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.
