>>> arch_setup_elf_property() when it was first exec()'d.  Later, it could
> >>>> use the new prctl(ARCH_X86_CET_DISABLE) to disable its shadow stack,
> >>>> then fork() and the child would not be using CET.  Right?
> >>>>
> >>>> What is ARCH_X86_CET_DISABLE used for, anyway?
> >>>
> >>> Both the parent and the child can do ARCH_X86_CET_DISABLE, if CET is
> >>> not locked.
> >>
> >> Could you please describe a real-world example of why
> >> ARCH_X86_CET_DISABLE exists?  What kinds of apps will use it, or *are*
> >> using it?  Why was it created in the first place?
> >
> > Currently, ld-linux turns off CET if the binary being loaded does not support
> > CET.
>
> Great!  Could this please be immortalized in the documentation for the
> prctl()?
>
> >>>>>> Does this *code* work?  Could you please indicate which JITs have been
> >>>>>> enabled to use the code in this series?  How much of the new ABI is in use?
> >>>>>
> >>>>> JIT does not necessarily use all of the ABI.  The JIT changes mainly fix stack
> >>>>> frames and insert ENDBRs.  I do not work on JIT.  What I found is LLVM JIT fixes
> >>>>> are tested and in the master branch.  Sljit fixes are in the release.
> >>>>
> >>>> Huh, so who is using the new prctl() ABIs?
> >>>
> >>> Any code can use the ABI, but JIT code CET-enabling part mostly do not use these
> >>> new prctl()'s, except, probably to get CET status.
> >>
> >> Which applications specifically are going to use the new prctl()s which
> >> this series adds?  How are they going to use them?
> >>
> >> "Any code can use them" is not a specific enough answer.
> >
> > We have four arch_ptctl() calls.  ARCH_X86_CET_DISABLE and ARCH_X86_CET_LOCK are
> > used by ld-linux.  ARCH_X86_CET_STATUS are used in many places to determine if
> > CET is on.  ARCH_X86_CET_ALLOC_SHSTK is used in ucontext related handling, but
> > it can be use by any application to switch shadow stacks.
>
> Could some of this information be added to the documentation, please?
> It would also be nice to have some more details about how apps end up
> using ARCH_X86_CET_STATUS.  Why would they care that CET is on?

CET software spec is at

https://gitlab.com/x86-psABIs/x86-64-ABI/-/wikis/Intel-CET-extension

My CET presentation at 2018 LPC is at

https://www.linuxplumbersconf.org/event/2/contributions/147/attachments/72/83/CET-LPC-2018.pdf

I am working on an updated CET presentation for 2020 LPC.  Let me know
if you want to see the early draft.

-- 
H.J.