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

[David Laight]

From: Andy Lutomirski
> Sent: 28 April 2021 16:15
> 
> On Wed, Apr 28, 2021 at 7:57 AM H.J. Lu <hjl.tools@xxxxxxxxx> wrote:
> >
> > On Wed, Apr 28, 2021 at 7:52 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Apr 28, 2021 at 7:48 AM David Laight <David.Laight@xxxxxxxxxx> 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?

> > > > 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?

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.

	David

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