Re: [PATCH v26 0/9] Control-flow Enforcement: Indirect Branch Tracking

H.J. Lu
2021-04-28T17:15:50Z

On Wed, Apr 28, 2021 at 9:25 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>
> On 4/28/2021 8:33 AM, David Laight wrote:
> > From: Andy Lutomirski
> >> Sent: 28 April 2021 16:15
> >>
> >> On Wed, Apr 28, 2021 at 7:57 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >>>
> >>> On Wed, Apr 28, 2021 at 7:52 AM Andy Lutomirski <luto@kernel.org> wrote:
> >>>>
> >>>> On Wed, Apr 28, 2021 at 7:48 AM David Laight <David.Laight@aculab.com> wrote:
> >>>>>
> >>>>> From: Yu-cheng Yu
> >>>>>> Sent: 27 April 2021 21:47
> >>>>>>
> >>>>>> Control-flow Enforcement (CET) is a new Intel processor feature that blocks
> >>>>>> return/jump-oriented programming attacks. Details are in "Intel 64 and
> >>>>>> IA-32 Architectures Software Developer's Manual" [1].
> >>>>> ...
> >>>>>
> >>>>> Does this feature require that 'binary blobs' for out of tree drivers
> >>>>> be compiled by a version of gcc that adds the ENDBRA instructions?
> >>>>>
> >>>>> If enabled for userspace, what happens if an old .so is dynamically
> >>>>> loaded?
> >>>
> >>> CET will be disabled by ld.so in this case.
> >>
> >> What if a program starts a thread and then dlopens a legacy .so?
> >
> > Or has shadow stack enabled and opens a .so that uses retpolines?
> >
>
> When shadow stack is enabled, retpolines are not necessary. I don't
> know if glibc has been updated for detection of this case. H.J.?
>
> >>>>> Or do all userspace programs and libraries have to have been compiled
> >>>>> with the ENDBRA instructions?
> >>>
> >>> Correct. ld and ld.so check this.
> >>>
> >>>> If you believe that the userspace tooling for the legacy IBT table
> >>>> actually works, then it should just work. Yu-cheng, etc: how well
> >>>> tested is it?
> >>>>
> >>>
> >>> Legacy IBT bitmap isn't unused since it doesn't cover legacy codes
> >>> generated by legacy JITs.
> >>>
> >>
> >> How does ld.so decide whether a legacy JIT is in use?
> >
> > What if your malware just precedes its 'jump into the middle of a function'
> > with a %ds segment override?
> >
>
> Do you mean far jump? It is not tracked by ibt, which tracks near
> indirect jump. The details can be found in Intel SDM.
>
> > I may have a real problem here.
> > We currently release program/library binaries that run on Linux
> > distributions that go back as far as RHEL6 (2.6.32 kernel era).
> > To do this everything is compiled on a userspace of the same vintage.
> > I'm not at all sure a new enough gcc to generate the ENDBR64 instructions
> > will run on the relevant system - and may barf on the system headers
> > even if we got it to run.
> > I really don't want to have to build multiple copies of everything.
>
> This is likely OK. We have tested many combinations. Should you run
> into any issue, please let glibc people know.
>

If you have a Tiger Lake laptop, you can install the CET kernel on
Fedora 34 or Ubuntu 20.10/21.04.

--
H.J.