Re: [PATCH 0/2] aarch64: Enable pointer auth in PLT

[Ramana Radhakrishnan <ramana dot gcc at googlemail dot com>]

On Fri, Jun 21, 2019 at 12:33 PM Szabolcs Nagy <Szabolcs.Nagy@arm.com> wrote:
>
> On 21/06/2019 11:46, Florian Weimer wrote:
> > * Szabolcs Nagy:
> >
> >> On 21/06/2019 10:12, Florian Weimer wrote:
> >>> * Szabolcs Nagy:
> >>>
> >>>> Pointer authentication is an armv8.3-a extension and it can be used
> >>>> to harden PLTGOT entries when that is not read-only protected.
> >>>>
> >>>> binutils bfd linker now supports creating binaries with -z pac-plt
> >>>> and then PLT entries authenticate the pointers loaded from PLTGOT.
> >>>>
> >>>> To support such binaries the glibc dynamic linker has to "sign" the
> >>>> pointers in the PLTGOT. On cpus without pointer authentication support,
> >>>> both the sign and authentication operations are nops, so such binaries
> >>>> may appear to work now, but on a new cpu they will crash without glibc
> >>>> support.
> >>>
> >>> I think you need to enable this feature in a different way, similar to
> >>> the non-executable stack.  Otherwise, people will have to decide whether
> >>> they want to build portable binaries or binaries with security
> >>> hardening, which is not desirable.
> >>
> >> with non-executable stack, a hardened binary works on
> >> an old system that uses executable stack.
> >>
> >> in this case a hardened binary does not work on an old
> >> system that's unaware of pointer auth.
> >
> > Does it work on an old system without kernel support for pointer
> > authentication?  With just support in the CPU?
>
> the binaries work on an old system if pauth is not
> enabled by the kernel.
>
> i think we don't have prctl or other control
> in linux to enable/disable pauth per process
> right now so it is not opt-in, the kernel just
> informs userspace with a HWCAP that it is enabled.

There was certainly talk about a prctl call for pauth with the Android
guys as they were interested in a model with zygote.

Ramana