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

Yu-cheng Yu
2020-05-18T14:21:22Z

On Mon, 2020-05-18 at 06:41 -0700, Dave Hansen 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/.

Sure. I will do that.

>
> > > > > 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()?

Yes.

>
> > > > > > > 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?

Yes.

Yu-cheng